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:
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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