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publicstaticintgetChildMeasureSpec(intspec, intpadding, intchildDimension) {
intspecMode = MeasureSpec.getMode(spec);
intspecSize = MeasureSpec.getSize(spec);
intsize = Math.max(0, specSize - padding);
intresultSize = 0;
intresultMode = 0;
switch (specMode) {
// Parent has imposed an exact size on us// 父布局给了确定的尺寸caseMeasureSpec.EXACTLY:
if (childDimension >= 0) {
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
} elseif (childDimension == LayoutParams.MATCH_PARENT) {
// Child wants to be our size. So be it.// 与父布局一样大resultSize = size;
resultMode = MeasureSpec.EXACTLY;
} elseif (childDimension == LayoutParams.WRAP_CONTENT) {
// Child wants to determine its own size. It can't be// bigger than us.// 子布局自己决定，不能超过父布局resultSize = size;
resultMode = MeasureSpec.AT_MOST;
}
break;
// Parent has imposed a maximum size on us// 父布局有一个确定的最大尺寸caseMeasureSpec.AT_MOST:
if (childDimension >= 0) {
// Child wants a specific size... so be itresultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
} elseif (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;
} elseif (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 becaseMeasureSpec.UNSPECIFIED:
if (childDimension >= 0) {
// Child wants a specific size... let him have itresultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
} elseif (childDimension == LayoutParams.MATCH_PARENT) {
// Child wants to be our size... find out how big it should// beresultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
resultMode = MeasureSpec.UNSPECIFIED;
} elseif (childDimension == LayoutParams.WRAP_CONTENT) {
// Child wants to determine its own size.... find out how// big it should beresultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
resultMode = MeasureSpec.UNSPECIFIED;
}
break;
}
//noinspection ResourceTypereturnMeasureSpec.makeMeasureSpec(resultSize, resultMode);
}
以上就是整个 View 和 ViewGroup 的整个 Measure 过程。
Layout
回到 ViewRootImpl 类中，执行 performLayout（）过程。
privatevoidperformLayout(WindowManager.LayoutParamslp, intdesiredWindowWidth,
intdesiredWindowHeight) {
try {
// 进行放置host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());
mInLayout = false;
intnumViewsRequestingLayout = mLayoutRequesters.size();
if (numViewsRequestingLayout > 0) {
// 放置子 ViewintnumValidRequests = validLayoutRequesters.size();
for (inti = 0; i < numValidRequests; ++i) {
finalViewview = validLayoutRequesters.get(i);
// view.requestLayout();
}
// 再次测量measureHierarchy(host, lp, mView.getContext().getResources(),
desiredWindowWidth, desiredWindowHeight);
mInLayout = true;
// 重新放置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'dvalidLayoutRequesters = getValidLayoutRequesters(mLayoutRequesters, true);
if (validLayoutRequesters != null) {
finalArrayList<View> finalRequesters = validLayoutRequesters;
// Post second-pass requests to the next frame// 再次检查是否仍有需要layout的View，如果有，就到下一帧再继续getRunQueue().post(newRunnable() {
@Overridepublicvoidrun() {
intnumValidRequests = finalRequesters.size();
for (inti = 0; i < numValidRequests; ++i) {
finalViewview = finalRequesters.get(i);
view.requestLayout();
}
mInLayout = false;
上述过程是先让 DecorView 进行放置，然后有需要就放置所有的子 View，调用 requestLayout 方法。然后再对整个 View Tree 进行测量，进行二次放置。
privatevoidperformDraw() {
// 主要是调用 draw 方法try {
booleancanUseAsync = draw(fullRedrawNeeded);
}
}
privatebooleandraw(booleanfullRedrawNeeded) {
if (!drawSoftware(surface, mAttachInfo, xOffset, yOffset,
scalingRequired, dirty, surfaceInsets)) {
returnfalse;
}
}
/** * @return true if drawing was successful, false if an error occurred */privatebooleandrawSoftware(Surfacesurface, AttachInfoattachInfo, intxoff, intyoff,
booleanscalingRequired, Rectdirty, RectsurfaceInsets) {
// Draw with software renderer.finalCanvascanvas;
// 获取画布canvas = mSurface.lockCanvas(dirty);
// 一系列设置之后mView.draw(canvas);
}
// DecorView.java@Overridepublicvoiddraw(Canvascanvas) {
super.draw(canvas);
if (mMenuBackground != null) {
mMenuBackground.draw(canvas);
}
}
// 还是调用 View 的 draw 方法publicvoiddraw(Canvascanvas) {
/* * 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 * 4. Draw children * 5. If necessary, draw the fading edges and restore layers * 6. Draw decorations (scrollbars for instance) */// Step 1, draw the background, if neededintsaveCount;
if (!dirtyOpaque) {
drawBackground(canvas);
}
// skip step 2 & 5 if possible (common case)finalintviewFlags = mViewFlags;
booleanhorizontalEdges = (viewFlags & FADING_EDGE_HORIZONTAL) != 0;
booleanverticalEdges = (viewFlags & FADING_EDGE_VERTICAL) != 0;
if (!verticalEdges && !horizontalEdges) {
// Step 3, draw the contentif (!dirtyOpaque) onDraw(canvas);
// Step 4, draw the childrendispatchDraw(canvas);
drawAutofilledHighlight(canvas);
// Overlay is part of the content and draws beneath Foregroundif (mOverlay != null && !mOverlay.isEmpty()) {
mOverlay.getOverlayView().dispatchDraw(canvas);
}
// Step 6, draw decorations (foreground, scrollbars)onDrawForeground(canvas);
// Step 7, draw the default focus highlightdrawDefaultFocusHighlight(canvas);
if (debugDraw()) {
debugDrawFocus(canvas);
}
// we're done...return;
}
}
在 draw 过程中，一般有 6 个步骤，2 - 5 可以省略。这里主要看 3 和 4 步。
/** * Implement this to do your drawing. * * @param canvas the canvas on which the background will be drawn */// 自定义 View 实现， 3protectedvoidonDraw(Canvascanvas) {
}
/** * Called by draw to draw the child views. This may be overridden * by derived classes to gain control just before its children are drawn * (but after its own view has been drawn). * @param canvas the canvas on which to draw the view */// 自定义 ViewGroup 实现protectedvoiddispatchDraw(Canvascanvas) {
}
View: working process
[TOC]
在上一篇文章Activity： Lifecycle and ActivityThread 分析了 Activity 启动后执行生命周期方法的基础上，走到了 performTraversals() 方法，这篇文章由此开始，讲解 View 的工作流程。
上面是该函数的三个主要过程，下面逐步去看一下每个过程分别做了什么。
View的基本介绍
ViewRoot 和 DecorView
ViewRoot 对应与 ViewRootImpl， 是连接 WindowManager 和 DecorView 的纽带，
View 的三大流程同时通过 ViewRoot 来完成。
在 ActivityThread 中，当 Activity 对象被创建完毕，会将 DecorView 添加到 Window 中，同时会创建 ViewRootImpl 对象，同 DecorView 建立关联。
Measure
这里的 measureSpec 应该很熟悉了，是一个32为的 int 型整数。高两位表示测量模式，低30位表示具体大小。
其中测量模式有三种：
UNSPECIFIED：父容器不作限制，子View想多大就多大，一般用于系统内部。
EXACTLY：精确模式，大小为SpecSize，父容器完全决定子View的大小，对应LayoutParams中的match_parent和具体数值。
AT_MOST：最大模式，大小不能大于SpecSize，也就是子View的大小有上限，对应于LayoutParams中的warp_content。
获得了 view 的测量数据后，
上述就是 View 的测量过程，ViewGroup 的测量过程类似，只不过还要测量子 View 的大小来确定自己的大小。
以 DecorView 为例：
以上就是整个 View 和 ViewGroup 的整个 Measure 过程。
Layout
回到 ViewRootImpl 类中，执行 performLayout（）过程。
上述过程是先让 DecorView 进行放置，然后有需要就放置所有的子 View，调用 requestLayout 方法。然后再对整个 View Tree 进行测量，进行二次放置。
因此这里知道， FrameLayout 放置子 View 就是根据其 Gravity 放置在对应的位置，然后根据顺序进行叠加放置。
Draw
在 draw 过程中，一般有 6 个步骤，2 - 5 可以省略。这里主要看 3 和 4 步。
看一下在 FrameLayout 的是如何处理的。
到此为止，整个 View 的绘制流程就分析完毕了。
自定义 View
总结
在 ViewRootImpl 的方法 performTraversals 中，会实现 view 的整个绘制过程。
测量：View 测量自身，ViewGroup 除了测量自己，还需测量子 View 来确定自己的大小
放置：ViewGroup 放置所有的 子View
绘制：使用 Canvas 进行绘制。
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