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下面是分析ListView初始化的源码流程分析，主要是ListVIew.onLayout过程与普通视图的layout过程完全不同，避免流程交代不清楚，以下是一个流程的思维导图。
思维导图是顺序是从左向右，从上向下。
一、 先看构造函数，上图中1.1就不分析了，主要是读取一些ListView参数，直接来看1.2 ViewGroup构造函数源码
private void initViewGroup() { ...... // 初始化保存当前ViewGroup中所有View的数组 mChildren = new View[ARRAY_INITIAL_CAPACITY]; // 初始时其Child个数为0 mChildrenCount = 0; ...... }
视图的创建过程的都会执行的三个步骤: onMeasure, onLayout, onDraw
二、接着2 即 ListView.onMeasure方法，只是获取当前ListView的宽高
@Override protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) { // Sets up mListPadding super.onMeasure(widthMeasureSpec, heightMeasureSpec); // 获取当前ListView总宽高 int widthSize = MeasureSpec.getSize(widthMeasureSpec); int heightSize = MeasureSpec.getSize(heightMeasureSpec); ...... setMeasuredDimension(widthSize , heightSize); mWidthMeasureSpec = widthMeasureSpec; }
三、步骤3是重点，AbsListView.onLayout的流程与普通View的不同
@Override protected void onLayout(boolean changed, int l, int t, int r, int b) { super.onLayout(changed, l, t, r, b); mInLayout = true; // 初始时changed = true if (changed) { int childCount = getChildCount(); for (int i = 0; i < childCount; i++) { // ? getChildAt(i).forceLayout(); } mRecycler.markChildrenDirty(); } if (mFastScroller != null && mItemCount != mOldItemCount) { mFastScroller.onItemCountChanged(mOldItemCount, mItemCount); } // ListView实现此方法 layoutChildren(); mInLayout = false; mOverscrollMax = (b - t) / OVERSCROLL_LIMIT_DIVISOR; }
四、步骤4.1 具体分析ListVIew.layoutChildren
@Override protected void layoutChildren() { // 默认为false final boolean blockLayoutRequests = mBlockLayoutRequests; if (!blockLayoutRequests) { // 目的是为了阻止没必要的layout操作，提交效率 mBlockLayoutRequests = true; } else { return; } try { super.layoutChildren(); // 为什么此处要请求重绘？ invalidate(); ...... int childCount = getChildCount(); ...... boolean dataChanged = mDataChanged; if (dataChanged) { handleDataChanged(); } ...... // 把所有child view放置到RecycleBin // 满足条件的话可以重用这些child view final int firstPosition = mFirstPosition; // ListView中Item复用使用此数据结构 final RecycleBin recycleBin = mRecycler; // reset the focus restoration View focusLayoutRestoreDirectChild = null; // Don‘t put header or footer views into the Recycler. Those are // already cached in mHeaderViews; if (dataChanged) { for (int i = 0; i < childCount; i++) { recycleBin.addScrapView(getChildAt(i), firstPosition+i); } } else { // 创建childCount个数的View放入RecycleBin类activeViews数组中 recycleBin.fillActiveViews(childCount, firstPosition); } ...... // Clear out old views detachAllViewsFromParent(); recycleBin.removeSkippedScrap(); switch (mLayoutMode) { ...... default: if (childCount == 0) { if (!mStackFromBottom) { final int position = lookForSelectablePosition(0, true); setSelectedPositionInt(position); // 此方法是重点，以下具体分析 sel = fillFromTop(childrenTop); } else { final int position = lookForSelectablePosition(mItemCount - 1, false); setSelectedPositionInt(position); sel = fillUp(mItemCount - 1, childrenBottom); } } else { if (mSelectedPosition >= 0 && mSelectedPosition < mItemCount) { sel = fillSpecific(mSelectedPosition, oldSel == null ? childrenTop : oldSel.getTop()); } else if (mFirstPosition < mItemCount) { sel = fillSpecific(mFirstPosition, oldFirst == null ? childrenTop : oldFirst.getTop()); } else { sel = fillSpecific(0, childrenTop); } } break; } // Flush any cached views that did not get reused above recycleBin.scrapActiveViews(); ...... invokeOnItemScrollListener(); } finally { if (!blockLayoutRequests) { mBlockLayoutRequests = false; } }
五、 分析步骤4.2 ListView.fillFromTop源码
// 从上向下在ListView中填充Item View private View fillFromTop(int nextTop) { mFirstPosition = Math.min(mFirstPosition, mSelectedPosition); mFirstPosition = Math.min(mFirstPosition, mItemCount - 1); if (mFirstPosition < 0) { mFirstPosition = 0; } // 具体操作在此 return fillDown(mFirstPosition, nextTop); }
六、查看步骤4.3 ListView.fillDown源码
// 在参数pos下面填充Item View private View fillDown(int pos, int nextTop) { View selectedView = null; // ListView getHeight也是这样计算的 int end = (mBottom - mTop); if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) { end -= mListPadding.bottom; } // 初始化时pos = 0，如果item总数少于一屏幕，执行mItemCount - pos次 // 如果item多余一屏幕，执行end - nextTop次 while (nextTop < end && pos < mItemCount) { // is this the selected item? boolean selected = pos == mSelectedPosition; // 获取Item View对象 View child = makeAndAddView(pos, nextTop, true, mListPadding.left, selected); nextTop = child.getBottom() + mDividerHeight; if (selected) { selectedView = child; } pos++; } setVisibleRangeHint(mFirstPosition, mFirstPosition + getChildCount() - 1); return selectedView; }
七、查看步骤4.4 ListView.makeAndAddView源码
// ListView都是通过此方法获取Item View // 具体Item View如何复用，是否需要创建新的Item View都有此方法处理 private View makeAndAddView(int position, int y, boolean flow, int childrenLeft, boolean selected) { View child; ListView的数据发生变化，肯定Item View之前已经创建好了，无需重新创建 if (!mDataChanged) { // 当前position复用之前创建的视图 child = mRecycler.getActiveView(position); if (child != null) { // 对复用的View针对当前需要进行配置 setupChild(child, position, y, flow, childrenLeft, selected, true); return child; } } // 创建或者重用视图 child = obtainView(position, mIsScrap); // This needs to be positioned and measured setupChild(child, position, y, flow, childrenLeft, selected, mIsScrap[0]); return child; }
八 查看步骤4.5 ListView.setupChild源码
private void setupChild(View child, int position, int y, boolean flowDown, int childrenLeft, boolean selected, boolean recycled) { // 判断当前Item View是否选中状态 final boolean isSelected = selected && shouldShowSelector(); final boolean updateChildSelected = isSelected != child.isSelected(); final int mode = mTouchMode; // 是否处于按下状态 final boolean isPressed = mode > TOUCH_MODE_DOWN && mode < TOUCH_MODE_SCROLL && mMotionPosition == position; final boolean updateChildPressed = isPressed != child.isPressed(); // 是否需要重新measure与layout final boolean needToMeasure = !recycled || updateChildSelected || child.isLayoutRequested(); // Respect layout params that are already in the view. Otherwise make some up... // noinspection unchecked AbsListView.LayoutParams p = (AbsListView.LayoutParams) child.getLayoutParams(); if (p == null) { p = (AbsListView.LayoutParams) generateDefaultLayoutParams(); } p.viewType = mAdapter.getItemViewType(position); if ((recycled && !p.forceAdd) || (p.recycledHeaderFooter && p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER)) { attachViewToParent(child, flowDown ? -1 : 0, p); } else { p.forceAdd = false; if (p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER) { p.recycledHeaderFooter = true; } // 向ListView（ViewGroup子类）添加当前Item View addViewInLayout(child, flowDown ? -1 : 0, p, true); } // 更新选中状态 if (updateChildSelected) { child.setSelected(isSelected); } // 更新按下状态 if (updateChildPressed) { child.setPressed(isPressed); } if (mChoiceMode != CHOICE_MODE_NONE && mCheckStates != null) { if (child instanceof Checkable) { ((Checkable) child).setChecked(mCheckStates.get(position)); } else if (getContext().getApplicationInfo().targetSdkVersion >= android.os.Build.VERSION_CODES.HONEYCOMB) { child.setActivated(mCheckStates.get(position)); } } if (needToMeasure) { int childWidthSpec = ViewGroup.getChildMeasureSpec(mWidthMeasureSpec, mListPadding.left + mListPadding.right, p.width); int lpHeight = p.height; int childHeightSpec; if (lpHeight > 0) { childHeightSpec = MeasureSpec.makeMeasureSpec(lpHeight, MeasureSpec.EXACTLY); } else { childHeightSpec = MeasureSpec.makeMeasureSpec(0, MeasureSpec.UNSPECIFIED); } // 与普通视图的measure流程不同，ListView是在此处执行具体的当前Item View measure child.measure(childWidthSpec, childHeightSpec); } else { cleanupLayoutState(child); } final int w = child.getMeasuredWidth(); final int h = child.getMeasuredHeight(); final int childTop = flowDown ? y : y - h; if (needToMeasure) { final int childRight = childrenLeft + w; final int childBottom = childTop + h; // 大小改变肯定位置也会发生变化，当前Item View重新进行layout child.layout(childrenLeft, childTop, childRight, childBottom); } else { child.offsetLeftAndRight(childrenLeft - child.getLeft()); child.offsetTopAndBottom(childTop - child.getTop()); } if (mCachingStarted && !child.isDrawingCacheEnabled()) { child.setDrawingCacheEnabled(true); } if (recycled && (((AbsListView.LayoutParams)child.getLayoutParams()).scrappedFromPosition) != position) { child.jumpDrawablesToCurrentState(); } }
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原文地址：http://www.cnblogs.com/duanxz/p/4391672.html
