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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作者:杰瑞教育出处:本文版权归烟台杰瑞教育科技有限公司和CSDN共有,欢迎转载,但未经作者同意必须保留此段声明,且在文章页面明显位置给出原文连接,否则保留追究法律责任的权利。
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