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View绘制过程详解

时间:2015-04-10 15:34:28      阅读:147      评论:0      收藏:0      [点我收藏+]

标签:布局   界面   view   

View绘制过程详解

界面窗口的根布局是DecorView,该类继承自FrameLayout.说到View绘制,想到的就是从这里入手,而FrameLayout继承自ViewGroup。感觉绘制肯定会在ViewGroup或者View中,
但是木有找到。发现ViewGroup实现ViewParent接口,而ViewParent有一个实现类是ViewRootImplViewGruop中会使用ViewRootImpl

/**
 * The top of a view hierarchy, implementing the needed protocol between View
 * and the WindowManager.  This is for the most part an internal implementation
 * detail of {@link WindowManagerGlobal}.
 *
 * {@hide}
 */
@SuppressWarnings({"EmptyCatchBlock", "PointlessBooleanExpression"})
public final class ViewRootImpl implements ViewParent,
        View.AttachInfo.Callbacks, HardwareRenderer.HardwareDrawCallbacks {

        }

View的绘制过程从ViewRootImpl.performTraversals()方法开始。
首先先说明一下,这部分代码比较多,逻辑也比较麻烦,很容易弄晕,如果感觉看起来费劲,就跳过这一块,直接到下面的Measure、Layout、Draw部分开始看。
我也没有全部弄清楚,我只是把里面的步骤标注了下。

private void performTraversals() {
    // ... 此处省略源代码N行

    // 是否需要Measure
    if (!mStopped) {
        boolean focusChangedDueToTouchMode = ensureTouchModeLocally(
                (relayoutResult&WindowManagerGlobal.RELAYOUT_RES_IN_TOUCH_MODE) != 0);
        if (focusChangedDueToTouchMode || mWidth != host.getMeasuredWidth()
                || mHeight != host.getMeasuredHeight() || contentInsetsChanged) {
            // 这里是获取widthMeasureSpec,这俩参数不是一般的尺寸数值,而是将模式和尺寸组合在一起的数值.
            // getRootMeasureSpec方法内部会使用MeasureSpec.makeMeasureSpec()方法来组装一个MeasureSpec,
            // 当lp.width参数等于MATCH_PARENT的时候,MeasureSpec的specMode就等于EXACTLY,当lp.width等于WRAP_CONTENT的时候,MeasureSpec的specMode就等于AT_MOST。
            // 并且MATCH_PARENT和WRAP_CONTENT时的specSize都是等于windowSize的,也就意味着根视图总是会充满全屏的。
            int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width);
            int childHeightMeasureSpec = getRootMeasureSpec(mHeight, lp.height);

            if (DEBUG_LAYOUT) Log.v(TAG, "Ooops, something changed!  mWidth="
                    + mWidth + " measuredWidth=" + host.getMeasuredWidth()
                    + " mHeight=" + mHeight
                    + " measuredHeight=" + host.getMeasuredHeight()
                    + " coveredInsetsChanged=" + contentInsetsChanged);

            // 调用PerformMeasure方法。
             // Ask host how big it wants to be
            performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);

            // Implementation of weights from WindowManager.LayoutParams
            // We just grow the dimensions as needed and re-measure if
            // needs be
            int width = host.getMeasuredWidth();
            int height = host.getMeasuredHeight();
            boolean measureAgain = false;

            if (lp.horizontalWeight > 0.0f) {
                width += (int) ((mWidth - width) * lp.horizontalWeight);
                childWidthMeasureSpec = MeasureSpec.makeMeasureSpec(width,
                        MeasureSpec.EXACTLY);
                measureAgain = true;
            }
            if (lp.verticalWeight > 0.0f) {
                height += (int) ((mHeight - height) * lp.verticalWeight);
                childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(height,
                        MeasureSpec.EXACTLY);
                measureAgain = true;
            }

            if (measureAgain) {
                if (DEBUG_LAYOUT) Log.v(TAG,
                        "And hey let‘s measure once more: width=" + width
                        + " height=" + height);
                performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
            }

            layoutRequested = true;
        }
    }

    final boolean didLayout = layoutRequested && !mStopped;
    boolean triggerGlobalLayoutListener = didLayout
            || mAttachInfo.mRecomputeGlobalAttributes;
    // 是否需要Layout
    if (didLayout) {
        // 调用performLayout方法。
        performLayout(lp, desiredWindowWidth, desiredWindowHeight);

        // By this point all views have been sized and positioned
        // We can compute the transparent area

        if ((host.mPrivateFlags & View.PFLAG_REQUEST_TRANSPARENT_REGIONS) != 0) {
            // start out transparent
            // TODO: AVOID THAT CALL BY CACHING THE RESULT?
            host.getLocationInWindow(mTmpLocation);
            mTransparentRegion.set(mTmpLocation[0], mTmpLocation[1],
                    mTmpLocation[0] + host.mRight - host.mLeft,
                    mTmpLocation[1] + host.mBottom - host.mTop);

            host.gatherTransparentRegion(mTransparentRegion);
            if (mTranslator != null) {
                mTranslator.translateRegionInWindowToScreen(mTransparentRegion);
            }

            if (!mTransparentRegion.equals(mPreviousTransparentRegion)) {
                mPreviousTransparentRegion.set(mTransparentRegion);
                mFullRedrawNeeded = true;
                // reconfigure window manager
                try {
                    mWindowSession.setTransparentRegion(mWindow, mTransparentRegion);
                } catch (RemoteException e) {
                }
            }
        }

        if (DBG) {
            System.out.println("======================================");
            System.out.println("performTraversals -- after setFrame");
            host.debug();
        }
    }

    // 是否需要Draw
    if (!cancelDraw && !newSurface) {
        if (!skipDraw || mReportNextDraw) {
            if (mPendingTransitions != null && mPendingTransitions.size() > 0) {
                for (int i = 0; i < mPendingTransitions.size(); ++i) {
                    mPendingTransitions.get(i).startChangingAnimations();
                }
                mPendingTransitions.clear();
            }
            // 调用performDraw方法
            performDraw();
        }
    } else {
        if (viewVisibility == View.VISIBLE) {
            // Try again
            scheduleTraversals();
        } else if (mPendingTransitions != null && mPendingTransitions.size() > 0) {
            for (int i = 0; i < mPendingTransitions.size(); ++i) {
                mPendingTransitions.get(i).endChangingAnimations();
            }
            mPendingTransitions.clear();
        }
    }

    mIsInTraversal = false;
}

从上面源码可以看出,performTraversals()方法中会依次做三件事:
- performMeasure(), 内部是mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);测量View大小。这里顺便提一下,这个mView是什么?它就是Window最顶成的View(DecorView),它是FrameLayout的子类。
- performLayout(), 内部是mView.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());视图布局,确定View位置。
- performDraw(), 内部是draw(fullRedrawNeeded); 绘制界面。

至此View绘制的三个过程已经展现:

Measure

performMeasure方法如下:

private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) {
    Trace.traceBegin(Trace.TRACE_TAG_VIEW, "measure");
    try {
        mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);
    } finally {
        Trace.traceEnd(Trace.TRACE_TAG_VIEW);
    }
}

performMeasure()方法中会调用View.measure()方法, 源码如下:

/**
 * <p>
 * This is called to find out how big a view should be. The parent
 * supplies constraint information in the width and height parameters.
 * </p>
 *
 * <p>
 * The actual measurement work of a view is performed in
 * {@link #onMeasure(int, int)}, called by this method. Therefore, only
 * {@link #onMeasure(int, int)} can and must be overridden by subclasses.
 * </p>
 *
 *
 * @param widthMeasureSpec Horizontal space requirements as imposed by the
 *        parent
 * @param heightMeasureSpec Vertical space requirements as imposed by the
 *        parent
 *
 * @see #onMeasure(int, int)
 */
public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
    boolean optical = isLayoutModeOptical(this);
    if (optical != isLayoutModeOptical(mParent)) {
        Insets insets = getOpticalInsets();
        int oWidth  = insets.left + insets.right;
        int oHeight = insets.top  + insets.bottom;
        widthMeasureSpec  = MeasureSpec.adjust(widthMeasureSpec,  optical ? -oWidth  : oWidth);
        heightMeasureSpec = MeasureSpec.adjust(heightMeasureSpec, optical ? -oHeight : oHeight);
    }

    // Suppress sign extension for the low bytes
    long key = (long) widthMeasureSpec << 32 | (long) heightMeasureSpec & 0xffffffffL;
    if (mMeasureCache == null) mMeasureCache = new LongSparseLongArray(2);

    if ((mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT ||
            widthMeasureSpec != mOldWidthMeasureSpec ||
            heightMeasureSpec != mOldHeightMeasureSpec) {

        // first clears the measured dimension flag
        mPrivateFlags &= ~PFLAG_MEASURED_DIMENSION_SET;

        resolveRtlPropertiesIfNeeded();

        int cacheIndex = (mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT ? -1 :
                mMeasureCache.indexOfKey(key);
        if (cacheIndex < 0 || sIgnoreMeasureCache) {
            // 调用onMeasure方法
            // measure ourselves, this should set the measured dimension flag back
            onMeasure(widthMeasureSpec, heightMeasureSpec);
            mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
        } else {
            long value = mMeasureCache.valueAt(cacheIndex);
            // Casting a long to int drops the high 32 bits, no mask needed
            setMeasuredDimensionRaw((int) (value >> 32), (int) value);
            mPrivateFlags3 |= PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
        }

        // flag not set, setMeasuredDimension() was not invoked, we raise
        // an exception to warn the developer
        if ((mPrivateFlags & PFLAG_MEASURED_DIMENSION_SET) != PFLAG_MEASURED_DIMENSION_SET) {
            // 重写onMeausre方法的时,必须调用setMeasuredDimension或者super.onMeasure方法,不然就会走到这里报错。
            // setMeasuredDimension中回去改变mPrivateFlags的值
            throw new IllegalStateException("onMeasure() did not set the"
                    + " measured dimension by calling"
                    + " setMeasuredDimension()");
        }

        mPrivateFlags |= PFLAG_LAYOUT_REQUIRED;
    }

    mOldWidthMeasureSpec = widthMeasureSpec;
    mOldHeightMeasureSpec = heightMeasureSpec;

    mMeasureCache.put(key, ((long) mMeasuredWidth) << 32 |
            (long) mMeasuredHeight & 0xffffffffL); // suppress sign extension
}

measure方法中会调用onMeasure方法。ViewGroup的子类会重写该方法来进行测量大小,因为mViewDecorView
DecorViewFrameLayout的子类。所以我们看一下FrameLayout.onMeasure方法:
FrameLayout.onMeasure源码如下:

/**
 * {@inheritDoc}
 */
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    int count = getChildCount();

    final boolean measureMatchParentChildren =
            MeasureSpec.getMode(widthMeasureSpec) != MeasureSpec.EXACTLY ||
            MeasureSpec.getMode(heightMeasureSpec) != MeasureSpec.EXACTLY;
    mMatchParentChildren.clear();

    int maxHeight = 0;
    int maxWidth = 0;
    int childState = 0;

    for (int i = 0; i < count; i++) {
        final View child = getChildAt(i);
        if (mMeasureAllChildren || child.getVisibility() != GONE) {
            // 调用该方法去测量每个子View
            measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);
            final LayoutParams lp = (LayoutParams) child.getLayoutParams();
            maxWidth = Math.max(maxWidth,
                    child.getMeasuredWidth() + lp.leftMargin + lp.rightMargin);
            maxHeight = Math.max(maxHeight,
                    child.getMeasuredHeight() + lp.topMargin + lp.bottomMargin);
            childState = combineMeasuredStates(childState, child.getMeasuredState());
            if (measureMatchParentChildren) {
                if (lp.width == LayoutParams.MATCH_PARENT ||
                        lp.height == LayoutParams.MATCH_PARENT) {
                    mMatchParentChildren.add(child);
                }
            }
        }
    }

    // Account for padding too
    maxWidth += getPaddingLeftWithForeground() + getPaddingRightWithForeground();
    maxHeight += getPaddingTopWithForeground() + getPaddingBottomWithForeground();

    // Check against our minimum height and width
    maxHeight = Math.max(maxHeight, getSuggestedMinimumHeight());
    maxWidth = Math.max(maxWidth, getSuggestedMinimumWidth());

    // Check against our foreground‘s minimum height and width
    final Drawable drawable = getForeground();
    if (drawable != null) {
        maxHeight = Math.max(maxHeight, drawable.getMinimumHeight());
        maxWidth = Math.max(maxWidth, drawable.getMinimumWidth());
    }

    setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
            resolveSizeAndState(maxHeight, heightMeasureSpec,
                    childState << MEASURED_HEIGHT_STATE_SHIFT));

    count = mMatchParentChildren.size();
    if (count > 1) {
        for (int i = 0; i < count; i++) {
            final View child = mMatchParentChildren.get(i);

            final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
            int childWidthMeasureSpec;
            int childHeightMeasureSpec;

            if (lp.width == LayoutParams.MATCH_PARENT) {
                childWidthMeasureSpec = MeasureSpec.makeMeasureSpec(getMeasuredWidth() -
                        getPaddingLeftWithForeground() - getPaddingRightWithForeground() -
                        lp.leftMargin - lp.rightMargin,
                        MeasureSpec.EXACTLY);
            } else {
                childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec,
                        getPaddingLeftWithForeground() + getPaddingRightWithForeground() +
                        lp.leftMargin + lp.rightMargin,
                        lp.width);
            }

            if (lp.height == LayoutParams.MATCH_PARENT) {
                childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(getMeasuredHeight() -
                        getPaddingTopWithForeground() - getPaddingBottomWithForeground() -
                        lp.topMargin - lp.bottomMargin,
                        MeasureSpec.EXACTLY);
            } else {
                childHeightMeasureSpec = getChildMeasureSpec(heightMeasureSpec,
                        getPaddingTopWithForeground() + getPaddingBottomWithForeground() +
                        lp.topMargin + lp.bottomMargin,
                        lp.height);
            }

            child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
        }
    }
}

我们看到内部会调用measureChildWithMargins()方法,该方法源码如下:

/**
 * Ask one of the children of this view to measure itself, taking into
 * account both the MeasureSpec requirements for this view and its padding
 * and margins. The child must have MarginLayoutParams The heavy lifting is
 * done in getChildMeasureSpec.
 *
 * @param child The child to measure
 * @param parentWidthMeasureSpec The width requirements for this view
 * @param widthUsed Extra space that has been used up by the parent
 *        horizontally (possibly by other children of the parent)
 * @param parentHeightMeasureSpec The height requirements for this view
 * @param heightUsed Extra space that has been used up by the parent
 *        vertically (possibly by other children of the parent)
 */
protected void measureChildWithMargins(View child,
        int parentWidthMeasureSpec, int widthUsed,
        int parentHeightMeasureSpec, int heightUsed) {
    final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();

    final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
            mPaddingLeft + mPaddingRight + lp.leftMargin + lp.rightMargin
                    + widthUsed, lp.width);
    final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
            mPaddingTop + mPaddingBottom + lp.topMargin + lp.bottomMargin
                    + heightUsed, lp.height);

    child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}

里面就是对该子View调用了measure方法,我们假设这个View已经不是ViewGroup了,就会又和上面一样,又调用onMeasure方法,
下面我们直接看一下View.onMeasure()方法:
View.onMeasure()方法的源码如下:

/**
 * <p>
 * Measure the view and its content to determine the measured width and the
 * measured height. This method is invoked by {@link #measure(int, int)} and
 * should be overriden by subclasses to provide accurate and efficient
 * measurement of their contents.
 * </p>
 *
 * <p>
 * <strong>CONTRACT:</strong> When overriding this method, you
 * <em>must</em> call {@link #setMeasuredDimension(int, int)} to store the
 * measured width and height of this view. Failure to do so will trigger an
 * <code>IllegalStateException</code>, thrown by
 * {@link #measure(int, int)}. Calling the superclass‘
 * {@link #onMeasure(int, int)} is a valid use.
 * </p>
 *
 * <p>
 * The base class implementation of measure defaults to the background size,
 * unless a larger size is allowed by the MeasureSpec. Subclasses should
 * override {@link #onMeasure(int, int)} to provide better measurements of
 * their content.
 * </p>
 *
 * <p>
 * If this method is overridden, it is the subclass‘s responsibility to make
 * sure the measured height and width are at least the view‘s minimum height
 * and width ({@link #getSuggestedMinimumHeight()} and
 * {@link #getSuggestedMinimumWidth()}).
 * </p>
 *
 * @param widthMeasureSpec horizontal space requirements as imposed by the parent.
 *                         The requirements are encoded with
 *                         {@link android.view.View.MeasureSpec}.
 * @param heightMeasureSpec vertical space requirements as imposed by the parent.
 *                         The requirements are encoded with
 *                         {@link android.view.View.MeasureSpec}.
 *
 * @see #getMeasuredWidth()
 * @see #getMeasuredHeight()
 * @see #setMeasuredDimension(int, int)
 * @see #getSuggestedMinimumHeight()
 * @see #getSuggestedMinimumWidth()
 * @see android.view.View.MeasureSpec#getMode(int)
 * @see android.view.View.MeasureSpec#getSize(int)
 */
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    // 如果不重写onMeasure方法,默认会调用getDefaultSize获取大小,下面会说getDefaultSize这个方法。
    setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
            getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}

setMeasuredDimension()方法如下:

/**
 * <p>This method must be called by {@link #onMeasure(int, int)} to store the
 * measured width and measured height. Failing to do so will trigger an
 * exception at measurement time.</p>
 *
 * @param measuredWidth The measured width of this view.  May be a complex
 * bit mask as defined by {@link #MEASURED_SIZE_MASK} and
 * {@link #MEASURED_STATE_TOO_SMALL}.
 * @param measuredHeight The measured height of this view.  May be a complex
 * bit mask as defined by {@link #MEASURED_SIZE_MASK} and
 * {@link #MEASURED_STATE_TOO_SMALL}.
 */
protected final void setMeasuredDimension(int measuredWidth, int measuredHeight) {
    boolean optical = isLayoutModeOptical(this);
    if (optical != isLayoutModeOptical(mParent)) {
        Insets insets = getOpticalInsets();
        int opticalWidth  = insets.left + insets.right;
        int opticalHeight = insets.top  + insets.bottom;

        measuredWidth  += optical ? opticalWidth  : -opticalWidth;
        measuredHeight += optical ? opticalHeight : -opticalHeight;
    }
    setMeasuredDimensionRaw(measuredWidth, measuredHeight);
}

setMeasuredDimensionRaw()方法如下:

/**
 * Sets the measured dimension without extra processing for things like optical bounds.
 * Useful for reapplying consistent values that have already been cooked with adjustments
 * for optical bounds, etc. such as those from the measurement cache.
 *
 * @param measuredWidth The measured width of this view.  May be a complex
 * bit mask as defined by {@link #MEASURED_SIZE_MASK} and
 * {@link #MEASURED_STATE_TOO_SMALL}.
 * @param measuredHeight The measured height of this view.  May be a complex
 * bit mask as defined by {@link #MEASURED_SIZE_MASK} and
 * {@link #MEASURED_STATE_TOO_SMALL}.
 */
private void setMeasuredDimensionRaw(int measuredWidth, int measuredHeight) {
    // 赋值给mMeasuredWidth,getMeasuredWidth就会调用该值。
    mMeasuredWidth = measuredWidth;
    mMeasuredHeight = measuredHeight;

    // 这就是重写onMeasure方法时如果不调用setMeasuredDimension方法时为什么会报错的原因。
    mPrivateFlags |= PFLAG_MEASURED_DIMENSION_SET;
}

我们接着看一下上面用到的getDefaultSize()方法,源码如下:

/**
 * Utility to return a default size. Uses the supplied size if the
 * MeasureSpec imposed no constraints. Will get larger if allowed
 * by the MeasureSpec.
 *
 * @param size Default size for this view
 * @param measureSpec Constraints imposed by the parent
 * @return The size this view should be.
 */
public static int getDefaultSize(int size, int measureSpec) {
    int result = size;
    // measureSpec值用于获取宽度(高度)的规格和大小,解析出对应的size和mode
    int specMode = MeasureSpec.getMode(measureSpec);
    int specSize = MeasureSpec.getSize(measureSpec);

    switch (specMode) {
    case MeasureSpec.UNSPECIFIED:
        result = size;
        break;
    case MeasureSpec.AT_MOST:
    case MeasureSpec.EXACTLY:
        result = specSize;
        break;
    }
    return result;
}

getDefaultSize方法又会使用到MeasureSpec类,文档中对MeasureSpec是这样介绍的A MeasureSpec is comprised of a size and a mode. There are three possible modes:
- MeasureSpec.EXACTLY The parent has determined an exact size for the child. The child is going to be given those bounds regardless of how big it wants to be.
理解成MATCH_PARENT或者在布局中指定了宽高值,如layout:width=’50dp’
- MeasureSpec.AT_MOST The child can be as large as it wants up to the specified size.理解成WRAP_CONTENT,这是的值是父View可以允许的最大的值,只要不超过这个值都可以。
- MeasureSpec.UNSPECIFIED The parent has not imposed any constraint on the child. It can be whatever size it wants. 这种情况比较少,一般用不到。

这里简单总结一下上面的过程:

performMeasure() {
    - 1.调用View.measure方法
    mView.measure():
        - 2.measure内部会调用onMeasure方法,但是因为这里mView是DecorView,所以会调用FrameLayout的onMeasure方法。
            onMeasure(FrameLayout)
            - 3. 内部设置ViewGroup的宽高
                setMeasuredDimension
                并且对每个子View进行遍历测量
                for (int i = 0; i < count; i++) {
                    final View child = getChildAt(i);
                    - 4. 对每个子View调用measureChildWithMargins方法
                    measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);  
                        -5. measureChildWithMargins内部调用子View的measure方法
                            meausre
                            - 6. measure方法内部又调用onMeasure方法
                                onMeasure(View)
                                - 7. onMeasure方法内部调用setMeasuredDimension
                                    setMeasuredDimension
                                    - 8. setMeasuredDimension内部调用setMeasuredDimensionRaw
                                        setMeasuredDimensionRaw
                }
}

从上面代码中能看到measurefinal的,我们可以重写onMeasure来实现measure过程。
到这里基本都讲完了,我们在开发中会按照需要重写onMeasure方法,然后调用setMeasuredDimension方法设置大小,
ps:譬如我们设置了setMeasuredDimension(10, 10),那么不管布局中怎么设置这个View的大小
都是没用的,最后显示出来大小都是10*10。

Layout

performLayout方法源码如下:

private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth,
        int desiredWindowHeight) {
    mLayoutRequested = false;
    mScrollMayChange = true;
    mInLayout = true;

    final View host = mView;
    if (DEBUG_ORIENTATION || DEBUG_LAYOUT) {
        Log.v(TAG, "Laying out " + host + " to (" +
                host.getMeasuredWidth() + ", " + host.getMeasuredHeight() + ")");
    }

    Trace.traceBegin(Trace.TRACE_TAG_VIEW, "layout");
    try {
        // 把刚才测量的宽高设置进来
        host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());

        mInLayout = false;
        int numViewsRequestingLayout = mLayoutRequesters.size();
        if (numViewsRequestingLayout > 0) {
            // requestLayout() was called during layout.
            // If no layout-request flags are set on the requesting views, there is no problem.
            // If some requests are still pending, then we need to clear those flags and do
            // a full request/measure/layout pass to handle this situation.
            ArrayList<View> validLayoutRequesters = getValidLayoutRequesters(mLayoutRequesters,
                    false);
            if (validLayoutRequesters != null) {
                // Set this flag to indicate that any further requests are happening during
                // the second pass, which may result in posting those requests to the next
                // frame instead
                mHandlingLayoutInLayoutRequest = true;

                // Process fresh layout requests, then measure and layout
                int numValidRequests = validLayoutRequesters.size();
                for (int i = 0; i < numValidRequests; ++i) {
                    final View view = validLayoutRequesters.get(i);
                    Log.w("View", "requestLayout() improperly called by " + view +
                            " during layout: running second layout pass");
                    view.requestLayout();
                }
                measureHierarchy(host, lp, mView.getContext().getResources(),
                        desiredWindowWidth, desiredWindowHeight);
                mInLayout = true;
                host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());

                mHandlingLayoutInLayoutRequest = false;

                // Check the valid requests again, this time without checking/clearing the
                // layout flags, since requests happening during the second pass get noop‘d
                validLayoutRequesters = getValidLayoutRequesters(mLayoutRequesters, true);
                if (validLayoutRequesters != null) {
                    final ArrayList<View> finalRequesters = validLayoutRequesters;
                    // Post second-pass requests to the next frame
                    getRunQueue().post(new Runnable() {
                        @Override
                        public void run() {
                            int numValidRequests = finalRequesters.size();
                            for (int i = 0; i < numValidRequests; ++i) {
                                final View view = finalRequesters.get(i);
                                Log.w("View", "requestLayout() improperly called by " + view +
                                        " during second layout pass: posting in next frame");
                                view.requestLayout();
                            }
                        }
                    });
                }
            }

        }
    } finally {
        Trace.traceEnd(Trace.TRACE_TAG_VIEW);
    }
    mInLayout = false;
}

内部会调用layout()方法,因为hostmViewViewGroup中重写了layout方法,并调用了super.layout.
所以我们直接看View.layout()方法,该方法源码如下:

/**
 * Assign a size and position to a view and all of its
 * descendants
 *
 * <p>This is the second phase of the layout mechanism.
 * (The first is measuring). In this phase, each parent calls
 * layout on all of its children to position them.
 * This is typically done using the child measurements
 * that were stored in the measure pass().</p>
 *
 * <p>Derived classes should not override this method.
 * Derived classes with children should override
 * onLayout. In that method, they should
 * call layout on each of their children.</p>
 *
 * @param l Left position, relative to parent
 * @param t Top position, relative to parent
 * @param r Right position, relative to parent
 * @param b Bottom position, relative to parent
 */
@SuppressWarnings({"unchecked"})
public void layout(int l, int t, int r, int b) {
    if ((mPrivateFlags3 & PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT) != 0) {
        onMeasure(mOldWidthMeasureSpec, mOldHeightMeasureSpec);
        mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
    }

    int oldL = mLeft;
    int oldT = mTop;
    int oldB = mBottom;
    int oldR = mRight;

    // 这部分是判断这个View的大小是否已经发生了变化,来判断是否需要重绘。
    boolean changed = isLayoutModeOptical(mParent) ?
            setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b);

    if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {
        // 内部调用onLayout方法
        onLayout(changed, l, t, r, b);
        mPrivateFlags &= ~PFLAG_LAYOUT_REQUIRED;

        ListenerInfo li = mListenerInfo;
        if (li != null && li.mOnLayoutChangeListeners != null) {
            ArrayList<OnLayoutChangeListener> listenersCopy =
                    (ArrayList<OnLayoutChangeListener>)li.mOnLayoutChangeListeners.clone();
            int numListeners = listenersCopy.size();
            for (int i = 0; i < numListeners; ++i) {
                listenersCopy.get(i).onLayoutChange(this, l, t, r, b, oldL, oldT, oldR, oldB);
            }
        }
    }

    mPrivateFlags &= ~PFLAG_FORCE_LAYOUT;
    mPrivateFlags3 |= PFLAG3_IS_LAID_OUT;
}

这里会调用onLayout方法,同样因为mViewFrameLayout的子类,所以我们要看FrameLayoutonLayout方法,
这里我们先看一下ViewGroup.onLayout方法:

/**
 * {@inheritDoc}
 */
@Override
protected abstract void onLayout(boolean changed,
        int l, int t, int r, int b);

是个抽象方法,所以ViewGroup的子类都需要实现该方法。
我们看一下FrameLayout.onLayout方法,源码如下:

 /**
 * {@inheritDoc}
 */
@Override
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
    layoutChildren(left, top, right, bottom, false /* no force left gravity */);
}

void layoutChildren(int left, int top, int right, int bottom,
                              boolean forceLeftGravity) {
    final int count = getChildCount();

    final int parentLeft = getPaddingLeftWithForeground();
    final int parentRight = right - left - getPaddingRightWithForeground();

    final int parentTop = getPaddingTopWithForeground();
    final int parentBottom = bottom - top - getPaddingBottomWithForeground();

    mForegroundBoundsChanged = true;

    for (int i = 0; i < count; i++) {
        final View child = getChildAt(i);
        if (child.getVisibility() != GONE) {
            final LayoutParams lp = (LayoutParams) child.getLayoutParams();

            final int width = child.getMeasuredWidth();
            final int height = child.getMeasuredHeight();

            int childLeft;
            int childTop;

            int gravity = lp.gravity;
            if (gravity == -1) {
                gravity = DEFAULT_CHILD_GRAVITY;
            }

            final int layoutDirection = getLayoutDirection();
            final int absoluteGravity = Gravity.getAbsoluteGravity(gravity, layoutDirection);
            final int verticalGravity = gravity & Gravity.VERTICAL_GRAVITY_MASK;

            switch (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) {
                case Gravity.CENTER_HORIZONTAL:
                    childLeft = parentLeft + (parentRight - parentLeft - width) / 2 +
                    lp.leftMargin - lp.rightMargin;
                    break;
                case Gravity.RIGHT:
                    if (!forceLeftGravity) {
                        childLeft = parentRight - width - lp.rightMargin;
                        break;
                    }
                case Gravity.LEFT:
                default:
                    childLeft = parentLeft + lp.leftMargin;
            }

            switch (verticalGravity) {
                case Gravity.TOP:
                    childTop = parentTop + lp.topMargin;
                    break;
                case Gravity.CENTER_VERTICAL:
                    childTop = parentTop + (parentBottom - parentTop - height) / 2 +
                    lp.topMargin - lp.bottomMargin;
                    break;
                case Gravity.BOTTOM:
                    childTop = parentBottom - height - lp.bottomMargin;
                    break;
                default:
                    childTop = parentTop + lp.topMargin;
            }
            //调用子View的layout方法
            child.layout(childLeft, childTop, childLeft + width, childTop + height);
        }
    }
}

View.layout方法,又会调用到View.onLayout方法,我们假设这个子View不是ViewGroup.
看一下View.onLayout方法源码如下:

/**
 * Called from layout when this view should
 * assign a size and position to each of its children.
 *
 * Derived classes with children should override
 * this method and call layout on each of
 * their children.
 * @param changed This is a new size or position for this view
 * @param left Left position, relative to parent
 * @param top Top position, relative to parent
 * @param right Right position, relative to parent
 * @param bottom Bottom position, relative to parent
 */
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
}

是一个空方法,这是因为Layout需要ViewGroup来控制进行。

这里也总结一下layout的过程。

private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth,
        int desiredWindowHeight) {
    - 1. host.layout
    host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());
        -2. layout方法会分别调用setFrame()和onLayout()方法
            setFrame()
            onLayout()
            -3. 因为host是mView也就是DecorView也就是FrameLayout的子类。FrameLayout的onLayout方法如下
                for (int i = 0; i < count; i++) {
                    final View child = getChildAt(i);
                    if (child.getVisibility() != GONE) {
                        -4. 遍历每个子View,并分别调用layout方法。
                        child.layout(childLeft, childTop, childLeft + width, childTop + height);
                    }
                }
}

Draw

绘制阶段是从ViewRootImpl中的performDraw方法开始的:

private void performDraw() {
    if (mAttachInfo.mDisplayState == Display.STATE_OFF && !mReportNextDraw) {
        return;
    }

    final boolean fullRedrawNeeded = mFullRedrawNeeded;
    mFullRedrawNeeded = false;

    mIsDrawing = true;
    Trace.traceBegin(Trace.TRACE_TAG_VIEW, "draw");
    try {
        // 开始draw了
        draw(fullRedrawNeeded);
    } finally {
        mIsDrawing = false;
        Trace.traceEnd(Trace.TRACE_TAG_VIEW);
    }

    // For whatever reason we didn‘t create a HardwareRenderer, end any
    // hardware animations that are now dangling
    if (mAttachInfo.mPendingAnimatingRenderNodes != null) {
        final int count = mAttachInfo.mPendingAnimatingRenderNodes.size();
        for (int i = 0; i < count; i++) {
            mAttachInfo.mPendingAnimatingRenderNodes.get(i).endAllAnimators();
        }
        mAttachInfo.mPendingAnimatingRenderNodes.clear();
    }

    if (mReportNextDraw) {
        mReportNextDraw = false;
        if (mAttachInfo.mHardwareRenderer != null) {
            mAttachInfo.mHardwareRenderer.fence();
        }

        if (LOCAL_LOGV) {
            Log.v(TAG, "FINISHED DRAWING: " + mWindowAttributes.getTitle());
        }
        if (mSurfaceHolder != null && mSurface.isValid()) {
            mSurfaceHolderCallback.surfaceRedrawNeeded(mSurfaceHolder);
            SurfaceHolder.Callback callbacks[] = mSurfaceHolder.getCallbacks();
            if (callbacks != null) {
                for (SurfaceHolder.Callback c : callbacks) {
                    if (c instanceof SurfaceHolder.Callback2) {
                        ((SurfaceHolder.Callback2)c).surfaceRedrawNeeded(
                                mSurfaceHolder);
                    }
                }
            }
        }
        try {
            mWindowSession.finishDrawing(mWindow);
        } catch (RemoteException e) {
        }
    }
}

内部会调用draw方法,draw方法源码如下:

private void draw(boolean fullRedrawNeeded) {
    Surface surface = mSurface;
    if (!surface.isValid()) {
        return;
    }

    if (DEBUG_FPS) {
        trackFPS();
    }

    if (!sFirstDrawComplete) {
        synchronized (sFirstDrawHandlers) {
            sFirstDrawComplete = true;
            final int count = sFirstDrawHandlers.size();
            for (int i = 0; i< count; i++) {
                mHandler.post(sFirstDrawHandlers.get(i));
            }
        }
    }

    scrollToRectOrFocus(null, false);

    if (mAttachInfo.mViewScrollChanged) {
        mAttachInfo.mViewScrollChanged = false;
        mAttachInfo.mTreeObserver.dispatchOnScrollChanged();
    }

    boolean animating = mScroller != null && mScroller.computeScrollOffset();
    final int curScrollY;
    if (animating) {
        curScrollY = mScroller.getCurrY();
    } else {
        curScrollY = mScrollY;
    }
    if (mCurScrollY != curScrollY) {
        mCurScrollY = curScrollY;
        fullRedrawNeeded = true;
    }

    final float appScale = mAttachInfo.mApplicationScale;
    final boolean scalingRequired = mAttachInfo.mScalingRequired;

    int resizeAlpha = 0;
    if (mResizeBuffer != null) {
        long deltaTime = SystemClock.uptimeMillis() - mResizeBufferStartTime;
        if (deltaTime < mResizeBufferDuration) {
            float amt = deltaTime/(float) mResizeBufferDuration;
            amt = mResizeInterpolator.getInterpolation(amt);
            animating = true;
            resizeAlpha = 255 - (int)(amt*255);
        } else {
            disposeResizeBuffer();
        }
    }

    final Rect dirty = mDirty;
    if (mSurfaceHolder != null) {
        // The app owns the surface, we won‘t draw.
        dirty.setEmpty();
        if (animating) {
            if (mScroller != null) {
                mScroller.abortAnimation();
            }
            disposeResizeBuffer();
        }
        return;
    }

    if (fullRedrawNeeded) {
        mAttachInfo.mIgnoreDirtyState = true;
        dirty.set(0, 0, (int) (mWidth * appScale + 0.5f), (int) (mHeight * appScale + 0.5f));
    }

    if (DEBUG_ORIENTATION || DEBUG_DRAW) {
        Log.v(TAG, "Draw " + mView + "/"
                + mWindowAttributes.getTitle()
                + ": dirty={" + dirty.left + "," + dirty.top
                + "," + dirty.right + "," + dirty.bottom + "} surface="
                + surface + " surface.isValid()=" + surface.isValid() + ", appScale:" +
                appScale + ", width=" + mWidth + ", height=" + mHeight);
    }

    mAttachInfo.mTreeObserver.dispatchOnDraw();

    int xOffset = 0;
    int yOffset = curScrollY;
    final WindowManager.LayoutParams params = mWindowAttributes;
    final Rect surfaceInsets = params != null ? params.surfaceInsets : null;
    if (surfaceInsets != null) {
        xOffset -= surfaceInsets.left;
        yOffset -= surfaceInsets.top;

        // Offset dirty rect for surface insets.
        dirty.offset(surfaceInsets.left, surfaceInsets.right);
    }

    if (!dirty.isEmpty() || mIsAnimating) {
        if (mAttachInfo.mHardwareRenderer != null && mAttachInfo.mHardwareRenderer.isEnabled()) {
            // Draw with hardware renderer.
            mIsAnimating = false;
            boolean invalidateRoot = false;
            if (mHardwareYOffset != yOffset || mHardwareXOffset != xOffset) {
                mHardwareYOffset = yOffset;
                mHardwareXOffset = xOffset;
                mAttachInfo.mHardwareRenderer.invalidateRoot();
            }
            mResizeAlpha = resizeAlpha;

            dirty.setEmpty();

            mBlockResizeBuffer = false;
            mAttachInfo.mHardwareRenderer.draw(mView, mAttachInfo, this);
        } else {
            // If we get here with a disabled & requested hardware renderer, something went
            // wrong (an invalidate posted right before we destroyed the hardware surface
            // for instance) so we should just bail out. Locking the surface with software
            // rendering at this point would lock it forever and prevent hardware renderer
            // from doing its job when it comes back.
            // Before we request a new frame we must however attempt to reinitiliaze the
            // hardware renderer if it‘s in requested state. This would happen after an
            // eglTerminate() for instance.
            if (mAttachInfo.mHardwareRenderer != null &&
                    !mAttachInfo.mHardwareRenderer.isEnabled() &&
                    mAttachInfo.mHardwareRenderer.isRequested()) {

                try {
                    mAttachInfo.mHardwareRenderer.initializeIfNeeded(
                            mWidth, mHeight, mSurface, surfaceInsets);
                } catch (OutOfResourcesException e) {
                    handleOutOfResourcesException(e);
                    return;
                }

                mFullRedrawNeeded = true;
                scheduleTraversals();
                return;
            }

            // draw的部分在这里。。。内部会用canvas去画
            if (!drawSoftware(surface, mAttachInfo, xOffset, yOffset, scalingRequired, dirty)) {
                return;
            }
        }
    }

    if (animating) {
        mFullRedrawNeeded = true;
        scheduleTraversals();
    }
}

我们看一下drawSoftware方法:

/**
 * @return true if drawing was successful, false if an error occurred
 */
private boolean drawSoftware(Surface surface, AttachInfo attachInfo, int xoff, int yoff,
        boolean scalingRequired, Rect dirty) {

    // Draw with software renderer.
    final Canvas canvas;
    try {
        final int left = dirty.left;
        final int top = dirty.top;
        final int right = dirty.right;
        final int bottom = dirty.bottom;

        canvas = mSurface.lockCanvas(dirty);

        // The dirty rectangle can be modified by Surface.lockCanvas()
        //noinspection ConstantConditions
        if (left != dirty.left || top != dirty.top || right != dirty.right
                || bottom != dirty.bottom) {
            attachInfo.mIgnoreDirtyState = true;
        }

        // TODO: Do this in native
        canvas.setDensity(mDensity);
    } catch (Surface.OutOfResourcesException e) {
        handleOutOfResourcesException(e);
        return false;
    } catch (IllegalArgumentException e) {
        Log.e(TAG, "Could not lock surface", e);
        // Don‘t assume this is due to out of memory, it could be
        // something else, and if it is something else then we could
        // kill stuff (or ourself) for no reason.
        mLayoutRequested = true;    // ask wm for a new surface next time.
        return false;
    }

    try {
        if (DEBUG_ORIENTATION || DEBUG_DRAW) {
            Log.v(TAG, "Surface " + surface + " drawing to bitmap w="
                    + canvas.getWidth() + ", h=" + canvas.getHeight());
            //canvas.drawARGB(255, 255, 0, 0);
        }

        // If this bitmap‘s format includes an alpha channel, we
        // need to clear it before drawing so that the child will
        // properly re-composite its drawing on a transparent
        // background. This automatically respects the clip/dirty region
        // or
        // If we are applying an offset, we need to clear the area
        // where the offset doesn‘t appear to avoid having garbage
        // left in the blank areas.
        if (!canvas.isOpaque() || yoff != 0 || xoff != 0) {
            canvas.drawColor(0, PorterDuff.Mode.CLEAR);
        }

        dirty.setEmpty();
        mIsAnimating = false;
        attachInfo.mDrawingTime = SystemClock.uptimeMillis();
        mView.mPrivateFlags |= View.PFLAG_DRAWN;

        if (DEBUG_DRAW) {
            Context cxt = mView.getContext();
            Log.i(TAG, "Drawing: package:" + cxt.getPackageName() +
                    ", metrics=" + cxt.getResources().getDisplayMetrics() +
                    ", compatibilityInfo=" + cxt.getResources().getCompatibilityInfo());
        }
        try {
            canvas.translate(-xoff, -yoff);
            if (mTranslator != null) {
                mTranslator.translateCanvas(canvas);
            }
            canvas.setScreenDensity(scalingRequired ? mNoncompatDensity : 0);
            attachInfo.mSetIgnoreDirtyState = false;

            // 内部会去调用View.draw();
            mView.draw(canvas);
        } finally {
            if (!attachInfo.mSetIgnoreDirtyState) {
                // Only clear the flag if it was not set during the mView.draw() call
                attachInfo.mIgnoreDirtyState = false;
            }
        }
    } finally {
        try {
            surface.unlockCanvasAndPost(canvas);
        } catch (IllegalArgumentException e) {
            Log.e(TAG, "Could not unlock surface", e);
            mLayoutRequested = true;    // ask wm for a new surface next time.
            //noinspection ReturnInsideFinallyBlock
            return false;
        }

        if (LOCAL_LOGV) {
            Log.v(TAG, "Surface " + surface + " unlockCanvasAndPost");
        }
    }
    return true;
}

代码中调用了mView.draw()方法,所以我们看一下FrameLayout.draw()方法:

/**
 * {@inheritDoc}
 */
@Override
public void draw(Canvas canvas) {
    super.draw(canvas);

    if (mForeground != null) {
        final Drawable foreground = mForeground;

        if (mForegroundBoundsChanged) {
            mForegroundBoundsChanged = false;
            final Rect selfBounds = mSelfBounds;
            final Rect overlayBounds = mOverlayBounds;

            final int w = mRight-mLeft;
            final int h = mBottom-mTop;

            if (mForegroundInPadding) {
                selfBounds.set(0, 0, w, h);
            } else {
                selfBounds.set(mPaddingLeft, mPaddingTop, w - mPaddingRight, h - mPaddingBottom);
            }

            final int layoutDirection = getLayoutDirection();
            Gravity.apply(mForegroundGravity, foreground.getIntrinsicWidth(),
                    foreground.getIntrinsicHeight(), selfBounds, overlayBounds,
                    layoutDirection);
            foreground.setBounds(overlayBounds);
        }

        foreground.draw(canvas);
    }
}

内部调用了super.draw(),而ViewGroup没有重写该方法,所以直接看Viewdraw()方法.
View.draw()方法如下:

/**
 * Manually render this view (and all of its children) to the given Canvas.
 * The view must have already done a full layout before this function is
 * called.  When implementing a view, implement
 * {@link #onDraw(android.graphics.Canvas)} instead of overriding this method.
 * If you do need to override this method, call the superclass version.
 *
 * @param canvas The Canvas to which the View is rendered.
 */
public void draw(Canvas canvas) {
    final int privateFlags = mPrivateFlags;
    final boolean dirtyOpaque = (privateFlags & PFLAG_DIRTY_MASK) == PFLAG_DIRTY_OPAQUE &&
            (mAttachInfo == null || !mAttachInfo.mIgnoreDirtyState);
    mPrivateFlags = (privateFlags & ~PFLAG_DIRTY_MASK) | PFLAG_DRAWN;

    // 这里注释说的很明白了,draw的6个步骤。
    /*
     * Draw traversal performs several drawing steps which must be executed
     * in the appropriate order:
     *
     *      1. Draw the background
     *      2. If necessary, save the canvas‘ layers to prepare for fading
     *      3. Draw view‘s content, 调用onDraw方法绘制自身
     *      4. Draw children, 调用dispatchDraw方法绘制子View
     *      5. If necessary, draw the fading edges and restore layers
     *      6. Draw decorations (scrollbars for instance)
     */

    // Step 1, draw the background, if needed
    int saveCount;

    if (!dirtyOpaque) {
        drawBackground(canvas);
    }

    // skip step 2 & 5 if possible (common case)
    final int viewFlags = mViewFlags;
    boolean horizontalEdges = (viewFlags & FADING_EDGE_HORIZONTAL) != 0;
    boolean verticalEdges = (viewFlags & FADING_EDGE_VERTICAL) != 0;
    if (!verticalEdges && !horizontalEdges) {
        // Step 3, draw the content
        if (!dirtyOpaque) onDraw(canvas);

        // Step 4, draw the children
        dispatchDraw(canvas);

        // Step 6, draw decorations (scrollbars)
        onDrawScrollBars(canvas);

        if (mOverlay != null && !mOverlay.isEmpty()) {
            mOverlay.getOverlayView().dispatchDraw(canvas);
        }

        // we‘re done...
        return;
    }

    /*
     * Here we do the full fledged routine...
     * (this is an uncommon case where speed matters less,
     * this is why we repeat some of the tests that have been
     * done above)
     */

    boolean drawTop = false;
    boolean drawBottom = false;
    boolean drawLeft = false;
    boolean drawRight = false;

    float topFadeStrength = 0.0f;
    float bottomFadeStrength = 0.0f;
    float leftFadeStrength = 0.0f;
    float rightFadeStrength = 0.0f;

    // Step 2, save the canvas‘ layers
    int paddingLeft = mPaddingLeft;

    final boolean offsetRequired = isPaddingOffsetRequired();
    if (offsetRequired) {
        paddingLeft += getLeftPaddingOffset();
    }

    int left = mScrollX + paddingLeft;
    int right = left + mRight - mLeft - mPaddingRight - paddingLeft;
    int top = mScrollY + getFadeTop(offsetRequired);
    int bottom = top + getFadeHeight(offsetRequired);

    if (offsetRequired) {
        right += getRightPaddingOffset();
        bottom += getBottomPaddingOffset();
    }

    final ScrollabilityCache scrollabilityCache = mScrollCache;
    final float fadeHeight = scrollabilityCache.fadingEdgeLength;
    int length = (int) fadeHeight;

    // clip the fade length if top and bottom fades overlap
    // overlapping fades produce odd-looking artifacts
    if (verticalEdges && (top + length > bottom - length)) {
        length = (bottom - top) / 2;
    }

    // also clip horizontal fades if necessary
    if (horizontalEdges && (left + length > right - length)) {
        length = (right - left) / 2;
    }

    if (verticalEdges) {
        topFadeStrength = Math.max(0.0f, Math.min(1.0f, getTopFadingEdgeStrength()));
        drawTop = topFadeStrength * fadeHeight > 1.0f;
        bottomFadeStrength = Math.max(0.0f, Math.min(1.0f, getBottomFadingEdgeStrength()));
        drawBottom = bottomFadeStrength * fadeHeight > 1.0f;
    }

    if (horizontalEdges) {
        leftFadeStrength = Math.max(0.0f, Math.min(1.0f, getLeftFadingEdgeStrength()));
        drawLeft = leftFadeStrength * fadeHeight > 1.0f;
        rightFadeStrength = Math.max(0.0f, Math.min(1.0f, getRightFadingEdgeStrength()));
        drawRight = rightFadeStrength * fadeHeight > 1.0f;
    }

    saveCount = canvas.getSaveCount();

    int solidColor = getSolidColor();
    if (solidColor == 0) {
        final int flags = Canvas.HAS_ALPHA_LAYER_SAVE_FLAG;

        if (drawTop) {
            canvas.saveLayer(left, top, right, top + length, null, flags);
        }

        if (drawBottom) {
            canvas.saveLayer(left, bottom - length, right, bottom, null, flags);
        }

        if (drawLeft) {
            canvas.saveLayer(left, top, left + length, bottom, null, flags);
        }

        if (drawRight) {
            canvas.saveLayer(right - length, top, right, bottom, null, flags);
        }
    } else {
        scrollabilityCache.setFadeColor(solidColor);
    }

    // Step 3, draw the content
    if (!dirtyOpaque) onDraw(canvas);

    // Step 4, draw the children
    dispatchDraw(canvas);

    // Step 5, draw the fade effect and restore layers
    final Paint p = scrollabilityCache.paint;
    final Matrix matrix = scrollabilityCache.matrix;
    final Shader fade = scrollabilityCache.shader;

    if (drawTop) {
        matrix.setScale(1, fadeHeight * topFadeStrength);
        matrix.postTranslate(left, top);
        fade.setLocalMatrix(matrix);
        p.setShader(fade);
        canvas.drawRect(left, top, right, top + length, p);
    }

    if (drawBottom) {
        matrix.setScale(1, fadeHeight * bottomFadeStrength);
        matrix.postRotate(180);
        matrix.postTranslate(left, bottom);
        fade.setLocalMatrix(matrix);
        p.setShader(fade);
        canvas.drawRect(left, bottom - length, right, bottom, p);
    }

    if (drawLeft) {
        matrix.setScale(1, fadeHeight * leftFadeStrength);
        matrix.postRotate(-90);
        matrix.postTranslate(left, top);
        fade.setLocalMatrix(matrix);
        p.setShader(fade);
        canvas.drawRect(left, top, left + length, bottom, p);
    }

    if (drawRight) {
        matrix.setScale(1, fadeHeight * rightFadeStrength);
        matrix.postRotate(90);
        matrix.postTranslate(right, top);
        fade.setLocalMatrix(matrix);
        p.setShader(fade);
        canvas.drawRect(right - length, top, right, bottom, p);
    }

    canvas.restoreToCount(saveCount);

    // Step 6, draw decorations (scrollbars)
    onDrawScrollBars(canvas);

    if (mOverlay != null && !mOverlay.isEmpty()) {
        mOverlay.getOverlayView().dispatchDraw(canvas);
    }
}

上面会调用onDrawdispatchDraw方法。
我们先看一下View.onDraw方法:

/**
 * Implement this to do your drawing.
 *
 * @param canvas the canvas on which the background will be drawn
 */
protected void onDraw(Canvas canvas) {
}

是空方法,这是也很好理解,因为每个View的展现都不一样,例如TextViewProgressBar等,
所以View不会去实现onDraw方法,具体是要子类去根据自己的显示要求实现该方法。

再看一下dispatchDraw方法,这个方法是用来绘制子View的,所以要看ViewGroup.dispatchDraw方法,View.dispatchDraw是空的。

/**
 * {@inheritDoc}
 */
@Override
protected void dispatchDraw(Canvas canvas) {
    boolean usingRenderNodeProperties = canvas.isRecordingFor(mRenderNode);
    final int childrenCount = mChildrenCount;
    final View[] children = mChildren;
    int flags = mGroupFlags;

    if ((flags & FLAG_RUN_ANIMATION) != 0 && canAnimate()) {
        final boolean cache = (mGroupFlags & FLAG_ANIMATION_CACHE) == FLAG_ANIMATION_CACHE;

        final boolean buildCache = !isHardwareAccelerated();
        for (int i = 0; i < childrenCount; i++) {
            final View child = children[i];
            if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE) {
                final LayoutParams params = child.getLayoutParams();
                attachLayoutAnimationParameters(child, params, i, childrenCount);
                bindLayoutAnimation(child);
                if (cache) {
                    child.setDrawingCacheEnabled(true);
                    if (buildCache) {
                        child.buildDrawingCache(true);
                    }
                }
            }
        }

        final LayoutAnimationController controller = mLayoutAnimationController;
        if (controller.willOverlap()) {
            mGroupFlags |= FLAG_OPTIMIZE_INVALIDATE;
        }

        controller.start();

        mGroupFlags &= ~FLAG_RUN_ANIMATION;
        mGroupFlags &= ~FLAG_ANIMATION_DONE;

        if (cache) {
            mGroupFlags |= FLAG_CHILDREN_DRAWN_WITH_CACHE;
        }

        if (mAnimationListener != null) {
            mAnimationListener.onAnimationStart(controller.getAnimation());
        }
    }

    int clipSaveCount = 0;
    final boolean clipToPadding = (flags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK;
    if (clipToPadding) {
        clipSaveCount = canvas.save();
        canvas.clipRect(mScrollX + mPaddingLeft, mScrollY + mPaddingTop,
                mScrollX + mRight - mLeft - mPaddingRight,
                mScrollY + mBottom - mTop - mPaddingBottom);
    }

    // We will draw our child‘s animation, let‘s reset the flag
    mPrivateFlags &= ~PFLAG_DRAW_ANIMATION;
    mGroupFlags &= ~FLAG_INVALIDATE_REQUIRED;

    boolean more = false;
    final long drawingTime = getDrawingTime();

    if (usingRenderNodeProperties) canvas.insertReorderBarrier();
    // Only use the preordered list if not HW accelerated, since the HW pipeline will do the
    // draw reordering internally
    final ArrayList<View> preorderedList = usingRenderNodeProperties
            ? null : buildOrderedChildList();
    final boolean customOrder = preorderedList == null
            && isChildrenDrawingOrderEnabled();
    for (int i = 0; i < childrenCount; i++) {
        int childIndex = customOrder ? getChildDrawingOrder(childrenCount, i) : i;
        final View child = (preorderedList == null)
                ? children[childIndex] : preorderedList.get(childIndex);
        if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE || child.getAnimation() != null) {
            // 调用drawChild方法
            more |= drawChild(canvas, child, drawingTime);
        }
    }
    if (preorderedList != null) preorderedList.clear();

    // Draw any disappearing views that have animations
    if (mDisappearingChildren != null) {
        final ArrayList<View> disappearingChildren = mDisappearingChildren;
        final int disappearingCount = disappearingChildren.size() - 1;
        // Go backwards -- we may delete as animations finish
        for (int i = disappearingCount; i >= 0; i--) {
            final View child = disappearingChildren.get(i);
            more |= drawChild(canvas, child, drawingTime);
        }
    }
    if (usingRenderNodeProperties) canvas.insertInorderBarrier();

    if (debugDraw()) {
        onDebugDraw(canvas);
    }

    if (clipToPadding) {
        canvas.restoreToCount(clipSaveCount);
    }

    // mGroupFlags might have been updated by drawChild()
    flags = mGroupFlags;

    if ((flags & FLAG_INVALIDATE_REQUIRED) == FLAG_INVALIDATE_REQUIRED) {
        invalidate(true);
    }

    if ((flags & FLAG_ANIMATION_DONE) == 0 && (flags & FLAG_NOTIFY_ANIMATION_LISTENER) == 0 &&
            mLayoutAnimationController.isDone() && !more) {
        // We want to erase the drawing cache and notify the listener after the
        // next frame is drawn because one extra invalidate() is caused by
        // drawChild() after the animation is over
        mGroupFlags |= FLAG_NOTIFY_ANIMATION_LISTENER;
        final Runnable end = new Runnable() {
           public void run() {
               notifyAnimationListener();
           }
        };
        post(end);
    }
}

可以看到上面的方法中会调用drawChild方法,该方法如下:

/**
 * Draw one child of this View Group. This method is responsible for getting
 * the canvas in the right state. This includes clipping, translating so
 * that the child‘s scrolled origin is at 0, 0, and applying any animation
 * transformations.
 *
 * @param canvas The canvas on which to draw the child
 * @param child Who to draw
 * @param drawingTime The time at which draw is occurring
 * @return True if an invalidate() was issued
 */
protected boolean drawChild(Canvas canvas, View child, long drawingTime) {
    return child.draw(canvas, this, drawingTime);
}

这里也简单总结一下draw的过程:

// 1. ViewRootImpl.performDraw()
private void performDraw() {
    // 2. ViewRootImpl.draw()
    draw(fullRedrawNeeded); 
        // 3. ViewRootImpl.drawSoftware
        drawSoftware
            // 4. 内部调用mView.draw,也就是FrameLayout.draw(). 
            mView.draw()(FrameLayout)
                // 5. FrameLayout.draw方法内部会调用super.draw方法,也就是View.draw方法.
                super.draw(canvas);
                    // 6. View.draw方法内部会分别调用onDraw绘制自己以及dispatchDraw绘制子View.
                    onDraw
                    // 绘制子View
                    dispatchDraw
                        // 7. dispatchDraw方法内部会遍历所有子View.
                        for (int i = 0; i < childrenCount; i++) {
                            // 8. 对每个子View分别调用drawChild方法
                            drawChild()
                                // 9. drawChild方法内部会对该子View调用draw方法,进行绘制。然后draw又会调用onDraw等,循环就开始了。 
                                    child.draw()
                        }
}

最后补充一个小问题: getWidth()getMeasuredWidth()有什么区别呢?
一般情况下这两个的值是相同的,getMeasureWidth()方法在measure()过程结束后就可以获取到了,而getWidth()方法要在layout()过程结束后才能获取到。
而且getMeasureWidth()的值是通过setMeasuredDimension()设置的,但是getWidth()的值是通过视图右边的坐标减去左边的坐标计算出来的。如果我们在layout的时候将宽高
不传getMeasureWidth的值,那么这时候getWidth()getMeasuredWidth的值就不会再相同了,当然一般也不会这么干…


  • 邮箱 :charon.chui@gmail.com
  • Good Luck!

View绘制过程详解

标签:布局   界面   view   

原文地址:http://blog.csdn.net/charon_chui/article/details/44980013

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