ViewRootImpl与ViewGroup，View关系

本篇解决两个问题：

ViewTree中的继承关系如何建立的?
mAttachInfo是如何分发的？

1：ViewTree的继承关系如何构建的？

在ActivityThread中触发了handleResumeActivity():

 public void handleResumeActivity(IBinder token, boolean finalStateRequest, boolean isForward,
            String reason){
            ...
            ViewManager wm = a.getWindowManager();
            ...
            if (a.mVisibleFromClient) {
                if (!a.mWindowAdded) {
                    a.mWindowAdded = true;
                    wm.addView(decor, l);
                } else {
                    // The activity will get a callback for this {@link LayoutParams} change
                    // earlier. However, at that time the decor will not be set (this is set
                    // in this method), so no action will be taken. This call ensures the
                    // callback occurs with the decor set.
                    a.onWindowAttributesChanged(l);
                }
            }
            ...

}

这里的ViewManager是一个接口，我们知道WindowManager继承了该接口，同时WindowManager的实现类WindowManagerImpl则将具体实现代理给了WindowManagerGlobal来实现。因此会调用到WindowManagerGlobal的addView()：

  public void addView(View view, ViewGroup.LayoutParams params,
            Display display, Window parentWindow) {
      ...
             root = new ViewRootImpl(view.getContext(), display);
            view.setLayoutParams(wparams);

            mViews.add(view);
            mRoots.add(root);
            mParams.add(wparams);

            // do this last because it fires off messages to start doing things
            try {
                root.setView(view, wparams, panelParentView);
            } catch (RuntimeException e) {
                // BadTokenException or InvalidDisplayException, clean up.
                if (index >= 0) {
                    removeViewLocked(index, true);
                }
                throw e;
            }
}

ViewRootImpl是WindowManager和View之间的桥梁。而ViewRootImpl实现了ViewParent接口。ViewGroup也实现了ViewParent接口。

ViewRootImpl在setView()中调用了View的assignParent()，将ViewRootImpl传入了DecorView中，ViewRootImpl也就成了DecorView的父亲。而普通view的这个方法，则是在ViewGroup的addView()中调用，这样每个View的parent就都赋值了。

    void assignParent(ViewParent parent) {
        if (mParent == null) {
            mParent = parent;
        } else if (parent == null) {
            mParent = null;
        } else {
            throw new RuntimeException("view " + this + " being added, but"
                    + " it already has a parent");
        }
    }

因此我们在Activity的onCreate()中调用：

   final View viewById = findViewById(R.id.tv);
        viewById.post(new Runnable() {
            @Override
            public void run() {
                ViewParent parent = viewById.getParent();
                while (parent!=null){
                    Log.e("parentName:",parent.getClass().getName());
                    parent = parent.getParent();
                }
            }
        });

打印的结果：

E/parentName:: androidx.constraintlayout.widget.ConstraintLayout
E/parentName:: androidx.appcompat.widget.ContentFrameLayout
E/parentName:: androidx.appcompat.widget.FitWindowsLinearLayout
E/parentName:: android.widget.FrameLayout
E/parentName:: android.widget.LinearLayout
E/parentName:: com.android.internal.policy.DecorView
E/parentName:: android.view.ViewRootImpl

2：mAttachInfo是如何分发的？

在ViewRootImpl的构造方法中就将自己传入到了View的内部类AttachInfo中。

 mAttachInfo = new View.AttachInfo(mWindowSession, mWindow, display, this, mHandler, this,
                context);

那AttachInfo是什么呢？注释中这么说的

 /**
     * {@hide}
     *
     * Not available for general use. If you need help, hang up and then dial one of the following
     * public APIs:
     *
     * @see #isAttachedToWindow() for current attach state
     * @see #onAttachedToWindow() for subclasses performing work when becoming attached
     * @see #onDetachedFromWindow() for subclasses performing work when becoming detached
     * @see OnAttachStateChangeListener for other code performing work on attach/detach
     * @see #getHandler() for posting messages to this view's UI thread/looper
     * @see #getParent() for interacting with the parent chain
     * @see #getWindowToken() for the current window token
     * @see #getRootView() for the view at the root of the attached hierarchy
     * @see #getDisplay() for the Display this view is presented on
     * @see #getRootWindowInsets() for the current insets applied to the whole attached window
     * @see #hasWindowFocus() for whether the attached window is currently focused
     * @see #getWindowVisibility() for checking the visibility of the attached window
     */
    @UnsupportedAppUsage(maxTargetSdk = Build.VERSION_CODES.P)
    AttachInfo mAttachInfo;

我们可以看到很多重要的属性，都保存在该类中。那么mAttachInfo是何时赋值的呢？在View中的该方法中进行赋值：

void dispatchAttachedToWindow(AttachInfo info, int visibility){
        mAttachInfo = info;
}

该方法由父ViewGroup来调用

 void dispatchAttachedToWindow(AttachInfo info, int visibility){
 mGroupFlags |= FLAG_PREVENT_DISPATCH_ATTACHED_TO_WINDOW;
        super.dispatchAttachedToWindow(info, visibility);
        mGroupFlags &= ~FLAG_PREVENT_DISPATCH_ATTACHED_TO_WINDOW;

        final int count = mChildrenCount;
        final View[] children = mChildren;
        for (int i = 0; i < count; i++) {
            final View child = children[i];
            child.dispatchAttachedToWindow(info,
                    combineVisibility(visibility, child.getVisibility()));
        }
        final int transientCount = mTransientIndices == null ? 0 : mTransientIndices.size();
        for (int i = 0; i < transientCount; ++i) {
            View view = mTransientViews.get(i);
            view.dispatchAttachedToWindow(info,
                    combineVisibility(visibility, view.getVisibility()));
        }
}

首先将当前ViewGroup的继承父类View中的mAttachInfo进行了赋值，然后对于每一个ChildView中的mAttachInfo进行了赋值。

在ViewRootImpl.setView()中，会调用requestLayout()。

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

scheduleTraversals()开始分发遍历任务，

    void scheduleTraversals() {
        if (!mTraversalScheduled) {
            mTraversalScheduled = true;
            mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();
            mChoreographer.postCallback(
                    Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
            if (!mUnbufferedInputDispatch) {
                scheduleConsumeBatchedInput();
            }
            notifyRendererOfFramePending();
            pokeDrawLockIfNeeded();
        }
    }

 final class TraversalRunnable implements Runnable {
        @Override
        public void run() {
            doTraversal();
        }
    }

将任务交给了Choreographer来处理。该类从GPU渲染出来的数据的buffer中取出，交给屏幕展示。

    void doTraversal() {
        if (mTraversalScheduled) {
            mTraversalScheduled = false;
            mHandler.getLooper().getQueue().removeSyncBarrier(mTraversalBarrier);

            if (mProfile) {
                Debug.startMethodTracing("ViewAncestor");
            }

            performTraversals();

            if (mProfile) {
                Debug.stopMethodTracing();
                mProfile = false;
            }
        }
    }

  private void performTraversals() {
     ...
//  这个host就是DecorView。而所有的mAttachInfo的分发也就是从这里开始的。
     host.dispatchAttachedToWindow(mAttachInfo, 0);
    ...
     performMeasure(childWidthMeasureSpec,     childHeightMeasureSpec);
...
            performLayout(lp, mWidth, mHeight);
...
            performDraw();
...

}

我们可以看到注释的这句

host.dispatchAttachedToWindow(mAttachInfo, 0);

将attachInfo分发给了DecorView，由此触发了所有子ViewGroup的分发。其实这些View拿到的mAttachInfo是同一个。

ViewRootImpl与ViewGroup，View关系