说到Android系统View的绘制流程，大家一定知道是分为测量（Measure）、布局（Layout）和绘制（Draw）三个阶段，这篇文章主要聊一聊在这三个步骤之前的源码执行流程，页面启动后是怎样通过代码执行这三个方法。

之前看过startActivity启动一个Activity最终都会经过ActivityThread类来调用Activity的生命周期。所以我们就从ActivityThread中调用onResume的performResumeActivity方法开始。

1、ActivityThread类的performResumeActivity方法

首先通过performResumeActivity调用Activity的onResume方法，之前在“四大组件”专栏中分析过这个方法。

final ActivityClientRecord r = performResumeActivity(token, finalStateRequest, reason);

继续向下看：

ViewManager wm = a.getWindowManager();

此处省略好多行。。。。

wm.addView(decor，l)，

这里第一个参数就是DecorView的实例，执行顺序在performResumeActivity后面.

补充一下，此处的wm类型是ViewManager，ViewManager是个接口：

/** Interface to let you add and remove child views to an Activity. To get an instance* of this class, call {@link android.content.Context#getSystemService(java.lang.String) Context.getSystemService()}.*/
public interface ViewManager
{/*** Assign the passed LayoutParams to the passed View and add the view to the window.* <p>Throws {@link android.view.WindowManager.BadTokenException} for certain programming* errors, such as adding a second view to a window without removing the first view.* <p>Throws {@link android.view.WindowManager.InvalidDisplayException} if the window is on a* secondary {@link Display} and the specified display can't be found* (see {@link android.app.Presentation}).* @param view The view to be added to this window.* @param params The LayoutParams to assign to view.*/public void addView(View view, ViewGroup.LayoutParams params);public void updateViewLayout(View view, ViewGroup.LayoutParams params);public void removeView(View view);
}

WindowManager接口继承ViewManager，顺着可以找到WindowManager的实现类WindowManagerImpl。

2、来到WindowManagerImpl类

@Override
public void addView(@NonNull View view, @NonNull ViewGroup.LayoutParams params) {applyDefaultToken(params);mGlobal.addView(view, params, mContext.getDisplayNoVerify(), mParentWindow,mContext.getUserId());
}

3、进入WindowManagerGlobal类

addView方法最底下：

root = new ViewRootImpl(view.getContext(), display);view.setLayoutParams(wparams);mViews.add(view);
mRoots.add(root);
mParams.add(wparams);// do this last because it fires off messages to start doing things
try {root.setView(view, wparams, panelParentView, userId);
} catch (RuntimeException e) {// BadTokenException or InvalidDisplayException, clean up.if (index >= 0) {removeViewLocked(index, true);}throw e;
}

这里创建ViewRootImpl并且会调用ViewRootImpl的setView方法，这里传入的view就是上面wm.addView(decor,l)时传入的decorView他也会一直透传到ViewRootImpl的setView方法。

4、ViewRootImpl的setView方法

public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView,int userId) {synchronized (this) {if (mView == null) {mView = view;

下面省略好多行。。。。

上面的view就是从ActivityThread传递过来的DecorView实例，因此mView 就是DecorView，add方法下面滑油三个比较重要的点：requestLayout()和mInputEventReceiver以及view.assignParent(this)。

4.1 requestLayout调用scheduleTraversals

@Override
public void requestLayout() {if (!mHandlingLayoutInLayoutRequest) {checkThread();mLayoutRequested = true;scheduleTraversals();}
}@UnsupportedAppUsage
void scheduleTraversals() {if (!mTraversalScheduled) {mTraversalScheduled = true;mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();mChoreographer.postCallback(Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);notifyRendererOfFramePending();pokeDrawLockIfNeeded();}
}

4.2 进入Choreographer类

上面mChoreographer.postCallBack, 这里会调用Choreographer类的postCallBack - postCallBackDelayed - postCallbackDelayedInternal - scheduleFrameLocked

@UnsupportedAppUsage
@TestApi
public void postCallback(int callbackType, Runnable action, Object token) {postCallbackDelayed(callbackType, action, token, 0);
}@UnsupportedAppUsage
@TestApi
public void postCallbackDelayed(int callbackType,Runnable action, Object token, long delayMillis) {if (action == null) {throw new IllegalArgumentException("action must not be null");}if (callbackType < 0 || callbackType > CALLBACK_LAST) {throw new IllegalArgumentException("callbackType is invalid");}postCallbackDelayedInternal(callbackType, action, token, delayMillis);
}private void postCallbackDelayedInternal(int callbackType,Object action, Object token, long delayMillis) {if (DEBUG_FRAMES) {Log.d(TAG, "PostCallback: type=" + callbackType+ ", action=" + action + ", token=" + token+ ", delayMillis=" + delayMillis);}synchronized (mLock) {final long now = SystemClock.uptimeMillis();final long dueTime = now + delayMillis;mCallbackQueues[callbackType].addCallbackLocked(dueTime, action, token);if (dueTime <= now) {scheduleFrameLocked(now);} else {Message msg = mHandler.obtainMessage(MSG_DO_SCHEDULE_CALLBACK, action);msg.arg1 = callbackType;msg.setAsynchronous(true);mHandler.sendMessageAtTime(msg, dueTime);}}
}private void scheduleFrameLocked(long now) {if (!mFrameScheduled) {mFrameScheduled = true;if (USE_VSYNC) {if (DEBUG_FRAMES) {Log.d(TAG, "Scheduling next frame on vsync.");}// If running on the Looper thread, then schedule the vsync immediately,// otherwise post a message to schedule the vsync from the UI thread// as soon as possible.if (isRunningOnLooperThreadLocked()) {scheduleVsyncLocked();} else {Message msg = mHandler.obtainMessage(MSG_DO_SCHEDULE_VSYNC);msg.setAsynchronous(true);mHandler.sendMessageAtFrontOfQueue(msg);}} else {final long nextFrameTime = Math.max(mLastFrameTimeNanos / TimeUtils.NANOS_PER_MS + sFrameDelay, now);if (DEBUG_FRAMES) {Log.d(TAG, "Scheduling next frame in " + (nextFrameTime - now) + " ms.");}Message msg = mHandler.obtainMessage(MSG_DO_FRAME);msg.setAsynchronous(true);mHandler.sendMessageAtTime(msg, nextFrameTime);}}
}

上面scheduleFrameLocked方法通过handler发了MSG_DO_FRAME的msg消息。

private final class FrameHandler extends Handler {public FrameHandler(Looper looper) {super(looper);}@Overridepublic void handleMessage(Message msg) {switch (msg.what) {case MSG_DO_FRAME:doFrame(System.nanoTime(), 0);break;case MSG_DO_SCHEDULE_VSYNC:doScheduleVsync();break;case MSG_DO_SCHEDULE_CALLBACK:doScheduleCallback(msg.arg1);break;}}
}

 handleMessage中doFrame方法调用 doCallbacks，然后调用CallbackRecord

类的run方法 c.run(frameTimeNanos);

5、进入CallbackRecord

private static final class CallbackRecord {public CallbackRecord next;public long dueTime;public Object action; // Runnable or FrameCallbackpublic Object token;@UnsupportedAppUsagepublic void run(long frameTimeNanos) {if (token == FRAME_CALLBACK_TOKEN) {((FrameCallback)action).doFrame(frameTimeNanos);} else {((Runnable)action).run();}}
}

通过((Runnable)action).run()看到是又回到了4.1中        mChoreographer.postCallback(
                Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);可以看到是返回了ViewRootImpl的TraversalRunnable的实例mTraversalRunnable。

6、返回ViewRootImpl 

((Runnable)action).run();  又调回到ViewRootImpl的TraversalRunnable的run方法调：

final class TraversalRunnable implements Runnable {@Overridepublic void run() {doTraversal();}
}
final TraversalRunnable mTraversalRunnable = new TraversalRunnable();

 doTraversal方法调用performTraversals()方法

performTraversals()方法，measure测量、layout布局、draw绘制，都是在这个方法开启的：
  预测量

windowSizeMayChange |= measureHierarchy(host, lp, res,
        desiredWindowWidth, desiredWindowHeight);

布局窗口
 relayoutResult = relayoutWindow(params, viewVisibility, insetsPending);

控件树测量

 performMeasure(childWidthMeasureSpec, childHeightMeasureSpec)

布局控件
 performLayout(lp,mWidth,mHeight)  

绘制控件
 performDraw()

以measure控件树测量为例看看怎样调用到View的measure方法：

private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) {if (mView == null) {return;}Trace.traceBegin(Trace.TRACE_TAG_VIEW, "measure");try {mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);} finally {Trace.traceEnd(Trace.TRACE_TAG_VIEW);}
}

但是DecorView类没有measure(int widthMeasureSpec, int heightMeasureSpec)方法，因此执行顶级父类View的measure方法：

onMeasure(widthMeasureSpec, heightMeasureSpec);

View的measure方法会调用onMeasure，DecorView中有此方法因此来到DecorView：

super.onMeasure(widthMeasureSpec, heightMeasureSpec);

这里super又会调用父类FrameLayout中的onMeasure方法，方法很长只截取部分：

FrameLayout根据它的MeasureSpec调用measureChildWithMargin方法来对每一个子View进行测量：

for (int i = 0; i < count; i++) {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);}}}
}

DecorView委托父类对所有子view完成测量。

7、ViewRootImpl的setView方法

再说剩下的两个重要的点，后面再细致的单独分析：
     res=mWindowSession.addToDisplayAsUser(),与requestLayout同在setView方法，负责将窗口添加到WMS上，主要通过mWindowSession与WMS进行系统通信
     mInputEventReceiver = new WindowInputEventReceiver(mInputChannel,Looper.myLooper()),与requestLayout同在setView方法，View事件输入的接收器，接收来自于系统的事件；
     view.assignParent(this),与requestLayout同在setView方法，assignParent方法在View类实现，将ViewRootImpl自己设置为DecorView的parent，所以ViewTree上的Root就是ViewRootImpl。

才疏学浅，如有错误，欢迎指正，多谢。