本文实例讲述了Android View刷新机制。分享给大家供大家参考，具体如下：
一、总体说明
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。
二、代码分析
1).ViewGroup的addView方法，理解参数的意义和传递
invalidate调用父类View的方法
addViewInner方法主要做的事情是
view的dispatchAttachedToWindow(AttachInfo info, int visibility)方法
1).View的invalidate方法，这是一个从下第向上回溯的过程，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。
void invalidate(boolean invalidateCache) {
if (ViewDebug.TRACE_HIERARCHY) {
ViewDebug.trace(this, ViewDebug.HierarchyTraceType.INVALIDATE);
}
if (skipInvalidate()) {
return;
}
if ((mPrivateFlags & (DRAWN | HAS_BOUNDS)) == (DRAWN | HAS_BOUNDS) ||
(invalidateCache && (mPrivateFlags & DRAWING_CACHE_VALID) == DRAWING_CACHE_VALID) ||
(mPrivateFlags & INVALIDATED) != INVALIDATED || isOpaque() != mLastIsOpaque) {
mLastIsOpaque = isOpaque();
mPrivateFlags &= ~DRAWN;
mPrivateFlags |= DIRTY;
if (invalidateCache) {
mPrivateFlags |= INVALIDATED;
mPrivateFlags &= ~DRAWING_CACHE_VALID;
}
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);//调用子类的方法完成
}
}
}
2)ViewGrop的invalidateChild方法
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) {
}
由ViewRoot对象的performTraversals()方法调用draw()方法发起绘制该View树，值得注意的是每次发起绘图时，并不会重新绘制每个View树的视图，而只会重新绘制那些“需要重绘”的视图，View类内部变量包含了一个标志位DRAWN，当该视图需要重绘时，就会为该View添加该标志位。
调用流程 ：
mView.draw()开始绘制，draw()方法实现的功能如下：
1 、绘制该View的背景
2 、为显示渐变框做一些准备操作(见5，大多数情况下，不需要改渐变框)
3、调用onDraw()方法绘制视图本身 (每个View都需要重载该方法，ViewGroup不需要实现该方法)
4、调用dispatchDraw ()方法绘制子视图(如果该View类型不为ViewGroup，即不包含子视图，不需要重载该
方法)值得说明的是，ViewGroup类已经为我们重写了dispatchDraw ()的功能实现，应用程序一般不需要重写该
方法，但可以重载父类函数实现具体的功能。
4.1 dispatchDraw()方法内部会遍历每个子视图，调用drawChild()去重新回调每个子视图的draw()方法(注意，这个 地方“需要重绘”的视图才会调用draw()方法)。值得说明的是，ViewGroup类已经为我们重写了dispatch
Draw()的功能实现，应用程序一般不需要重写该方法，但可以重载父类函数实现具体的功能。
希望本文所述对大家Android程序设计有所帮助。
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