Android(安卓)View刷新机制

在Android的布局体系中，父View负责刷新、布局显示子View；而当子View需要刷新时，则是通知父View来完成。这种处理逻辑在View的代码中明确的表现出来：

  void invalidate(boolean invalidateCache) {            final AttachInfo ai = mAttachInfo;            final ViewParent p = mParent;            //noinspection PointlessBooleanExpression,ConstantConditions            if (!HardwareRenderer.RENDER_DIRTY_REGIONS) {                if (p != null && ai != null && ai.mHardwareAccelerated) {                    // fast-track for GL-enabled applications; just invalidate the whole hierarchy                    // with a null dirty rect, which tells the ViewAncestor to redraw everything                    p.invalidateChild(this, null);                    return;                }            }            if (p != null && ai != null) {                final Rect r = ai.mTmpInvalRect;                r.set(0, 0, mRight - mLeft, mBottom - mTop);                // Don't call invalidate -- we don't want to internally scroll                // our own bounds                p.invalidateChild(this, r);            }        }    }

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();}

另外：

Invalidate()方法不能放在线程中，所以需要把Invalidate()方法放在Handler中。在MyThread中只需要在规定时间内发送一个Message给handler，当Handler接收到消息就调用Invalidate()方法。

postInvalidate()方法就可以放在线程中做处理，就不需要Handler。

而上面的新线程MyThread可以放在OnCreate()中开始，也可以放在OnStart()中开始。

Invalidate()方法和postInvalidate()都可以在主线程中调用而刷新视图。

Invalidate()方法在SDK中是这样描述的：Invalidatethe whole view. If the view is visible, onDraw(Canvas) will be called at somepoint in the future. This must be called from a UI thread. To call from anon-UI thread, call postInvalidate(). 当Invalidate()被调用的时候，View的OnDraw()就会被调用，Invalidate()必须是在UI线程中被调用，如果在新线程中更新视图的就调用postInvalidate()。

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