从前文《 源码解析：dialog, popupwindow, 和activity 的第一个view是怎么来的？》中知道了activity第一个view或者说根view或者说mDecorView 其实就是一个FrameLayout，以及是在系统handleResume的时候加入到系统windowManager中的，并由framework中的ViewRootImpl 接管，通过ViewRootImpl.setView() 开始整个显示过程的。这次着重梳理一下view的显示过程（onAttach, onMeasure, onLayout, onDraw ）在源码中的过程。

从ViewRootImpl.setview 开始。

    public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView) {
        synchronized (this) {
            if (mView == null) {
                mView = view;

                requestLayout();
                if ((mWindowAttributes.inputFeatures
                        & WindowManager.LayoutParams.INPUT_FEATURE_NO_INPUT_CHANNEL) == 0) {
                    mInputChannel = new InputChannel();
                }
                try {
                    mOrigWindowType = mWindowAttributes.type;
                    res = sWindowSession.add(mWindow, mSeq, mWindowAttributes,
                            getHostVisibility(), mAttachInfo.mContentInsets,
                            mInputChannel);
                } catch (RemoteException e) {
                    mAdded = false;
                    mView = null;
                    mAttachInfo.mRootView = null;
                    mInputChannel = null;
                    mFallbackEventHandler.setView(null);
                    unscheduleTraversals();
                    throw new RuntimeException("Adding window failed", e);
                } finally {
                    if (restore) {
                        attrs.restore();
                    }
                }

    public void requestLayout() {
        checkThread();
        mLayoutRequested = true;
        scheduleTraversals();
    }

    public void scheduleTraversals() {
        if (!mTraversalScheduled) {
            mTraversalScheduled = true;

            //noinspection ConstantConditions
            if (ViewDebug.DEBUG_LATENCY && mLastTraversalFinishedTimeNanos != 0) {
                final long now = System.nanoTime();
                Log.d(TAG, "Latency: Scheduled traversal, it has been "
                        + ((now - mLastTraversalFinishedTimeNanos) * 0.000001f)
                        + "ms since the last traversal finished.");
            }

            sendEmptyMessage(DO_TRAVERSAL);
        }
    }

    @Override
    public void handleMessage(Message msg) {
        switch (msg.what) {
        case DO_TRAVERSAL:

            performTraversals();
	}

这个函数比较长，不适合把全部函数代码都 贴上来。就分段叙述。

1. 函数刚开始的部分，初始化了一些后面会用到的变量和标志位。

        final View host = mView;
        WindowManager.LayoutParams lp = mWindowAttributes;
        final View.AttachInfo attachInfo = mAttachInfo;
        CompatibilityInfo compatibilityInfo = mCompatibilityInfo.get();
        Rect frame = mWinFrame;
        mTraversalScheduled = false;
        mWillDrawSoon = true;
        boolean windowSizeMayChange = false;
        boolean fullRedrawNeeded = mFullRedrawNeeded;
        boolean newSurface = false;
        boolean surfaceChanged = false;

2. 接下来，是一个重要的判断， 如果是初次执行，则调用host.dispatchAttachedToWindow(attachInfo, 0);

        if (mFirst) {
            // 略去一大堆赋值
            mLastConfiguration.setTo(host.getResources().getConfiguration());
            host.dispatchAttachedToWindow(attachInfo, 0);
            //Log.i(TAG, "Screen on initialized: " + attachInfo.mKeepScreenOn);

            host.fitSystemWindows(mAttachInfo.mContentInsets);

        } else {
            desiredWindowWidth = frame.width();
            desiredWindowHeight = frame.height();
            if (desiredWindowWidth != mWidth || desiredWindowHeight != mHeight) {
                if (DEBUG_ORIENTATION) Log.v(TAG,
                        "View " + host + " resized to: " + frame);
                fullRedrawNeeded = true;
                mLayoutRequested = true;
                windowSizeMayChange = true;
            }
        }

2.1. onAttachedToWindow();

2.2. listener.onViewAttachedToWindow(this);

2.3 onWindowVisibilityChanged(vis);