View的更新原理

我知道android的view布局是树形布局的。子View需要刷新的自身界面的时候，把需求告诉父亲，则父View负责刷新、布局显示子View，从上到下。

public void invalidate() {
    final ViewParent p = mParent;
    final AttachInfo ai = mAttachInfo;
    if (p != null && ai != null) {
        final Rect r = ai.mTmpInvalRect;

        // 设置刷新区域为自己的尺寸 
        r.set(0, 0, mRight - mLeft, mBottom - mTop);
        p.invalidateChild(this, r);
    }
}

从上面的代码我们可以看出：子View调用invalidate时，首先找到自己父View(View的成员变量mParent记录自己的父View)，然后将AttachInfo中保存的信息告诉父View刷新自己。

View的父子关系的建立分为两种情况：

(1) View加入ViewGroup中

private void addViewInner(View child, int index, LayoutParams params, boolean preventRequestLayout) {

         ...

            // tell our children
        if (preventRequestLayout) {
            child.assignParent(this);
        } else {
            child.mParent = this;
        }

       ...

}

2)DecorView注册给WindowManagerImpl时，产生一个ViewRoot作为其父View。

public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView){

    .....

    view.assignParent(this);

    ....

}

AttachInfo是在View第一次attach到Window时，ViewRoot传给自己的子View的。这个AttachInfo之后，会顺着布局体系一直传递到最底层的View。

View.java

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

    .....
}

ViewGroup.java

void dispatchAttachedToWindow(AttachInfo info, int visibility) {
    super.dispatchAttachedToWindow(info, visibility);

    for (int i = 0; i < count; i++) {
        children[i].dispatchAttachedToWindow(info, visibility);
    }
}

并且在新的View被加入ViewGroup时，也会将该AttachInfo传给加入的View

ViewGroup.java

private void addViewInner(View child, int index, LayoutParams params, boolean preventRequestLayout) {

    child.dispatchAttachedToWindow(mAttachInfo, (mViewFlags&VISIBILITY_MASK));

}

到这里明白了mParent与AttachInfo代表的意义，可以继续刷新过程的分析。

在invalidate中，调用父View的invalidateChild，这是一个从第向上回溯的过程，每一层的父View都将自己的显示区域与传入的刷新Rect做交集。

public final void invalidateChild(View child, final Rect dirty) {
    ViewParent parent = this;

    final AttachInfo attachInfo = mAttachInfo;
    if (attachInfo != null) {
        final int[] location = attachInfo.mInvalidateChildLocation;

        // 需要刷新的子View的位置 
        location[CHILD_LEFT_INDEX] = child.mLeft;
        location[CHILD_TOP_INDEX] = child.mTop;

        // If the child is drawing an animation, we want to copy this flag onto
        // ourselves and the parent to make sure the invalidate request goes through
        final boolean drawAnimation = (child.mPrivateFlags & DRAW_ANIMATION) == DRAW_ANIMATION;

        // Check whether the child that requests the invalidate is fully opaque
        final boolean isOpaque = child.isOpaque() && !drawAnimation && child.getAnimation() != null;

        // Mark the child as dirty, using the appropriate flag
        // Make sure we do not set both flags at the same time
        final int opaqueFlag = isOpaque ? DIRTY_OPAQUE : DIRTY;

        do {
            View view = null;
            if (parent instanceof View) {
                view = (View) parent;
            }

            if (drawAnimation) {
                if (view != null) {
                        view.mPrivateFlags |= DRAW_ANIMATION;
                } else if (parent instanceof ViewRoot) {
                        ((ViewRoot) parent).mIsAnimating = true;
                }
            }

                // If the parent is dirty opaque or not dirty, mark it dirty with the opaque
                // flag coming from the child that initiated the invalidate
            if (view != null && (view.mPrivateFlags & DIRTY_MASK) != DIRTY) {
                view.mPrivateFlags = (view.mPrivateFlags & ~DIRTY_MASK) | opaqueFlag;
            }

            parent = parent.invalidateChildInParent(location, dirty);
        } while (parent != null);
    }
}


 

public ViewParent invalidateChildInParent(final int[] location, final Rect dirty) {
    if ((mPrivateFlags & DRAWN) == DRAWN) {
        if ((mGroupFlags & (FLAG_OPTIMIZE_INVALIDATE | FLAG_ANIMATION_DONE)) !=
                        FLAG_OPTIMIZE_INVALIDATE) {

            // 根据父View的位置，偏移刷新区域 
            dirty.offset(location[CHILD_LEFT_INDEX] - mScrollX, location[CHILD_TOP_INDEX] - mScrollY);

            final int left = mLeft;
            final int top = mTop;

            //计算实际可刷新区域 
            if (dirty.intersect(0, 0, mRight - left, mBottom - top) ||
                        (mPrivateFlags & DRAW_ANIMATION) == DRAW_ANIMATION) {
                mPrivateFlags &= ~DRAWING_CACHE_VALID;

                location[CHILD_LEFT_INDEX] = left;
                location[CHILD_TOP_INDEX] = top;
                return mParent;
            }
        } else {
            mPrivateFlags &= ~DRAWN & ~DRAWING_CACHE_VALID;

            location[CHILD_LEFT_INDEX] = mLeft;
            location[CHILD_TOP_INDEX] = mTop;

           dirty.set(0, 0, mRight - location[CHILD_LEFT_INDEX],
                        mBottom - location[CHILD_TOP_INDEX]);

                return mParent;
            }
        }

        return null;
}

这个向上回溯的过程直到ViewRoot那里结束，由ViewRoot对这个最终的刷新区域做刷新。

ViewRoot.java

public void invalidateChild(View child, Rect dirty) {

    scheduleTraversals();

}