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[操作系统]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();……@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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作者:杰瑞教育
出处:http://www.cnblogs.com/jerehedu/ 
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