Android如何强制刷新view--比较偏的考试题目

今天学习Android VIEW的刷新机制，之前一直以为是调用VIEW的刷新就自己调用刷新函数。今天学习了一下view的刷新机制，还是表示学习到很多的知识啊。

感想就是自己要多阅读android的源代码，其实很多的消息传递等等的机制，都是通过阅读android的源代码得到的，所以有事没事就去看源代码玩吧~

好了，来到正题，关键的一句话就是：

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。

步骤就是：

1、调用子View的invalidate（）

2、跳转到上一层的invalidateChild函数中区

3、在一次调用invalidateChildInParent的函数一次层层刷新

4、具体的刷新后续操作，我就不清楚了，调用invalidate最终在代码上就在invalidateChild终止了的，所以表示有点点不清晰，求各位大牛介绍一下吧。。。。。？在此谢过了。。

让我来阅读源代码：

首先在View类中：

/**
* Invalidate the whole view. If the view is visible, {@link #onDraw} will
* be called at some point in the future. This must be called from a
* UI thread. To call from a non-UI thread, call {@link #postInvalidate()}.
*/
public void invalidate() {
if (ViewDebug.TRACE_HIERARCHY) {
ViewDebug.trace(this, ViewDebug.HierarchyTraceType.INVALIDATE);
}

if ((mPrivateFlags & (DRAWN | HAS_BOUNDS)) == (DRAWN | HAS_BOUNDS)) {
mPrivateFlags &= ~DRAWN & ~DRAWING_CACHE_VALID;

final ViewParent p = mParent; //获得父类View的对象
final AttachInfo ai = mAttachInfo;//获得匹配
if (p != null && ai != null) {
final Rect r = ai.mTmpInvalRect;
r.set(0, 0, mRight - mLeft, mBottom - mTop);//设置本View的尺寸，其实就是大小没有设置位置

// Don't call invalidate -- we don't want to internally scroll
// our own bounds
p.invalidateChild(this, r); //调用父类的刷新函数
}
}
}

下面我们来到Viewgroup对象：

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。

/**
* Don't call or override this method. It is used for the implementation of
* the view hierarchy.
*/
public final void invalidateChild(View child, final Rect dirty) {
if (ViewDebug.TRACE_HIERARCHY) {
ViewDebug.trace(this, ViewDebug.HierarchyTraceType.INVALIDATE_CHILD);
}

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

// 刷新子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);
}

/**
* Don't call or override this method. It is used for the implementation of
* the view hierarchy.
*
* This implementation returns null if this ViewGroup does not have a parent,
* if this ViewGroup is already fully invalidated or if the dirty rectangle
* does not intersect with this ViewGroup's bounds.
*/
public ViewParent invalidateChildInParent(final int[] location, final Rect dirty) {
if (ViewDebug.TRACE_HIERARCHY) {
ViewDebug.trace(this, ViewDebug.HierarchyTraceType.INVALIDATE_CHILD_IN_PARENT);
}

if ((mPrivateFlags & DRAWN) == DRAWN) {
if ((mGroupFlags & (FLAG_OPTIMIZE_INVALIDATE | FLAG_ANIMATION_DONE)) !=
FLAG_OPTIMIZE_INVALIDATE) {

// 由父类的的位置，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;

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;
}

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MyThre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：Invalidate the whole view. If the view is visible, onDraw(Canvas) will be called at some point in the future. This must be called from a UI thread. To call from a non-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()。

简言之，如果确定是在main thread中调用调用话，使用 invaludate()

否则要调用 postInvalidate()

另外，横竖屏切换使用重新构造 activity的。所以一定会重新刷新view 。

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