invalidate()函数的主要作用是请求View树进行重绘

invalidate()函数的主要作用是请求View树进行重绘，该函数可以由应用程序调用，或者由系统函数间接调用，例如setEnable(), setSelected(), setVisiblity()都会间接调用到invalidate()来请求View树重绘，更新View树的显示。注：requestLayout()和requestFocus()函数也会引起视图重绘
下面我们通过源码来了解invalidate()函数的工作原理，首先我们来看View类中invalidate()的实现过程：

1. 
   
2. 
   
3. public void invalidate() {
   
4. invalidate(true);
   
5. }
   

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invalidate()函数会转而调用invalidate(true)，继续往下看：

1. 
   
2. 
   
3. void invalidate(boolean invalidateCache) {
   
4. if (ViewDebug.TRACE_HIERARCHY) {
   
5. ViewDebug.trace(this, ViewDebug.HierarchyTraceType.INVALIDATE);
   
6. }
   
7. if (skipInvalidate()) {
   
8. return;
   
9. }
   
10. if ((mPrivateFlags & (DRAWN | HAS_BOUNDS)) == (DRAWN | HAS_BOUNDS) ||
    
11. (invalidateCache && (mPrivateFlags & DRAWING_CACHE_VALID) == DRAWING_CACHE_VALID) ||
    
12. (mPrivateFlags & INVALIDATED) != INVALIDATED || isOpaque() != mLastIsOpaque) {
    
13. mLastIsOpaque = isOpaque();
    
14. mPrivateFlags &= ~DRAWN;
    
15. mPrivateFlags |= DIRTY;
    
16. if (invalidateCache) {
    
17. mPrivateFlags |= INVALIDATED;
    
18. mPrivateFlags &= ~DRAWING_CACHE_VALID;
    
19. }
    
20. final AttachInfo ai = mAttachInfo;
    
21. final ViewParent p = mParent;
    
22. //noinspection PointlessBooleanExpression,ConstantConditions
    
23. if (!HardwareRenderer.RENDER_DIRTY_REGIONS) {
    
24. if (p != null && ai != null && ai.mHardwareAccelerated) {
    
25. // fast-track for GL-enabled applications; just invalidate the whole hierarchy
    
26. // with a null dirty rect, which tells the ViewAncestor to redraw everything
    
27. p.invalidateChild(this, null);
    
28. return;
    
29. }
    
30. }
    
31. if (p != null && ai != null) {
    
32. final Rect r = ai.mTmpInvalRect;
    
33. r.set(0, 0, mRight - mLeft, mBottom - mTop);
    
34. // Don't call invalidate -- we don't want to internally scroll
    
35. // our own bounds
    
36. p.invalidateChild(this, r);
    
37. }
    
38. }
    
39. }
    

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下面我们来具体进行分析invalidate(true)函数的执行流程：


1、首先调用skipInvalidate()，该函数主要判断该View是否不需要重绘，如果不许要重绘则直接返回，不需要重绘的条件是该View不可见并且未进行动画

2、接下来的if语句是来进一步判断View是否需要绘制，其中表达式 (mPrivateFlags & (DRAWN | HAS_BOUNDS)) == (DRAWN | HAS_BOUNDS)的意思指的是如果View需要重绘并且其大小不为0，其余几个本人也未完全理解，还望高手指点～～如果需要重绘，则处理相关标志位

3、对于开启硬件加速的应用程序，则调用父视图的invalidateChild函数绘制整个区域，否则只绘制dirty区域（r变量所指的区域），这是一个向上回溯的过程，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。

接下来看invalidateChild()的 实现过程：

1. 
   
2. public final void invalidateChild(View child, final Rect dirty) {
   
3. if (ViewDebug.TRACE_HIERARCHY) {
   
4. ViewDebug.trace(this, ViewDebug.HierarchyTraceType.INVALIDATE_CHILD);
   
5. }
   
6. ViewParent parent = this;
   
7. final AttachInfo attachInfo = mAttachInfo;
   
8. if (attachInfo != null) {
   
9. // If the child is drawing an animation, we want to copy this flag onto
   
10. // ourselves and the parent to make sure the invalidate request goes
    
11. // through
    
12. final boolean drawAnimation = (child.mPrivateFlags & DRAW_ANIMATION) == DRAW_ANIMATION;
    
13. if (dirty == null) {
    
14. if (child.mLayerType != LAYER_TYPE_NONE) {
    
15. mPrivateFlags |= INVALIDATED;
    
16. mPrivateFlags &= ~DRAWING_CACHE_VALID;
    
17. child.mLocalDirtyRect.setEmpty();
    
18. }
    
19. do {
    
20. View view = null;
    
21. if (parent instanceof View) {
    
22. view = (View) parent;
    
23. if (view.mLayerType != LAYER_TYPE_NONE) {
    
24. view.mLocalDirtyRect.setEmpty();
    
25. if (view.getParent() instanceof View) {
    
26. final View grandParent = (View) view.getParent();
    
27. grandParent.mPrivateFlags |= INVALIDATED;
    
28. grandParent.mPrivateFlags &= ~DRAWING_CACHE_VALID;
    
29. }
    
30. }
    
31. if ((view.mPrivateFlags & DIRTY_MASK) != 0) {
    
32. // already marked dirty - we're done
    
33. break;
    
34. }
    
35. }
    
36. if (drawAnimation) {
    
37. if (view != null) {
    
38. view.mPrivateFlags |= DRAW_ANIMATION;
    
39. } else if (parent instanceof ViewRootImpl) {
    
40. ((ViewRootImpl) parent).mIsAnimating = true;
    
41. }
    
42. }
    
43. if (parent instanceof ViewRootImpl) {
    
44. ((ViewRootImpl) parent).invalidate();
    
45. parent = null;
    
46. } else if (view != null) {
    
47. if ((view.mPrivateFlags & DRAWN) == DRAWN ||
    
48. (view.mPrivateFlags & DRAWING_CACHE_VALID) == DRAWING_CACHE_VALID) {
    
49. view.mPrivateFlags &= ~DRAWING_CACHE_VALID;
    
50. view.mPrivateFlags |= DIRTY;
    
51. parent = view.mParent;
    
52. } else {
    
53. parent = null;
    
54. }
    
55. }
    
56. } while (parent != null);
    
57. } else {
    
58. // Check whether the child that requests the invalidate is fully opaque
    
59. final boolean isOpaque = child.isOpaque() && !drawAnimation &&
    
60. child.getAnimation() == null;
    
61. // Mark the child as dirty, using the appropriate flag
    
62. // Make sure we do not set both flags at the same time
    
63. int opaqueFlag = isOpaque ? DIRTY_OPAQUE : DIRTY;
    
64. if (child.mLayerType != LAYER_TYPE_NONE) {
    
65. mPrivateFlags |= INVALIDATED;
    
66. mPrivateFlags &= ~DRAWING_CACHE_VALID;
    
67. child.mLocalDirtyRect.union(dirty);
    
68. }
    
69. final int[] location = attachInfo.mInvalidateChildLocation;
    
70. location[CHILD_LEFT_INDEX] = child.mLeft;
    
71. location[CHILD_TOP_INDEX] = child.mTop;
    
72. Matrix childMatrix = child.getMatrix();
    
73. if (!childMatrix.isIdentity()) {
    
74. RectF boundingRect = attachInfo.mTmpTransformRect;
    
75. boundingRect.set(dirty);
    
76. //boundingRect.inset(-0.5f, -0.5f);
    
77. childMatrix.mapRect(boundingRect);
    
78. dirty.set((int) (boundingRect.left - 0.5f),
    
79. (int) (boundingRect.top - 0.5f),
    
80. (int) (boundingRect.right + 0.5f),
    
81. (int) (boundingRect.bottom + 0.5f));
    
82. }
    
83. do {
    
84. View view = null;
    
85. if (parent instanceof View) {
    
86. view = (View) parent;
    
87. if (view.mLayerType != LAYER_TYPE_NONE &&
    
88. view.getParent() instanceof View) {
    
89. final View grandParent = (View) view.getParent();
    
90. grandParent.mPrivateFlags |= INVALIDATED;
    
91. grandParent.mPrivateFlags &= ~DRAWING_CACHE_VALID;
    
92. }
    
93. }
    
94. if (drawAnimation) {
    
95. if (view != null) {
    
96. view.mPrivateFlags |= DRAW_ANIMATION;
    
97. } else if (parent instanceof ViewRootImpl) {
    
98. ((ViewRootImpl) parent).mIsAnimating = true;
    
99. }
    
100. }
     
101. // If the parent is dirty opaque or not dirty, mark it dirty with the opaque
     
102. // flag coming from the child that initiated the invalidate
     
103. if (view != null) {
     
104. if ((view.mViewFlags & FADING_EDGE_MASK) != 0 &&
     
105. view.getSolidColor() == 0) {
     
106. opaqueFlag = DIRTY;
     
107. }
     
108. if ((view.mPrivateFlags & DIRTY_MASK) != DIRTY) {
     
109. view.mPrivateFlags = (view.mPrivateFlags & ~DIRTY_MASK) | opaqueFlag;
     
110. }
     
111. }
     
112. parent = parent.invalidateChildInParent(location, dirty);
     
113. if (view != null) {
     
114. // Account for transform on current parent
     
115. Matrix m = view.getMatrix();
     
116. if (!m.isIdentity()) {
     
117. RectF boundingRect = attachInfo.mTmpTransformRect;
     
118. boundingRect.set(dirty);
     
119. m.mapRect(boundingRect);
     
120. dirty.set((int) boundingRect.left, (int) boundingRect.top,
     
121. (int) (boundingRect.right + 0.5f),
     
122. (int) (boundingRect.bottom + 0.5f));
     
123. }
     
124. }
     
125. } while (parent != null);
     
126. }
     
127. }
     
128. }
     

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大概流程如下，我们主要关注dirty区域不是null（非硬件加速）的情况：
1、判断子视图是否是不透明的（不透明的条件是isOpaque()返回true，视图未进行动画以及child.getAnimation() == null），并将判断结果保存到变量isOpaque中，如果不透明则将变量opaqueFlag设置为DIRTY_OPAQUE，否则设置为DIRTY。

2、定义location保存子视图的左上角坐标

3、如果子视图正在动画，那么父视图也要添加动画标志，如果父视图是ViewGroup，那么给mPrivateFlags添加DRAW_ANIMATION标识，如果父视图是ViewRoot，则给其内部变量mIsAnimating赋值为true

4、设置dirty标识，如果子视图是不透明的，则父视图设置为DIRTY_OPAQUE，否则设置为DIRTY

5、调用parent.invalidateChildInparent()，这里的parent有可能是ViewGroup，也有可能是ViewRoot（最后一次while循环），首先来看ViewGroup, ViewGroup中该函数的主要作用是对dirty区域进行计算

以上过程的主体是一个do{}while{}循环，不断的将子视图的dirty区域与父视图做运算来确定最终要重绘的dirty区域，最终循环到ViewRoot（ViewRoot的parent为null）为止，并将dirty区域保存到ViewRoot的mDirty变量中

1. 
   
2. 
   
3. public ViewParent invalidateChildInParent(final int[] location, final Rect dirty) {
   
4. if (ViewDebug.TRACE_HIERARCHY) {
   
5. ViewDebug.trace(this, ViewDebug.HierarchyTraceType.INVALIDATE_CHILD_IN_PARENT);
   
6. }
   
7. if ((mPrivateFlags & DRAWN) == DRAWN ||
   
8. (mPrivateFlags & DRAWING_CACHE_VALID) == DRAWING_CACHE_VALID) {
   
9. if ((mGroupFlags & (FLAG_OPTIMIZE_INVALIDATE | FLAG_ANIMATION_DONE)) !=
   
10. FLAG_OPTIMIZE_INVALIDATE) {
    
11. dirty.offset(location[CHILD_LEFT_INDEX] - mScrollX,
    
12. location[CHILD_TOP_INDEX] - mScrollY);
    
13. final int left = mLeft;
    
14. final int top = mTop;
    
15. if ((mGroupFlags & FLAG_CLIP_CHILDREN) != FLAG_CLIP_CHILDREN ||
    
16. dirty.intersect(0, 0, mRight - left, mBottom - top) ||
    
17. (mPrivateFlags & DRAW_ANIMATION) == DRAW_ANIMATION) {
    
18. mPrivateFlags &= ~DRAWING_CACHE_VALID;
    
19. location[CHILD_LEFT_INDEX] = left;
    
20. location[CHILD_TOP_INDEX] = top;
    
21. if (mLayerType != LAYER_TYPE_NONE) {
    
22. mLocalDirtyRect.union(dirty);
    
23. }
    
24. return mParent;
    
25. }
    
26. } else {
    
27. mPrivateFlags &= ~DRAWN & ~DRAWING_CACHE_VALID;
    
28. location[CHILD_LEFT_INDEX] = mLeft;
    
29. location[CHILD_TOP_INDEX] = mTop;
    
30. if ((mGroupFlags & FLAG_CLIP_CHILDREN) == FLAG_CLIP_CHILDREN) {
    
31. dirty.set(0, 0, mRight - mLeft, mBottom - mTop);
    
32. } else {
    
33. // in case the dirty rect extends outside the bounds of this container
    
34. dirty.union(0, 0, mRight - mLeft, mBottom - mTop);
    
35. }
    
36. if (mLayerType != LAYER_TYPE_NONE) {
    
37. mLocalDirtyRect.union(dirty);
    
38. }
    
39. return mParent;
    
40. }
    
41. }
    
42. return null;
    
43. }
    

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该函数首先调用offset将子视图的坐标位置转换为在父视图（当前视图）的显示位置，这里主要考虑scroll后导致子视图在父视图中的显示区域会发生变化，接着调用union函数求得当前视图与子视图的交集,求得的交集必定是小于dirty的范围，因为子视图的dirty区域有可能超出其父视图（当前视图）的范围，最后返回当前视图的父视图。

再来看ViewRoot中invalidateChildInparent的执行过程：

1. 
   
2. public ViewParent invalidateChildInParent(final int[] location, final Rect dirty) {
   
3. invalidateChild(null, dirty);
   
4. return null;
   
5. }
   

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该函数仅仅调用了ViewRoot的invalidateChild，下面继续看invalidateChild的源码：

1. 
   
2. public void invalidateChild(View child, Rect dirty) {
   
3. checkThread();
   
4. if (DEBUG_DRAW) Log.v(TAG, "Invalidate child: " + dirty);
   
5. if (dirty == null) {
   
6. // Fast invalidation for GL-enabled applications; GL must redraw everything
   
7. invalidate();
   
8. return;
   
9. }
   
10. if (mCurScrollY != 0 || mTranslator != null) {
    
11. mTempRect.set(dirty);
    
12. dirty = mTempRect;
    
13. if (mCurScrollY != 0) {
    
14. dirty.offset(0, -mCurScrollY);
    
15. }
    
16. if (mTranslator != null) {
    
17. mTranslator.translateRectInAppWindowToScreen(dirty);
    
18. }
    
19. if (mAttachInfo.mScalingRequired) {
    
20. dirty.inset(-1, -1);
    
21. }
    
22. }
    
23. if (!mDirty.isEmpty() && !mDirty.contains(dirty)) {
    
24. mAttachInfo.mSetIgnoreDirtyState = true;
    
25. mAttachInfo.mIgnoreDirtyState = true;
    
26. }
    
27. mDirty.union(dirty);
    
28. if (!mWillDrawSoon) {
    
29. scheduleTraversals();
    
30. }
    
31. }
    

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具体分析如下： 1、判断此次调用是否在UI线程中进行

2、将dirty的坐标位置转换为ViewRoot的屏幕显示区域

3、更新mDirty变量，并调用scheduleTraversals发起重绘请求

至此一次invalidate()就结束了

总结：invalidate主要给需要重绘的视图添加DIRTY标记，并通过和父视图的矩形运算求得真正需要绘制的区域，并保存在ViewRoot中的mDirty变量中，最后调用scheduleTraversals发起重绘请求，scheduleTraversals会发送一个异步消息，最终调用performTraversals()执行重绘，performTraversals()的具体过程以后再分析。

 

转自：http://blog.sina.com.cn/s/blog_6400e5c50101e1dl.html