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://blog.csdn.net/jerehedu/article/details/47021541）。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：