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[Android][转]Android View绘制13问13答

时间:2016-03-26 07:09:18      阅读:258      评论:0      收藏:0      [点我收藏+]

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转自:http://www.androidchina.net/4458.html


1.view的绘制流程分几步,从哪开始?哪个过程结束以后能看到view?

答:从ViewRoot的performTraversals开始,经过measure,layout,draw 三个流程。draw流程结束以后就可以在屏幕上看到view了。

2.view的测量宽高和实际宽高有区别吗?

答:基本上百分之99的情况下都是可以认为没有区别的。有两种情况,有区别。第一种 就是有的时候会因为某些原因 view会多次测量,那第一次测量的宽高 肯定和最后实际的宽高 是不一定相等的,但是在这种情况下

最后一次测量的宽高和实际宽高是一致的。此外,实际宽高是在layout流程里确定的,我们可以在layout流程里 将实际宽高写死 写成硬编码,这样测量的宽高和实际宽高就肯定不一样了,虽然这么做没有意义 而且也不好。

3.view的measureSpec 由谁决定?顶级view呢?

答:由view自己的layoutparams和父容器  一起决定自己的measureSpec。一旦确定了spec,onMeasure中就可以确定view的宽高了。

顶级view就稍微特殊一点,对于decorView的测量在ViewRootImpl的源码里。

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//desire的这2个参数就代表屏幕的宽高,
  childWidthMeasureSpec = getRootMeasureSpec(desiredWindowWidth, lp.width);
  childHeightMeasureSpec = getRootMeasureSpec(desiredWindowHeight, lp.height);
  performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
 
  //decorView的measureSpec就是在这里确定的,其实比普通view的measurespec要简单的多
  //代码就不分析了 一目了然的东西
  private static int getRootMeasureSpec(int windowSize, int rootDimension) {
        int measureSpec;
        switch (rootDimension) {
 
        case ViewGroup.LayoutParams.MATCH_PARENT:
            // Window can‘t resize. Force root view to be windowSize.
            measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY);
            break;
        case ViewGroup.LayoutParams.WRAP_CONTENT:
            // Window can resize. Set max size for root view.
            measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.AT_MOST);
            break;
        default:
            // Window wants to be an exact size. Force root view to be that size.
            measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY);
            break;
        }
        return measureSpec;
}

4.对于普通view来说,他的measure过程中,与父view有关吗?如果有关,这个父view也就是viewgroup扮演了什么角色?

答:看源码:

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//对于普通view的measure来说 是由这个view的 父view ,也就是viewgroup来触发的。
//也就是下面这个measureChildWithMargins方法
 
protected void measureChildWithMargins(View child,
            int parentWidthMeasureSpec, int widthUsed,
            int parentHeightMeasureSpec, int heightUsed) {
         //第一步 先取得子view的 layoutParams 参数值
        final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
 
        //然后开始计算子view的spec的值,注意这里看到 计算的时候除了要用子view的 layoutparams参数以外
        //还用到了父view 也就是viewgroup自己的spec的值
        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);
}
 
//这个算view的spec的方法 看上去一大串 但是真的逻辑非常简单 就是根据父亲viewgroup
//的meaurespec 同时还有view自己的params来确定 view自己的measureSpec。
//注意这里的参数是padding,这个值的含义是 父容器已占用的控件的大小 所以view的Specsize
//的值 你们可以看到 是要减去这个padding的值的。总大小-已经用的 =可用的。 很好理解。
 
//然后就是下面的switch逻辑 要自己梳理清楚。其实也不难,主要是下面几条原则
//如果view采用固定宽高,也就是写死的数值那种。那就不管父亲的spec的值了,view的spec 就肯定是exactly 并且大小遵循layout参数里设置的大小。
 
//如果view的宽高是match_parent ,那么就要看父容器viewgroup的 spec的值了,如果父view的spec是exactly模式,
//那view也肯定是exactly,并且大小就是父容器剩下的空间。如果父容器是at_most模式,那view也是at_most 并且不会超过剩余空间大小
 
//如果view的宽高是wrap_content, 那就不管父容器的spec了,view的spec一定是at_most 并且不会超过父view 剩余空间的大小。
 
public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
        int specMode = MeasureSpec.getMode(spec);
        int specSize = MeasureSpec.getSize(spec);
 
        int size = Math.max(0, specSize - padding);
 
        int resultSize = 0;
        int resultMode = 0;
 
        switch (specMode) {
        // Parent has imposed an exact size on us
        case MeasureSpec.EXACTLY:
            if (childDimension >= 0) {
                resultSize = childDimension;
                resultMode = MeasureSpec.EXACTLY;
            else if (childDimension == LayoutParams.MATCH_PARENT) {
                // Child wants to be our size. So be it.
                resultSize = size;
                resultMode = MeasureSpec.EXACTLY;
            else if (childDimension == LayoutParams.WRAP_CONTENT) {
                // Child wants to determine its own size. It can‘t be
                // bigger than us.
                resultSize = size;
                resultMode = MeasureSpec.AT_MOST;
            }
            break;
 
        // Parent has imposed a maximum size on us
        case MeasureSpec.AT_MOST:
            if (childDimension >= 0) {
                // Child wants a specific size... so be it
                resultSize = childDimension;
                resultMode = MeasureSpec.EXACTLY;
            else if (childDimension == LayoutParams.MATCH_PARENT) {
                // Child wants to be our size, but our size is not fixed.
                // Constrain child to not be bigger than us.
                resultSize = size;
                resultMode = MeasureSpec.AT_MOST;
            else if (childDimension == LayoutParams.WRAP_CONTENT) {
                // Child wants to determine its own size. It can‘t be
                // bigger than us.
                resultSize = size;
                resultMode = MeasureSpec.AT_MOST;
            }
            break;
 
        // Parent asked to see how big we want to be
        case MeasureSpec.UNSPECIFIED:
            if (childDimension >= 0) {
                // Child wants a specific size... let him have it
                resultSize = childDimension;
                resultMode = MeasureSpec.EXACTLY;
            else if (childDimension == LayoutParams.MATCH_PARENT) {
                // Child wants to be our size... find out how big it should
                // be
                resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
                resultMode = MeasureSpec.UNSPECIFIED;
            else if (childDimension == LayoutParams.WRAP_CONTENT) {
                // Child wants to determine its own size.... find out how
                // big it should be
                resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
                resultMode = MeasureSpec.UNSPECIFIED;
            }
            break;
        }
        return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
    }

5.view的meaure和onMeasure有什么关系?

答:看源码:

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//view的measure是final 方法 我们子类无法修改的。
 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) {
                // 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) {
                throw new IllegalStateException("View with id " + getId() + ": "
                        + getClass().getName() + "#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
    }
 
//不过可以看到的是在measure方法里调用了onMeasure方法
//所以就能知道 我们在自定义view的时候一定是重写这个方法!
 protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
                getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
    }

6.简要分析view的measure流程?

答:先回顾问题4,viewgroup 算出子view的spec以后 会调用子view的measure方法,而子view的measure方法 我们问题5也看过了实际上是调用的onMeasure方法

所以我们只要分析好onMeasure方法即可,注意onMeasure方法的参数 正是他的父view算出来的那2个spec的值(这里view的measure方法会把这个spec里的specSize值做略微的修改 这个部分 不做分析 因为measure方法修改specSize的部分很简单)。

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//可以看出来这个就是setMeasuredDimension方法的调用 这个方法看名字就知道就是确定view的测量宽高的
//所以我们分析的重点就是看这个getDefaultSize 方法 是怎么确定view的测量宽高的
 protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
                getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
    }
 
//这个方法特别简单 基本可以认为就是近似的返回spec中的specSize,除非你的specMode是UNSPECIFIED
//UNSPECIFIED 这个一般都是系统内部测量才用的到,这种时候返回size 也就是getSuggestedMinimumWidth的返回值
 public static int getDefaultSize(int size, int measureSpec) {
        int result = size;
        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;
}
 
//跟view的背景相关 这里不多做分析了
protected int getSuggestedMinimumWidth() {
        return (mBackground == null) ? mMinWidth : max(mMinWidth, mBackground.getMinimumWidth());
    }

7.自定义view中 如果onMeasure方法 没有对wrap_content 做处理 会发生什么?为什么?怎么解决?

答:如果没有对wrap_content做处理 ,那即使你在xml里设置为wrap_content.其效果也和match_parent相同。看问题4的分析。我们可以知道view自己的layout为wrap,那mode就是at_most(不管父亲view是什么specmode).

这种模式下宽高就是等于specSize(getDefaultSize函数分析可知),而这里的specSize显然就是parentSize的大小。也就是父容器剩余的大小。那不就和我们直接设置成match_parent是一样的效果了么?

解决方式就是在onMeasure里 针对wrap 来做特殊处理 比如指定一个默认的宽高,当发现是wrap_content 就设置这个默认宽高即可。

8.ViewGroup有onMeasure方法吗?为什么?

答:没有,这个方法是交给子类自己实现的。不同的viewgroup子类 肯定布局都不一样,那onMeasure索性就全部交给他们自己实现好了。

9.为什么在activity的生命周期里无法获得测量宽高?有什么方法可以解决这个问题吗?

答:因为measure的过程和activity的生命周期  没有任何关系。你无法确定在哪个生命周期执行完毕以后 view的measure过程一定走完。可以尝试如下几种方法 获取view的测量宽高。

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//重写activity的这个方法
public void onWindowFocusChanged(boolean hasFocus) {
        super.onWindowFocusChanged(hasFocus);
        if (hasFocus) {
            int width = tv.getMeasuredWidth();
            int height = tv.getMeasuredHeight();
            Log.v("burning""width==" + width);
            Log.v("burning""height==" + height);
 
        }
    }

或者重写这个方法

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@Override
    protected void onStart() {
        super.onStart();
        tv.post(new Runnable() {
            @Override
            public void run() {
                int width = tv.getMeasuredWidth();
                int height = tv.getMeasuredHeight();
            }
        });
    }

再或者:

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@Override
    protected void onStart() {
        super.onStart();
        ViewTreeObserver observer = tv.getViewTreeObserver();
        observer.addOnGlobalLayoutListener(new ViewTreeObserver.OnGlobalLayoutListener() {
            @Override
            public void onGlobalLayout() {
                int width = tv.getMeasuredWidth();
                int height = tv.getMeasuredHeight();
                tv.getViewTreeObserver().removeOnGlobalLayoutListener(this);
            }
        });
    }

10.layout和onLayout方法有什么区别?

答:layout是确定本身view的位置 而onLayout是确定所有子元素的位置。layout里面 就是通过serFrame方法设设定本身view的 四个顶点的位置。这4个位置以确定 自己view的位置就固定了

然后就调用onLayout来确定子元素的位置。view和viewgroup的onlayout方法都没有写。都留给我们自己给子元素布局

11.draw方法 大概有几个步骤?

答: 一共是4个步骤, 绘制背景———绘制自己——–绘制chrildren—-绘制装饰。

12.setWillNotDraw方法有什么用?

答:这个方法在view里。

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/**
     * If this view doesn‘t do any drawing on its own, set this flag to
     * allow further optimizations. By default, this flag is not set on
     * View, but could be set on some View subclasses such as ViewGroup.
     *
     * Typically, if you override {@link #onDraw(android.graphics.Canvas)}
     * you should clear this flag.
     *
     * @param willNotDraw whether or not this View draw on its own
     */
    public void setWillNotDraw(boolean willNotDraw) {
        setFlags(willNotDraw ? WILL_NOT_DRAW : 0, DRAW_MASK);
    }

用于设置标志位的 也就是说 如果你的自定义view 不需要draw的话,就可以设置这个方法为true。这样系统知道你这个view 不需要draw 可以优化执行速度。viewgroup 一般都默认设置这个为true,因为viewgroup多数都是只负责布局

不负责draw的。而view 这个标志位 默认一般都是关闭的。

13.自定义view 有哪些需要注意的点?

答:主要是要处理wrap_content 和padding。否则xml 那边设置这2个属性就根本没用了。还有不要在view中使用handler 因为人家已经提供了post方法。如果是继承自viewGroup,那在onMeasure和onLayout里面 也要考虑

padding和layout的影响。也就是说specSize 要算一下 。最后就是如果view的动画或者线程需要停止,可以考虑在onDetachedFromWindow里面来做。

针对上述的几点,给出几个简单的自定义view 供大家理解。

给出一个圆形的view 范例:

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package com.example.administrator.motioneventtest;
 
import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.util.AttributeSet;
import android.view.View;
 
/**
 * Created by Administrator on 2016/2/4.
 */
public class CircleView extends View {
 
    private int mColor = Color.RED;
    private Paint mPaint = new Paint(Paint.ANTI_ALIAS_FLAG);
 
    private void init() {
        mPaint.setColor(mColor);
    }
 
    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        int widthSpecMode = MeasureSpec.getMode(widthMeasureSpec);
        int widthSpecSize = MeasureSpec.getSize(widthMeasureSpec);
        int heightSpecMode = MeasureSpec.getMode(heightMeasureSpec);
        int heightSpecSize = MeasureSpec.getSize(heightMeasureSpec);
 
        //处理为wrap_content时的情况
        if (widthSpecMode == MeasureSpec.AT_MOST && heightSpecMode == MeasureSpec.AT_MOST) {
            setMeasuredDimension(200200);
        else if (widthSpecMode == MeasureSpec.AT_MOST) {
            setMeasuredDimension(200, heightSpecSize);
        else if (heightSpecMode == MeasureSpec.AT_MOST) {
            setMeasuredDimension(widthSpecSize, 200);
        }
 
    }
 
    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        //处理padding的情况
        final int paddingLeft = getPaddingLeft();
        final int paddingRight = getPaddingRight();
        final int paddingTop = getPaddingTop();
        final int paddingBottom = getPaddingBottom();
 
        int width = getWidth() - paddingLeft - paddingRight;
        int height = getHeight() - paddingTop - paddingBottom;
        int radius = Math.min(width, height) / 2;
        canvas.drawCircle(paddingLeft + width / 2, paddingTop + height / 2, radius, mPaint);
    }
 
    public CircleView(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        init();
    }
 
    public CircleView(Context context) {
        super(context);
        init();
 
    }
 
    public CircleView(Context context, AttributeSet attrs) {
        super(context, attrs);
        init();
    }
}

然后下面再给出一个范例,稍微复杂一点是自定义viewgroup了(主要是加强对onMeasure和onLayout的理解), 需求如下:

一个水平的viewgroup,内部的子元素 为了简单 我们假定他们的宽高都是一样的。来写一个这样的简单的viewgroup。

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package com.example.administrator.motioneventtest;
 
import android.content.Context;
import android.util.AttributeSet;
import android.util.Log;
import android.view.View;
import android.view.ViewGroup;
 
/**
 * Created by Administrator on 2016/2/4.
 */
//这里我们只处理了padding的状态 没有处理margin的状态,子view的margin 对measure和layout的影响
//就留给读者自己完成了
public class CustomHorizontalLayout extends ViewGroup {
 
    //设置默认的控件最小是多少 这里不提供自定义属性了 写死在代码里 你们可以自行拓展
    final int minHeight = 0;
    final int minWidth = 0;
 
    public CustomHorizontalLayout(Context context) {
        super(context);
    }
 
    public CustomHorizontalLayout(Context context, AttributeSet attrs) {
        super(context, attrs);
    }
 
    public CustomHorizontalLayout(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
    }
 
    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        int measureWidth = 0;
        int measureHeight = 0;
        final int childCount = getChildCount();
        measureChildren(widthMeasureSpec, heightMeasureSpec);
        int widthSpecMode = MeasureSpec.getMode(widthMeasureSpec);
        int widthSpecSize = MeasureSpec.getSize(widthMeasureSpec);
        int heightSpecMode = MeasureSpec.getMode(heightMeasureSpec);
        int heightSpecSize = MeasureSpec.getSize(heightMeasureSpec);
        final View childView = getChildAt(0);
        final int paddingLeft = getPaddingLeft();
        final int paddingRight = getPaddingRight();
        final int paddingTop = getPaddingTop();
        final int paddingBottom = getPaddingBottom();
        //没有子控件 时 我们的宽高要作特殊处理
        if (childCount == 0) {
            //当没有子控件时,如果长宽有一个为wrap 那么就让这个控件以最小的形式展现
            //这里我们最小设置为0
            if (widthSpecMode == MeasureSpec.AT_MOST || heightSpecMode == MeasureSpec.AT_MOST) {
                setMeasuredDimension(minWidth, minHeight);
            else {
                //否则根据我们的layout属性来
                setMeasuredDimension(getLayoutParams().width, getLayoutParams().height);
            }
 
        else if (widthSpecMode == MeasureSpec.AT_MOST && heightSpecMode == MeasureSpec.AT_MOST) {
            measureWidth = childView.getMeasuredWidth() * childCount;
            measureHeight = childView.getMeasuredHeight();
            setMeasuredDimension(paddingLeft + measureWidth + paddingRight, paddingTop + measureHeight + paddingBottom);
        else if (heightSpecMode == MeasureSpec.AT_MOST) {
            measureHeight = childView.getMeasuredHeight();
            setMeasuredDimension(paddingLeft + paddingRight + widthSpecSize, paddingTop + paddingBottom + measureHeight);
        else if (widthSpecMode == MeasureSpec.AT_MOST) {
            measureWidth = childView.getMeasuredWidth() * childCount;
            setMeasuredDimension(paddingLeft + paddingRight + measureWidth, paddingTop + paddingBottom + heightSpecSize);
        }
    }
 
    @Override
    protected void onLayout(boolean changed, int l, int t, int r, int b) {
        final int paddingLeft = getPaddingLeft();
        final int paddingRight = getPaddingRight();
        final int paddingTop = getPaddingTop();
        final int paddingBottom = getPaddingBottom();
        //左边初始位置为0
        int childLeft = 0 + paddingLeft;
        final int childCount = getChildCount();
        for (int i = 0; i < childCount; i++) {
            final View childView = getChildAt(i);
            if (childView.getVisibility() != View.GONE) {
                final int childWidth = childView.getMeasuredWidth();
                childView.layout(childLeft, 0 + paddingTop, childLeft + childWidth, paddingTop + childView.getMeasuredHeight());
                childLeft += childWidth;
            }
        }
    }
}

[Android][转]Android View绘制13问13答

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原文地址:http://blog.csdn.net/andrewyls/article/details/50978594

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