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Android GUI之View绘制流程

时间:2015-07-27 12:22:46      阅读:194      评论:0      收藏:0      [点我收藏+]

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  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:

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  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/ 
本文版权归烟台杰瑞教育科技有限公司和博客园共有,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。
 

Android GUI之View绘制流程

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原文地址:http://www.cnblogs.com/jerehedu/p/4679534.html

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