上一篇文章最后结束在ViewRootImpl.setView()函数，这个函数之后发生了什么事情，我们接着分析。
1. ViewRootImpl简介
ViewRootImpl作为视图层次中的顶层，实现了View和WindowManager之间需要的协议，与SystemServer进程的WindowManagerService有交互，具体实现了WindowManagerGlobal内部的大部分功能。
1.1 ViewRootImpl的定义：
public final class ViewRootImpl implements ViewParent, View.AttachInfo.Callbacks, HardwareRenderer.HardwareDrawCallbacks { ...... final IWindowSession mWindowSession; final W mWindow; //W继承于IWindow.Stub类。 View mView; // 由setView()函数赋值。 private final Surface mSurface = new Surface(); //mSurface可能存在多线程访问，需要加锁保护。 ...... public ViewRootImpl(Context context, Display display) { ...... mWindowSession = WindowManagerGlobal.getWindowSession(); mWindow = new W(this); ...... }} mWindowSession将被用于建立Activity的ViewRootImpl和WindowManagerService的关系。创建代码如下：
getWindowSession()@WindowManagerGlobal.java
public static IWindowSession getWindowSession() { synchronized (WindowManagerGlobal.class) { if (sWindowSession == null) { try { InputMethodManager imm = InputMethodManager.getInstance(); IWindowManager windowManager = getWindowManagerService(); //得到WindowMangerService的Binder代理. sWindowSession = windowManager.openSession(imm.getClient(), imm.getInputContext()); ///openSession操作是一个使用Binder通信的跨进程调用。 ...... } catch (RemoteException e) { ...... } } return sWindowSession; }} getWindowManagerService()@WindowManagerGlobal.java
public static IWindowManager getWindowManagerService() { synchronized (WindowManagerGlobal.class) { if (sWindowManagerService == null) { sWindowManagerService = IWindowManager.Stub.asInterface(ServiceManager.getService("window")); } return sWindowManagerService; }} openSession()@WindowManagerService.java
public IWindowSession openSession(IInputMethodClient client, IInputContext inputContext) { ...... Session session = new Session(this, client, inputContext); return session;} 可以看到，通过WindowManagerService.openSession()获得一个Session对象。类Session继承于IWindowSession.Stub，并实现了IBinder接口。它支持Binder通信，并属于Bn端，即响应请求的服务端。
1.2 ViewRootImpl与WindowManagerService的关系
ViewRootImpl通过mWindowSession（IWindowSession）与WMS进程进行跨进程通信。
ViewRootImpl的mWindow是基于Binder通信的类，而且W是IWindow的Bn端，用于响应请求。
它们之间属于跨进程通信，关系如下图：
每个APP进程都会和WMS建立一个IWindowSession会话，APP进程利用这个会话与WMS通信。WMS利用IWindow进行事件通知，每当发生一些事件（按键触摸事件）时，WMS就会告诉某个IWindow.
事件分发大致流程：
(1) WMS所在的SystemServer进程接收到按键事件。
(2) WMS找到UI位于屏幕顶端的进程所对应的IWindow对象，这个是Bp端对象。
(3) 调用这个IWindow对象的dispatchKey。 IWindow对象的Bn端位于ViewRootImpl中，ViewRootImpl根据内部View的位置信息找到真正处理这个事件的View，最后调用dispatchKey函数完成事件处理。
每个按键事件都会被转化成一个消息，由系统将这个消息加入到对应进程的消息队列中。该进程在处理消息派发时，根据消息的句柄找到对应的Window，进而由Window处理掉这个消息。
2. ViewRootImpl.setView()函数
setView()@ViewRootImpl.java
public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView) { synchronized (this) { if (mView == null) { mView = view; //传进来的view是PhoneWindow的DecorView，保存起来。 mViewLayoutDirectionInitial = mView.getRawLayoutDirection(); mFallbackEventHandler.setView(view); mWindowAttributes.copyFrom(attrs); ...... mSoftInputMode = attrs.softInputMode; // InputMode可以影响输入法弹出时视图应该怎么显示。 ...... mAdded = true; int res; /* = WindowManagerImpl.ADD_OKAY; */ //在被加到WindowManager之前调度第一次layout,确保收到系统事件之前重新进行了布局。 requestLayout(); if ((mWindowAttributes.inputFeatures & WindowManager.LayoutParams.INPUT_FEATURE_NO_INPUT_CHANNEL) == 0) { mInputChannel = new InputChannel(); //创建InputChannel对象 } try { mOrigWindowType = mWindowAttributes.type; mAttachInfo.mRecomputeGlobalAttributes = true; collectViewAttributes(); res = mWindowSession.addToDisplay(mWindow, mSeq, mWindowAttributes, //注意第一个参数是mWindow. 继承IWindow.Stub类。 getHostVisibility(), mDisplay.getDisplayId(), mAttachInfo.mContentInsets, mInputChannel); } catch (RemoteException e) { ...... } // 创建InputQueue和WindowInputEventReceiver与窗口输入事件相关的对象。 if (mInputChannel != null) { if (mInputQueueCallback != null) { mInputQueue = new InputQueue(); mInputQueueCallback.onInputQueueCreated(mInputQueue); } mInputEventReceiver = new WindowInputEventReceiver(mInputChannel, Looper.myLooper()); } mAddedTouchMode = (res & WindowManagerGlobal.ADD_FLAG_IN_TOUCH_MODE) != 0; // 根据res设置touch mode。 mAppVisible = (res & WindowManagerGlobal.ADD_FLAG_APP_VISIBLE) != 0; // 根据res设置visible属性。 ...... } }} 3. performTraversals()函数
requestLayout()函数的功能就是触发UI绘制。结合时序图。调用3至调用15，是消息发送传递及Callback设置过程。最后会由TraversalRunnable的run函数中调用ViewRootImpl.doTraversal()函数去调用performTraversals()开始进行UI绘制。
performTraversals()函数很复杂，我们先专注于时序图展示的内容。
performTraversals()@ViewRootImpl.java
private void performTraversals() { ...... final View host = mView; if (mFirst || windowShouldResize || insetsChanged || viewVisibilityChanged || params != null) { ...... try {
...... relayoutResult = relayoutWindow(params, viewVisibility, insetsPending); } ...... if (!mStopped) { boolean focusChangedDueToTouchMode = ensureTouchModeLocally( (relayoutResult&WindowManagerGlobal.RELAYOUT_RES_IN_TOUCH_MODE) != 0); if (focusChangedDueToTouchMode || mWidth != host.getMeasuredWidth() || mHeight != host.getMeasuredHeight() || contentInsetsChanged) { int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width); //获取测量模式。lp.width和lp.height在创建ViewGroup实例时等于MATCH_PARENT int childHeightMeasureSpec = getRootMeasureSpec(mHeight, lp.height); // Ask host how big it wants to be performMeasure(childWidthMeasureSpec, childHeightMeasureSpec); // 根据测量模式进行测量。 ...... layoutRequested = true; } } } final boolean didLayout = layoutRequested && !mStopped; boolean triggerGlobalLayoutListener = didLayout || attachInfo.mRecomputeGlobalAttributes; if (didLayout) { performLayout(lp, desiredWindowWidth, desiredWindowHeight); // By this point all views have been sized and positioned ...... } ...... boolean cancelDraw = attachInfo.mTreeObserver.dispatchOnPreDraw() || viewVisibility != View.VISIBLE; if (!cancelDraw && !newSurface) { if (!skipDraw || mReportNextDraw) { ...... performDraw(); } } ......} 3.1 relayoutWindow()函数
relayoutWindow()@ViewRootImpl.java
private int relayoutWindow(WindowManager.LayoutParams params, int viewVisibility, boolean insetsPending) throws RemoteException { ...... // 调用WMS的relayout函数。通过该函数修改一个window的参数，提供新的参数，它将返回屏幕上window的新的一帧(忽略位置)，和该window的surface。如果window当前是隐藏的，那么surface是无效的，否则就可以使用它来画window的内容。 int relayoutResult = mWindowSession.relayout( mWindow, // 需要修改参数的window mSeq, // 排序的序列号 params, // 要应用到window的新参数 (int) (mView.getMeasuredWidth() * appScale + 0.5f), //window想要的宽 (int) (mView.getMeasuredHeight() * appScale + 0.5f), //window想要的高 viewVisibility, // window的root view的可见性 insetsPending ? WindowManagerGlobal.RELAYOUT_INSETS_PENDING : 0, mWinFrame, // outFrame, 放着屏幕上新position/size的Rect mPendingOverscanInsets, mPendingContentInsets, mPendingVisibleInsets, mPendingConfiguration, mSurface // 存放新显示surface的对象。 ); ...... return relayoutResult;} 3.2 performMeasure()
在执行performMesure()时，需要传入测量模式，这里简单介绍下MeasureSpec。MeasureSpec封装了从父view传 给子view的布局要求，每个MeasureSpec代表对宽度或高度的要求。每个MeasureSpec值由大小和模式组成，高16位表示mode，低 16位表示size，即父view的size。
MeasureSpec有三种可能的模式： UNSPECIFIED： 父view没有强加任何限制给子view，子view可以有它想要的size。 EXACTLY：父view决定子view的确切size，子view将被指定这些边界，而忽视它想要的size. AT_MOST: 父view对子view没有限制，子view可以获得它自己指定的size。对于DecorView来说，mode一般为EXACTLY，size为屏幕的宽高。
看下父view中是如何得到MeasureSpec的：
getRootMeasureSpec()@ViewRootImpl.java
// 根据layout params计算window中root view的MeasureSpec.// windowSize: window可用的宽或高// window的宽或高的layout params. 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.EXACTLY: 父控件决定子控件的确切大小，子控件与父控件一样大，子控件要求的大小被忽视。 measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY); break; case ViewGroup.LayoutParams.WRAP_CONTENT: // Window can resize. Set max size for root view. // MeasureSpec.AT_MOST：子控件的大小可以为它指定的大小。 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; } performMeasure()@ViewRootImpl.java
private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) { Trace.traceBegin(Trace.TRACE_TAG_VIEW, "measure"); try { mView.measure(childWidthMeasureSpec, childHeightMeasureSpec); // 调用mView的measure。 } finally { Trace.traceEnd(Trace.TRACE_TAG_VIEW); }} measure()@View.java
// 这个函数计算出view的大小，然后设置实际的宽和高。每个view的实际宽高都是由父view和自身共同决定的。// 实际测量工作在onMeasure()函数中执行，子类也能重载onMesure()函数实现对自身的测量。public final void measure(int widthMeasureSpec, int heightMeasureSpec) { // final方法，子类不可以重写 ......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, 测量自己，这个调用需要重新设置measured dimension flag。 onMeasure(widthMeasureSpec, heightMeasureSpec); 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) { // 子类重写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} onMeasure()@View.java
// 提供view和它的内容的测量宽高，应该由子类重写这个函数来提供准确有效的测量值。这个函数的两个参数都是由父view传过来的，代表父view的规格。// 子类重写这个函数的时候必须调用setMeasuredDimension()保存这个view的测量宽和高，确保测量的宽和高至少是view的最小宽高。protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) { setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec), getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));} setMeasuredDimension()@View.java
// 设置测量结果及flags. 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; } // 赋值给mMeasuredWidth，getMeasuredWidth()函数返回的就是这个值。 // measure的目的就是对view树中的每个view的mMeasuredWidth和mMesuredHeight赋值。这两个值被赋值，该view的测量工作才算结束。 mMeasuredWidth = measuredWidth; mMeasuredHeight = measuredHeight; // 设置flag, 这就是重写onMeasure方法时如果不调用setMeasuredDimension方法时, measure会抛出异常的原因。 mPrivateFlags |= PFLAG_MEASURED_DIMENSION_SET;} View，ViewGroup，FrameLayout，DecorView的关系如下：
DecorView.onMesure()@PhoneWindow.java
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) { final DisplayMetrics metrics = getContext().getResources().getDisplayMetrics(); final boolean isPortrait = metrics.widthPixels < metrics.heightPixels; final int widthMode = getMode(widthMeasureSpec); final int heightMode = getMode(heightMeasureSpec); boolean fixedWidth = false; if (widthMode == AT_MOST) { final TypedValue tvw = isPortrait ? mFixedWidthMinor : mFixedWidthMajor; if (tvw != null && tvw.type != TypedValue.TYPE_NULL) { final int w; if (tvw.type == TypedValue.TYPE_DIMENSION) { w = (int) tvw.getDimension(metrics); } else if (tvw.type == TypedValue.TYPE_FRACTION) { w = (int) tvw.getFraction(metrics.widthPixels, metrics.widthPixels); } else { w = 0; } if (w > 0) { final int widthSize = MeasureSpec.getSize(widthMeasureSpec); widthMeasureSpec = MeasureSpec.makeMeasureSpec(Math.min(w, widthSize), EXACTLY); fixedWidth = true; } } } if (heightMode == AT_MOST) { final TypedValue tvh = isPortrait ? mFixedHeightMajor : mFixedHeightMinor; if (tvh != null && tvh.type != TypedValue.TYPE_NULL) { final int h; if (tvh.type == TypedValue.TYPE_DIMENSION) { h = (int) tvh.getDimension(metrics); } else if (tvh.type == TypedValue.TYPE_FRACTION) { h = (int) tvh.getFraction(metrics.heightPixels, metrics.heightPixels); } else { h = 0; } if (h > 0) { final int heightSize = MeasureSpec.getSize(heightMeasureSpec); heightMeasureSpec = MeasureSpec.makeMeasureSpec(Math.min(h, heightSize), EXACTLY); } } } super.onMeasure(widthMeasureSpec, heightMeasureSpec); // 调用FrameLayout.onMeasure(). int width = getMeasuredWidth(); boolean measure = false; widthMeasureSpec = MeasureSpec.makeMeasureSpec(width, EXACTLY); if (!fixedWidth && widthMode == AT_MOST) { final TypedValue tv = isPortrait ? mMinWidthMinor : mMinWidthMajor; if (tv.type != TypedValue.TYPE_NULL) { final int min; if (tv.type == TypedValue.TYPE_DIMENSION) { min = (int)tv.getDimension(metrics); } else if (tv.type == TypedValue.TYPE_FRACTION) { min = (int)tv.getFraction(metrics.widthPixels, metrics.widthPixels); } else { min = 0; } if (width < min) { widthMeasureSpec = MeasureSpec.makeMeasureSpec(min, EXACTLY); measure = true; } } } if (measure) { super.onMeasure(widthMeasureSpec, heightMeasureSpec); }} onMesure()@FrameLayout.java
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) { int count = getChildCount(); // 获得子view的数目 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++) { // 测量每个子View final View child = getChildAt(i); if (mMeasureAllChildren || child.getVisibility() != GONE) { 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()); } // 设置自己的size, 即ViewGroup的size. 可以看到，它的size由父view及子view共同决定。 setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState), resolveSizeAndState(maxHeight, heightMeasureSpec, childState << MEASURED_HEIGHT_STATE_SHIFT)); count = mMatchParentChildren.size(); if (count > 1) { // 对于match parent的child进行测量。 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); } }} measureChildWithMargins()@ViewGroup.java
// 要求view的一个child测量自己，同时考虑MeasureSpec要求和padding和margins.protected void measureChildWithMargins(View child, int parentWidthMeasureSpec, int widthUsed, int parentHeightMeasureSpec, int heightUsed) { // 获取子view的LayoutParams final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams(); // 调整MeasureSpec，通过传进来的父view的MeasureSpec及子view自身的LayoutParams，共同决定子view的MeasureSpec. 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); // 调用子view的measure()，又重新开始上面measure的流程。子view的measure()中会回调子view的onMeasure()函数。 child.measure(childWidthMeasureSpec, childHeightMeasureSpec); } getChildMeasureSpec()@ViewGroup.java
public static int getChildMeasureSpec(int spec, int padding, int childDimension) { //获取parent view的mode和size int specMode = MeasureSpec.getMode(spec); int specSize = MeasureSpec.getSize(spec); //parent size与padding的差值(即parent中除去padding后剩余的大小)，若小于0则直接返回0. int size = Math.max(0, specSize - padding); int resultSize = 0; int resultMode = 0; switch (specMode) { //根据parent mode进行处理 // Parent has imposed an exact size on us case MeasureSpec.EXACTLY: if (childDimension >= 0) { // 若child的layout_width或者layout_height在xml或者代码中指定了大于等于0的值， // 则设置child的size值为指定的值，mode为EXACTLY. resultSize = childDimension; resultMode = MeasureSpec.EXACTLY; } else if (childDimension == LayoutParams.MATCH_PARENT) { // Child wants to be our size. So be it. // 若child的layout_width或者layout_height在xml或者代码中指定为MATCH_PARENT， // 则设置child的size值为size，mode为EXACTLY. 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. // 若child的layout_width或者layout_height在xml或者代码中指定为WRAP_CONTENT， // 则设置child的size值为指定的值，mode为AT_MOST. resultSize = size; resultMode = MeasureSpec.AT_MOST; } break; // Parent has imposed a maximum size on us // 下面两个分支类似 case MeasureSpec.AT_MOST: ......break; // Parent asked to see how big we want to be case MeasureSpec.UNSPECIFIED: ......break; } //使用mode和size生成MeasureSpec返回。 return MeasureSpec.makeMeasureSpec(resultSize, resultMode);} 可以看到，getChildMeasureSpec()是通过其父view提供的MeasureSpec参数得到specMode和specSize，然后根据计算出来的specMode以及子view的childDimension（layout_width或layout_height）来计算自身的measureSpec，如果其本身包含子视图，则计算出来的measureSpec将作为调用其子视图measure函数的参数，同时也作为自身调用setMeasuredDimension()的参数，如果其不包含子视图则默认情况下最终会调用onMeasure的默认实现，并最终调用到setMeasuredDimension()。
performMeasure()小结：
(1) mView.measure() , 即调用View.measure()方法。
(2) View.measure()内部调用onMeasure()方法，因为这里的mView是DecorView，调用的是DecorView.onMeasure()， 这个函数内部调用super.onMeasure()。因为DecorView继承于FrameLayout，所以super.onMeasure()即调用FrameLayout的onMeasure()方法。
(3) FrameLayout.onMeasure()对每个子view进行遍历测量，对每个子view调用measureChildWithMargins()方法测量子view。
(4) measureChildWithMargins()内部调用getChildMeasureSpec()计算出传给子view的MeasureSpec，并调用子view的measure()方法。
(5) FrameLayout.onMeasure()中根据父view及(3)测量出来的子view的MeasureSpec信息调用setMeasuredDimension()设置自身的size。
(6) 使用View的getMeasuredWidth()和getMeasuredHeight()方法来获取view测量的宽高，必须保证这两个方法在onMeasure流程之后被调用才能返回有效值。
(7) ViewGroup的子类必须要求LayoutParams继承于MarginLayoutParams，否则无法使用layout_margin参数。
3.3 performLayout()
performLayout()@ViewRootImpl.java
private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth, int desiredWindowHeight) { mLayoutRequested = false; mScrollMayChange = true; mInLayout = true; final View host = mView; Trace.traceBegin(Trace.TRACE_TAG_VIEW, "layout"); try { // 把performMeasure测量的结果传入layout()函数 host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight()); mInLayout = false; ...... } finally { Trace.traceEnd(Trace.TRACE_TAG_VIEW); } mInLayout = false;} 这个函数内调用mView.layout()方法，而ViewGroup中重写了layout()方法，并调用了super.layout(), 即View.layout()。
layout()@View.java// 给view及它的子孙的size及position赋值。// 这是布局机制的第二阶段(第一阶段是measuring)。在这个阶段，每个父view都会调用其所有子view的layout()来放置它们。// 子类中不应该重载这个方法，若子类中有children, 则该子类应该重载onLayout()函数，在这个函数中调用每个child的layout.public void layout(int l, int t, int r, int b) { ......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，来判断是否需要重新layout。判断的方式就是通过将l,t,r,b参数传给setFrame()赋值给 // mLeft，mTop, mRight，mBottom。 // isLayoutModeOptical(): 如果mParent是一个使用可见范围进行编排的ViewGroup, 返回true. boolean changed = isLayoutModeOptical(mParent) ? setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b); if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) { // 调用onLayout方法 onLayout(changed, l, t, r, b); mPrivateFlags &= ~PFLAG_LAYOUT_REQUIRED; ...... } mPrivateFlags &= ~PFLAG_FORCE_LAYOUT; mPrivateFlags3 |= PFLAG3_IS_LAID_OUT;} 我们先来看View的onLayout()方法，
onLayout()@View.java
protected void onLayout(boolean changed, int left, int top, int right, int bottom) { } 这是一个空方法，这是因为Layout需要ViewGroup来控制进行。
再来看ViewGroup的onLayout()方法：
protected abstract void onLayout(boolean changed, int l, int t, int r, int b);
onLayout在ViewGroup中是个抽象方法，所以ViewGroup的子类都需要实现该方法。FrameLayout继承于ViewGroup，我们来看下FrameLayout.onLayout()的实现。
onLayout()@FrameLayout.java
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) { // 获取子view的LayoutParams final LayoutParams lp = (LayoutParams) child.getLayoutParams(); // measure过程计算处理的子view的宽高 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; // 根据absoluteGravity计算childLeft 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; } // 根据verticalGravity计算childTop 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child的layout方法设置child的位置。 child.layout(childLeft, childTop, childLeft + width, childTop + height); } }} FrameLayout.onLayout()函数也展示了一般的自定义ViewGroup重载onLayout函数的实现方式。onLayout()函数的目的就是确定子view们在父view中的具体位置，通常做法是用一个for循环调用每个子view的layout(l, t, r, b)函数，传入不同的l，t，r，b值确定子view的位置。
performLayout()小结：
(1) 调用host.layout(), 即View.layout()。
(2) View.layout()中调用setFrame()或者setOpticalFrame()赋值给View的mLeft，mTop，mRight，mBottom变量并判断位置是否变化，若变化则调用onLayout()。
(3) 因为host是mView就是DecorView，也就是FrameLayout的子类，所以实际上是FrameLayout.onLayout()被调用。
(4) 在FrameLayout.onLayout()函数中遍历每个子view, 并调用每个子view的layout()设置子view的位置。
(5) View.layout()函数可以被重载，ViewGroup.layout不能被重载，ViewGroup的onLayout为abstract的，子类必须重载实现自己的位置逻辑。
(6) measure()过程的结果是得到每个view的measuredWidth和measuredHeight，layout()过程的结果是得到每个view的mLeft，mTop，mRight，mBottom.这些值都是相对于父view而言的。
(7) layout_XXX的布局属性都是针对包含子view的ViewGroup的，当对一个没有父容器的view设置相关layout_XXX属性是没有意义的。
(8) 使用View的getWidth()和getHeight()方法来获取view测量的宽高时，必须保证这两个方法在onLayout流程之后被调用才能返回有效值。
3.4 performDraw()
performDraw()@ViewRootImpl.java
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 draw(fullRedrawNeeded); } finally { mIsDrawing = false; Trace.traceEnd(Trace.TRACE_TAG_VIEW); } ......} draw()@ViewRootImpl.java
private void draw(boolean fullRedrawNeeded) { Surface surface = mSurface; if (!surface.isValid()) { return; } ......if (!dirty.isEmpty() || mIsAnimating) { if (attachInfo.mHardwareRenderer != null && attachInfo.mHardwareRenderer.isEnabled()) { ...... attachInfo.mHardwareRenderer.draw(mView, attachInfo, this, animating ? null : mCurrentDirty); // 使用硬件绘制 } else { ......// 使用软件进行绘制。 if (!drawSoftware(surface, attachInfo, yoff, scalingRequired, dirty)) { return; } } } if (animating) { mFullRedrawNeeded = true; scheduleTraversals(); // 如果在动画中，重新发送绘制消息 }} drawSoftware()@ViewRootImpl.java
private boolean drawSoftware(Surface surface, AttachInfo attachInfo, int yoff, boolean scalingRequired, Rect dirty) { // 使用软件渲染绘制。 Canvas canvas; try { int left = dirty.left; int top = dirty.top; int right = dirty.right; int bottom = dirty.bottom; canvas = mSurface.lockCanvas(dirty); //从mSurface中lock一块Canvas. The dirty rectangle can be modified by Surface.lockCanvas() //no inspection Constant Conditions 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) { // 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 { // 如果该位图格式包含alpha通道，需要在绘制之前把它清除。这样child就可以在透明背景下重新合成绘制。 // 如果使用了偏移，需要清理偏移不会出现的区域，以免垃圾遗留在空白区域上。 if (!canvas.isOpaque() || yoff != 0) { canvas.drawColor(0, PorterDuff.Mode.CLEAR); } dirty.setEmpty(); mIsAnimating = false; attachInfo.mDrawingTime = SystemClock.uptimeMillis(); mView.mPrivateFlags |= View.PFLAG_DRAWN; try { canvas.translate(0, -yoff); if (mTranslator != null) { mTranslator.translateCanvas(canvas); } canvas.setScreenDensity(scalingRequired ? mNoncompatDensity : 0); attachInfo.mSetIgnoreDirtyState = false; mView.draw(canvas); // 调用View.draw()由view自己在canvas上进行绘制。 drawAccessibilityFocusedDrawableIfNeeded(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); // unlock canvas } catch (IllegalArgumentException e) { mLayoutRequested = true; // ask wm for a new surface next time. //noinspection ReturnInsideFinallyBlock return false; } } return true;} draw()@View.java
// 在给定的canvas上手动渲染view及所有子view.这个函数被调用之前，该view必须已经完整走了一遍layout.// 当实现一个view时，应该实现onDraw()而不是重载这个函数。如果要重载这个函数，调用super.draw()。public void draw(Canvas canvas) { if (mClipBounds != null) { canvas.clipRect(mClipBounds); } 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; /* * 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 needed int saveCount; if (!dirtyOpaque) { final Drawable background = mBackground; // 获取xml中通过android:background属性或者代码中setBackgroundColor()，setBackgroundResource()等进行赋值的背景Drawable. if (background != null) { final int scrollX = mScrollX; final int scrollY = mScrollY; // 根据layout过程确定的view的位置设置背景绘制区域。 if (mBackgroundSizeChanged) { background.setBounds(0, 0, mRight - mLeft, mBottom - mTop); mBackgroundSizeChanged = false; } if ((scrollX | scrollY) == 0) { background.draw(canvas); // 调用Drawble.draw()完成绘制工作。 } else { canvas.translate(scrollX, scrollY); background.draw(canvas); canvas.translate(-scrollX, -scrollY); } } } // 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) { // 没有边缘效果 // Step 3, draw the content if (!dirtyOpaque) onDraw(canvas); // 对view的内容进行绘制 // Step 4, draw the children dispatchDraw(canvas); // 对当前所有子view进行绘制，如果当前的view没有子view，就不需要绘制。// Step 6, draw decorations (scrollbars). scroolbars第一次唤醒时进行绘制。 // 注意，每一个view都是有水平和垂直滚动条的，只是大多数情况不显示而已。 onDrawScrollBars(canvas); if (mOverlay != null && !mOverlay.isEmpty()) { mOverlay.getOverlayView().dispatchDraw(canvas); } // we're done... return; } ...... } View.onDraw()方法是一个空方法，而ViewGroup也没有重写该方法，需要子类去实现具体的onDraw()逻辑，这是因为每个view的内容是各不相同的。
View.dispatchDraw()是一个空函数，有子view时才进行绘制，看下在ViewGroup中的实现。
dispatchDraw()@ViewGroup.java
protected void dispatchDraw(Canvas canvas) { final int count = mChildrenCount; final View[] children = mChildren; int flags = mGroupFlags; ...... if ((flags & FLAG_USE_CHILD_DRAWING_ORDER) == 0) { for (int i = 0; i < count; i++) { final View child = children[i]; if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE || child.getAnimation() != null) { more |= drawChild(canvas, child, drawingTime); } } } else { for (int i = 0; i < count; i++) { final View child = children[getChildDrawingOrder(count, i)]; if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE || child.getAnimation() != null) { more |= drawChild(canvas, child, drawingTime); } } } // Draw any disappearing views that have animations if (mDisappearingChildren != null) { final ArrayList 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); } } ......// mGroupFlags might have been updated by drawChild() flags = mGroupFlags; if ((flags & FLAG_INVALIDATE_REQUIRED) == FLAG_INVALIDATE_REQUIRED) { invalidate(true); } ...... } drawChild()@ViewGroup.java
protected boolean drawChild(Canvas canvas, View child, long drawingTime) { return child.draw(canvas, this, drawingTime); //调用View.draw()，child的绘制重新走上面流程。 } performDraw()小结：
(1) View默认不会绘制内容的，需要子类自己去实现实际的绘制。
(2) View的绘制是利用onDraw()函数传入的Canvas进行的。
(3) 看源码中有很多关于动画的逻辑，需要区分View动画和ViewGroup布局动画。View动画是指View自身的动画，可以通过setAnimation()进行设置。ViewGroup布局动画是针对ViewGroup显示内部子view时设置的动画，可以在xml文件中对ViewGroup设置layoutAnimation属性。
(4) 子view在获取canvas的剪切区时不用关注padding，padding会被自动处理掉。
(5) 默认情况下子ViewGroup.drawChild()的顺序与子view被添加的顺序一致，可以通过ViewGroupDrawingOrder()函数提供不同的顺序。
3.5 View.invalidate()和View.postInvalidate()介绍
上一篇文章中，ViewGroup.addView()调用了invalidate()函数，现在来介绍下这个函数的内容。
3.5.1 View.invalidate()的作用
invalidate()的作用是废止整个view，如果这个view是可见的，那么在将来某个时间点onDraw()会被调用。这个函数必须在UI线程中调用，在非UI线程可以调用postInvalidate()。
这个函数有三种具体实现：
(1) void invalidate(boolean invalidateCache) : 完整的废止会将drawing cache一起废止, 当view的内容或者dimensions不变的时候，这个函数可以设置不废止cache。
(2) void invalidate(Rect dirty) : 标记由dirty定义的区域为需要绘制的区域。
(3) void invalidate(int l, int t, int r, int b) : 标记由rect(l,t,r,b)定义的区域，dirty rect的坐标是相对于view的。
3.5.2 常见会引起invalidate()调用的原因
(1) 直接调用invalidate()方法请求重新draw，只会绘制调用者本身。
(2) 调用setSelection()方法请求重新draw，只会绘制调用者本身。
(3) 调用setVisibility()方法, 当view的状态从INVISIBLE变成VISIBLE时会间接调用invalidate方法，从而绘制该view。当view的状态从 INVISIBLEVISIBLE变为GONE状态时会间接调用requestLayout和invalidate()方法，同时由于view树大小发生 了变化，所以会请求measure过程以及draw过程，同样只绘制需要重新绘制的view。
(4) 调用setEnabled()方法请求重新draw，不会重新绘制任何view包括该调用者本身。
(5) 调用requestFocus()方法请求view树的draw过程，只绘制需要重绘的view。
3.5.3 invalidate()源码分析
View.invalidate()会调用父view的invalidateChild()函数，实际上调用的是ViewGroup.invalidateChild()函数。
invalidateChild()@ViewGroup.java
public final void invalidateChild(View child, final Rect dirty) { ViewParent parent = this; ...... do { ...... parent = parent.invalidateChildInParent(location, dirty); //层层循环向上调用，知道ViewRootImpl返回null. ...... } while (parent != null); ......} invalidateChild()实际上是一个从当前view向上父view回溯的过程。每一层的父view都将自己的显示区域与传入的dirty Rect做交集。
invalidateChild()函数调用最后会传到ViewRootImpl.invalidateChindInParent()。
invalidateChindInParent()@ViewRootImpl
public ViewParent invalidateChildInParent(int[] location, Rect dirty) { checkThread(); ...... final Rect localDirty = mDirty; ...... localDirty.union(dirty.left, dirty.top, dirty.right, dirty.bottom); // 融合dirty rect与当前的rect. ...... scheduleTraversals(); // 回到上文第3点， 会异步触发执行performTraversals()。 ...... return null;} ViewRootImpl.invalidateChindInParent()直接返回null，也就是ViewGroup.invalidateChild()中do-while循环结束的条件。
调用invalidate()的实质就是从当前view层层上传到父级，直到ViewRootImpl触发scheduleTraversals()函数，整个view树重新走一遍上面的绘制流程。
3.5.4 postInvalidate()函数
View.invalidate()是在UI线程里运行的，在其它线程中只能调用View.postInvalidate()，该函数最终会通过ViewRootImpl类的handler发送MSG_INVALIDATE消息，最终又在UI线程中执行View.invalidate()。
3.6 requestLayout()函数
requestLayout()@View.java
public void requestLayout() { ...... if (mParent != null && !mParent.isLayoutRequested()) { mParent.requestLayout(); } ......} 当view的requestLayout()函数被调用时，实际上是层层向父级传递，知道调用ViewRootImpl.requestLayout()为止。
requestLayout()@ViewRootImpl.java
public void requestLayout() { if (!mHandlingLayoutInLayoutRequest) { checkThread(); mLayoutRequested = true; scheduleTraversals(); }} scheduleTraversals()即时序图中的第3个调用，后面的流程与上面描述的类似了。需要注意的是requestLayout()函数会触发measure和layout，但不会draw，不会重新绘制包括调用者本身的任何view。
4. addToDisplay()@Session.java
通过addToDisplay()将window对象添加到WMS中，WMS将管理这个window。
public int addToDisplay(IWindow window, int seq, WindowManager.LayoutParams attrs, int viewVisibility, int displayId, Rect outContentInsets, InputChannel outInputChannel) { // 调用WindowManagerService的addWindow()函数 return mService.addWindow(this, window, seq, attrs, viewVisibility, displayId, outContentInsets, outInputChannel);} addWindow()@WindowManagerService.java
public int addWindow(Session session, IWindow client, int seq, WindowManager.LayoutParams attrs, int viewVisibility, int displayId, Rect outContentInsets, InputChannel outInputChannel) { ...... WindowState win = null; ...... // 创建一个WindowState， WMS中维护的是WindowState对象. win = new WindowState(this, session, client, token, attachedWindow, appOp[0], seq, attrs, viewVisibility, displayContent); ...... win.attach(); // 调用WindowState.attach()函数。 mWindowMap.put(client.asBinder(), win); ...... return res;} attach()@WindowState.java
void attach() { mSession.windowAddedLocked();} windowAddedLocked()@Session.java
void windowAddedLocked() { if (mSurfaceSession == null) { mSurfaceSession = new SurfaceSession(); //创建SurfaceSeesion对象 mService.mSessions.add(this); } mNumWindow++;}
（时序图中调用26以后就是在WindowManagerService中与底层交互，控制底层显示实际内容了，在本文中暂不考虑)