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Android GUI之View绘制流程

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： http://www.cnblogs.com/jerehedu/p/4607599.html#gui ）。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(); …… @Override public 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：

Android GUI之View绘制流程

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;     }