在上篇文章中，我们通过跟踪源码，我们了解了Activity、Window、DecorView以及View之间的关系（查看文章：）。那么整个Activity的界面到底是如何绘制出来的呢？既然DecorView作为Activity的顶层界面视图，那么整个界面的绘制工作应该从它开始，下面我们继续跟踪源码，看看是不是这样的。
Activity在启动过程中会调用主线程ActivityThread中的方法performLaunchActivity和handleResumeActivity。在方法handleResumeActivity中会将创建的DecorView和WindowManagerImpl对象关联起来，关键源码部分如下：
public final class ActivityThread { …… final void handleResumeActivity(IBinder token, boolean clearHide, boolean isForward, boolean reallyResume) { …… if (r.window == null && !a.mFinished && willBeVisible) { r.window = r.activity.getWindow(); View decor = r.window.getDecorView(); decor.setVisibility(View.INVISIBLE); ViewManager wm = a.getWindowManager(); WindowManager.LayoutParams l = r.window.getAttributes(); a.mDecor = decor; l.type = WindowManager.LayoutParams.TYPE_BASE_APPLICATION; l.softInputMode |= forwardBit; if (a.mVisibleFromClient) { a.mWindowAdded = true; wm.addView(decor, l); } } …… }} WindowManagerImpl关键代码：
public final class WindowManagerImpl implements WindowManager {private final WindowManagerGlobal mGlobal = WindowManagerGlobal.getInstance();……@Overridepublic void addView(View view, ViewGroup.LayoutParams params) { mGlobal.addView(view, params, mDisplay, mParentWindow);}……} WindowManagerGlobal关键代码：
public final class WindowManagerGlobal { …… public void addView(View view, ViewGroup.LayoutParams params, Display display, Window parentWindow) { …… ViewRootImpl root; View panelParentView = null;…… root.setView(view, wparams, panelParentView); …… }} 根据源码调用关系，可得下图：
从图中，我们可以看出在ActivityThread中生成的DecorView经过WindowManagerImpl、WindowManagerGlobal，最终调用了ViewRootImpl中的setView方法，将DecorView设置赋值给了ViewRootImpl中的mView属性。通过追踪ViewRootImpl我们发现最终调用了performTraversals方法，该方法关键代码如下：
private void performTraversals() { // cache mView since it is used so much below... final View host = mView; …… performMeasure(childWidthMeasureSpec, childHeightMeasureSpec); …… performLayout(lp, desiredWindowWidth, desiredWindowHeight); …… performDraw(); ……} 从上述源码中我们可以看出，performTraversals实际上依次调用了三个关键的方法，分别是performMeasure，performLayout、performDraw。
1、方法performMeasure，内部实际上调用了mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);不要忘了此处的mView正是我们传递进来的DecorView，该方法用于测量View的大小。关键源码如下：
private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) { Trace.traceBegin(Trace.TRACE_TAG_VIEW, "measure"); try { mView.measure(childWidthMeasureSpec, childHeightMeasureSpec); } finally { Trace.traceEnd(Trace.TRACE_TAG_VIEW); } } 2、方法performLayout，内部实际上调用了host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());该方法用于确定视图的位置。关键源码如下：
private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth, int desiredWindowHeight) { mLayoutRequested = false; mScrollMayChange = true; mInLayout = true; final View host = mView; …… try { host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight()); mInLayout = false; int numViewsRequestingLayout = mLayoutRequesters.size(); if (numViewsRequestingLayout > 0) { …… if (validLayoutRequesters != null) { // Set this flag to indicate that any further requests are happening during …… host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight()); …… } } } } finally { Trace.traceEnd(Trace.TRACE_TAG_VIEW); } mInLayout = false; } 3、方法performDraw，用于绘制视图，追踪源码发现，最终调用了mView.draw(canvas)方法，用于绘制。
经过上述过程基本上可以确定View的绘制流程，流程图具体如下：
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