RecyclerView 介绍 02 – 重要概念

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RecyclerView 介绍 02 – 重要概念
几个概念
1. RecyclerView是一个ViewGroup；
2. LayoutManager控制RecyclerView的ChildView的布局显示，childview由Recycler提供以及管理；
3. Recycler具有两级缓存，Scrap和RecycledViewPool，通过Detach以及Remove，对Viewholder进行转移以及状态改变；
4. RecycledViewPool可以由多个RecyclerView共享；
5. ViewHolder具有多种状态标记；
关于Recycler

Scrap中的ViewHolder，不用通过Adapter重新处理，只需要attach后回到LayoutManager就可以重用。

RecycledViewPool中的ViewHolder，数据往往是错误的，则需要通过Adapter重新绑定正确的数据后在回到LayoutManager。

当LayoutManager需要一个新的View时，Recycler会行检查scrap中是否有合适的ViewHolder，如果有直接返回给LayoutManager使用；如果没有，就需要从Pool里面寻找，然后右Adapter重新绑定数据后，返回到LayoutManager；如果pool还是没有，就需要由Adapter创建一个新的Viewholder。见如下代码：
1. View getViewForPosition(int position, boolean dryRun) {
2. if (position < 0 || position >= mState.getItemCount()) {
3. throw new IndexOutOfBoundsException("Invalid item position " + position
4. + "(" + position + "). Item count:" + mState.getItemCount());
5. }
6. boolean fromScrap = false;
7. ViewHolder holder = null;
8. // 0) If there is a changed scrap, try to find from there
9. if (mState.isPreLayout()) {
10. holder = getChangedScrapViewForPosition(position);
11. fromScrap = holder != null;
12. }
13. // 1) Find from scrap by position
14. if (holder == null) {
15. holder = getScrapViewForPosition(position, INVALID_TYPE, dryRun);
16. if (holder != null) {
17. if (!validateViewHolderForOffsetPosition(holder)) {
18. // recycle this scrap
19. if (!dryRun) {
20. // we would like to recycle this but need to make sure it is not used by
21. // animation logic etc.
22. holder.addFlags(ViewHolder.FLAG_INVALID);
23. if (holder.isScrap()) {
24. removeDetachedView(holder.itemView, false);
25. holder.unScrap();
26. } else if (holder.wasReturnedFromScrap()) {
27. holder.clearReturnedFromScrapFlag();
28. }
29. recycleViewHolderInternal(holder);
30. }
31. holder = null;
32. } else {
33. fromScrap = true;
34. }
35. }
36. }
37. if (holder == null) {
38. final int offsetPosition = mAdapterHelper.findPositionOffset(position);
39. if (offsetPosition < 0 || offsetPosition >= mAdapter.getItemCount()) {
40. throw new IndexOutOfBoundsException("Inconsistency detected. Invalid item "
41. + "position " + position + "(offset:" + offsetPosition + ")."
42. + "state:" + mState.getItemCount());
43. }
45. final int type = mAdapter.getItemViewType(offsetPosition);
46. // 2) Find from scrap via stable ids, if exists
47. if (mAdapter.hasStableIds()) {
48. holder = getScrapViewForId(mAdapter.getItemId(offsetPosition), type, dryRun);
49. if (holder != null) {
50. // update position
51. holder.mPosition = offsetPosition;
52. fromScrap = true;
53. }
54. }
55. if (holder == null && mViewCacheExtension != null) {
56. // We are NOT sending the offsetPosition because LayoutManager does not
57. // know it.
58. final View view = mViewCacheExtension
59. .getViewForPositionAndType(this, position, type);
60. if (view != null) {
61. holder = getChildViewHolder(view);
62. if (holder == null) {
63. throw new IllegalArgumentException("getViewForPositionAndType returned"
64. + " a view which does not have a ViewHolder");
65. } else if (holder.shouldIgnore()) {
66. throw new IllegalArgumentException("getViewForPositionAndType returned"
67. + " a view that is ignored. You must call stopIgnoring before"
68. + " returning this view.");
69. }
70. }
71. }
72. if (holder == null) { // fallback to recycler
73. // try recycler.
74. // Head to the shared pool.
75. if (DEBUG) {
76. Log.d(TAG, "getViewForPosition(" + position + ") fetching from shared "
77. + "pool");
78. }
79. holder = getRecycledViewPool()
80. .getRecycledView(mAdapter.getItemViewType(offsetPosition));
81. if (holder != null) {
82. holder.resetInternal();
83. if (FORCE_INVALIDATE_DISPLAY_LIST) {
84. invalidateDisplayListInt(holder);
85. }
86. }
87. }
88. if (holder == null) {
89. holder = mAdapter.createViewHolder(RecyclerView.this,
90. mAdapter.getItemViewType(offsetPosition));
91. if (DEBUG) {
92. Log.d(TAG, "getViewForPosition created new ViewHolder");
93. }
94. }
95. }
96. boolean bound = false;
97. if (mState.isPreLayout() && holder.isBound()) {
98. // do not update unless we absolutely have to.
99. holder.mPreLayoutPosition = position;
100. } else if (!holder.isBound() || holder.needsUpdate() || holder.isInvalid()) {
101. if (DEBUG && holder.isRemoved()) {
102. throw new IllegalStateException("Removed holder should be bound and it should"
103. + " come here only in pre-layout. Holder: " + holder);
104. }
105. final int offsetPosition = mAdapterHelper.findPositionOffset(position);
106. mAdapter.bindViewHolder(holder, offsetPosition);
107. attachAccessibilityDelegate(holder.itemView);
108. bound = true;
109. if (mState.isPreLayout()) {
110. holder.mPreLayoutPosition = position;
111. }
112. }
114. final ViewGroup.LayoutParams lp = holder.itemView.getLayoutParams();
115. final LayoutParams rvLayoutParams;
116. if (lp == null) {
117. rvLayoutParams = (LayoutParams) generateDefaultLayoutParams();
118. holder.itemView.setLayoutParams(rvLayoutParams);
119. } else if (!checkLayoutParams(lp)) {
120. rvLayoutParams = (LayoutParams) generateLayoutParams(lp);
121. holder.itemView.setLayoutParams(rvLayoutParams);
122. } else {
123. rvLayoutParams = (LayoutParams) lp;
124. }
125. rvLayoutParams.mViewHolder = holder;
126. rvLayoutParams.mPendingInvalidate = fromScrap && bound;
127. return holder.itemView;
128. }
关于ViewHolder

在RecyclerView里面，view是有多重状态的，各种状态在ViewHolder里面定义。看看下面的代码：
1. public static abstract class ViewHolder {
2. public final View itemView;
3. int mPosition = NO_POSITION;
4. int mOldPosition = NO_POSITION;
5. long mItemId = NO_ID;
6. int mItemViewType = INVALID_TYPE;
7. int mPreLayoutPosition = NO_POSITION;
9. // The item that this holder is shadowing during an item change event/animation
10. ViewHolder mShadowedHolder = null;
11. // The item that is shadowing this holder during an item change event/animation
12. ViewHolder mShadowingHolder = null;
14. /**
15. * This ViewHolder has been bound to a position; mPosition, mItemId and mItemViewType
16. * are all valid.
17. */
18. static final int FLAG_BOUND = 1 << 0;
20. /**
21. * The data this ViewHolder's view reflects is stale and needs to be rebound
22. * by the adapter. mPosition and mItemId are consistent.
23. */
24. static final int FLAG_UPDATE = 1 << 1;
26. /**
27. * This ViewHolder's data is invalid. The identity implied by mPosition and mItemId
28. * are not to be trusted and may no longer match the item view type.
29. * This ViewHolder must be fully rebound to different data.
30. */
31. static final int FLAG_INVALID = 1 << 2;
33. /**
34. * This ViewHolder points at data that represents an item previously removed from the
35. * data set. Its view may still be used for things like outgoing animations.
36. */
37. static final int FLAG_REMOVED = 1 << 3;
39. /**
40. * This ViewHolder should not be recycled. This flag is set via setIsRecyclable()
41. * and is intended to keep views around during animations.
42. */
43. static final int FLAG_NOT_RECYCLABLE = 1 << 4;
45. /**
46. * This ViewHolder is returned from scrap which means we are expecting an addView call
47. * for this itemView. When returned from scrap, ViewHolder stays in the scrap list until
48. * the end of the layout pass and then recycled by RecyclerView if it is not added back to
49. * the RecyclerView.
50. */
51. static final int FLAG_RETURNED_FROM_SCRAP = 1 << 5;
53. /**
54. * This ViewHolder's contents have changed. This flag is used as an indication that
55. * change animations may be used, if supported by the ItemAnimator.
56. */
57. static final int FLAG_CHANGED = 1 << 6;
59. /**
60. * This ViewHolder is fully managed by the LayoutManager. We do not scrap, recycle or remove
61. * it unless LayoutManager is replaced.
62. * It is still fully visible to the LayoutManager.
63. */
64. static final int FLAG_IGNORE = 1 << 7;
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原文地址：https://www.cnblogs.com/halzhang/p/4445145.html