可以做彩字的网站,淘宝自己网站怎么建设,滨州网站建设电话,建个短视频网站在日常开发中我们常常使用LinearLayout作为布局Group#xff0c;本文从其源码实现出发分析测量流程。大家可以带着问题进入下面的分析流程#xff0c;看看是否能找到答案。
垂直测量
View的测量入口方法是onmeasure方法。LinearLayout的onMeasure方法根据其方向而做不同的处…在日常开发中我们常常使用LinearLayout作为布局Group本文从其源码实现出发分析测量流程。大家可以带着问题进入下面的分析流程看看是否能找到答案。
垂直测量
View的测量入口方法是onmeasure方法。LinearLayout的onMeasure方法根据其方向而做不同的处理。本文我们选择垂直布局测量看下。 protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {if (mOrientation VERTICAL) {measureVertical(widthMeasureSpec, heightMeasureSpec);} else {measureHorizontal(widthMeasureSpec, heightMeasureSpec);}}而measureVertical方法整体上分为2个部分。即两次测量。
第一次测量 for (int i 0; i count; i) {final View child getVirtualChildAt(i);if (child null) {mTotalLength measureNullChild(i);continue;}if (child.getVisibility() View.GONE) {//GONE的View不用测量注意INVISIBLE还是会测量的i getChildrenSkipCount(child, i);continue;}nonSkippedChildCount;if (hasDividerBeforeChildAt(i)) {//mTotalLength为总的使用高度这里加上分割线的高度mTotalLength mDividerHeight;}final LayoutParams lp (LayoutParams) child.getLayoutParams();//totalWeight为总的weight这里主要是统计所有使用weight的view的总weight为后续测量weight0的View作准备totalWeight lp.weight;final boolean useExcessSpace lp.height 0 lp.weight 0;if (heightMode MeasureSpec.EXACTLY useExcessSpace) {// 优化不必费心测量仅使用多余空间布局的子视图。如果我们有空间可供分配这些视图将在稍后进行测量。//这里是指当LinearLayout为精确模式子View使用了weight。那么本次就不用测量该View而只是将其margin统计到已使用的高度上final int totalLength mTotalLength;mTotalLength Math.max(totalLength, totalLength lp.topMargin lp.bottomMargin);skippedMeasure true;} else {if (useExcessSpace) {// heightMode 为 UNSPECIFIED 或 AT_MOST并且此子项仅使用多余的空间进行布局。使用 WRAP_CONTENT 进行测量以便我们能够找出视图的最佳高度。测量后我们将恢复原始高度 0lp.height LayoutParams.WRAP_CONTENT;}// 确定此子项想要有多大。如果此子项或先前的子项已指定weight权重则我们允许它使用所有可用空间如果需要我们稍后会缩小空间。final int usedHeight totalWeight 0 ? mTotalLength : 0;//测量子View的宽高measureChildBeforeLayout(child, i, widthMeasureSpec, 0,heightMeasureSpec, usedHeight);final int childHeight child.getMeasuredHeight();if (useExcessSpace) {// 恢复原始高度并记录我们为多余的子项分配了多少空间以便我们能够匹配精确测量的行为。lp.height 0;consumedExcessSpace childHeight;}final int totalLength mTotalLength;//更新当前总的高度mTotalLength Math.max(totalLength, totalLength childHeight lp.topMargin lp.bottomMargin getNextLocationOffset(child));if (useLargestChild) {//这个largestChildHeight可以理解成所有子View中的高度最大的View的高度主要是为了在当前View使用weight0但是父View的高度不确定的情况下确定子View的高度比如LinearLayout是wrap_content,largestChildHeight Math.max(childHeight, largestChildHeight);}}if ((baselineChildIndex 0) (baselineChildIndex i 1)) {mBaselineChildTop mTotalLength;}boolean matchWidthLocally false;if (widthMode ! MeasureSpec.EXACTLY lp.width LayoutParams.MATCH_PARENT) {// 线性布局的宽度将缩放并且至少有一个子视图表示它想要匹配我们的宽度。设置一个标志表示当我们知道我们的宽度时我们至少需要重新测量该视图。matchWidth true;matchWidthLocally true;}final int margin lp.leftMargin lp.rightMargin;final int measuredWidth child.getMeasuredWidth() margin;maxWidth Math.max(maxWidth, measuredWidth);childState combineMeasuredStates(childState, child.getMeasuredState());allFillParent allFillParent lp.width LayoutParams.MATCH_PARENT;if (lp.weight 0) {/** 计算最大宽度*/weightedMaxWidth Math.max(weightedMaxWidth,matchWidthLocally ? margin : measuredWidth);} else {alternativeMaxWidth Math.max(alternativeMaxWidth,matchWidthLocally ? margin : measuredWidth);}i getChildrenSkipCount(child, i);}从第一测量的源码得知其第一次测量主要是测量weight0的子View的宽高并为第二次测量做准备统计weight的和、统计已使用的高度(为weight能分配的高度做准备) 统计最大的子View的高度。
第二次测量 //remainingExcess为剩余可以为weight0的子View分配高度的总和int remainingExcess heightSize - mTotalLength (mAllowInconsistentMeasurement ? 0 : consumedExcessSpace);
if (skippedMeasure|| ((sRemeasureWeightedChildren || remainingExcess ! 0) totalWeight 0.0f)) {//如果有weight0的子View。则走这个分支float remainingWeightSum mWeightSum 0.0f ? mWeightSum : totalWeight;mTotalLength 0;for (int i 0; i count; i) {final View child getVirtualChildAt(i);if (child null || child.getVisibility() View.GONE) {continue;}final LayoutParams lp (LayoutParams) child.getLayoutParams();final float childWeight lp.weight;if (childWeight 0) {// 按照权重计算每个weight0的子View的高度final int share (int) (childWeight * remainingExcess / remainingWeightSum);remainingExcess - share;remainingWeightSum - childWeight;final int childHeight;if (mUseLargestChild heightMode ! MeasureSpec.EXACTLY) {childHeight largestChildHeight;} else if (lp.height 0 (!mAllowInconsistentMeasurement|| heightMode MeasureSpec.EXACTLY)) {// This child needs to be laid out from scratch using// only its share of excess space.childHeight share;} else {// This child had some intrinsic height to which we// need to add its share of excess space.childHeight child.getMeasuredHeight() share;}//生成子View的高度MeasureSpec模式为精确final int childHeightMeasureSpec MeasureSpec.makeMeasureSpec(Math.max(0, childHeight), MeasureSpec.EXACTLY);final int childWidthMeasureSpec getChildMeasureSpec(widthMeasureSpec,mPaddingLeft mPaddingRight lp.leftMargin lp.rightMargin,lp.width);//重新测量子Viewchild.measure(childWidthMeasureSpec, childHeightMeasureSpec);// Child may now not fit in vertical dimension.childState combineMeasuredStates(childState, child.getMeasuredState() (MEASURED_STATE_MASKMEASURED_HEIGHT_STATE_SHIFT));}final int margin lp.leftMargin lp.rightMargin;final int measuredWidth child.getMeasuredWidth() margin;maxWidth Math.max(maxWidth, measuredWidth);boolean matchWidthLocally widthMode ! MeasureSpec.EXACTLY lp.width LayoutParams.MATCH_PARENT;alternativeMaxWidth Math.max(alternativeMaxWidth,matchWidthLocally ? margin : measuredWidth);allFillParent allFillParent lp.width LayoutParams.MATCH_PARENT;final int totalLength mTotalLength;mTotalLength Math.max(totalLength, totalLength child.getMeasuredHeight() lp.topMargin lp.bottomMargin getNextLocationOffset(child));}// Add in our paddingmTotalLength mPaddingTop mPaddingBottom;// TODO: Should we recompute the heightSpec based on the new total length?} else {alternativeMaxWidth Math.max(alternativeMaxWidth,weightedMaxWidth);// We have no limit, so make all weighted views as tall as the largest child.// Children will have already been measured once.if (useLargestChild heightMode ! MeasureSpec.EXACTLY) {for (int i 0; i count; i) {final View child getVirtualChildAt(i);if (child null || child.getVisibility() View.GONE) {continue;}final LinearLayout.LayoutParams lp (LinearLayout.LayoutParams) child.getLayoutParams();float childExtra lp.weight;if (childExtra 0) {child.measure(MeasureSpec.makeMeasureSpec(child.getMeasuredWidth(),MeasureSpec.EXACTLY),MeasureSpec.makeMeasureSpec(largestChildHeight,MeasureSpec.EXACTLY));}}}}
