View的刷新机制

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：

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);
}
}

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();

}

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