RecyclerView 介绍 02 – 重要概念

几个概念

RecyclerView是一个ViewGroup；
LayoutManager控制RecyclerView的ChildView的布局显示，childview由Recycler提供以及管理；
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；
RecycledViewPool可以由多个RecyclerView共享；
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：

View getViewForPosition(int position, boolean dryRun) {
if (position < 0 || position >= mState.getItemCount()) {
throw new IndexOutOfBoundsException("Invalid item position " + position
+ "(" + position + "). Item count:" + mState.getItemCount());
}
boolean fromScrap = false;
ViewHolder holder = null;
// 0) If there is a changed scrap, try to find from there
if (mState.isPreLayout()) {
holder = getChangedScrapViewForPosition(position);
fromScrap = holder != null;
}
// 1) Find from scrap by position
if (holder == null) {
holder = getScrapViewForPosition(position, INVALID_TYPE, dryRun);
if (holder != null) {
if (!validateViewHolderForOffsetPosition(holder)) {
// recycle this scrap
if (!dryRun) {
// we would like to recycle this but need to make sure it is not used by
// animation logic etc.
holder.addFlags(ViewHolder.FLAG_INVALID);
if (holder.isScrap()) {
removeDetachedView(holder.itemView, false);
holder.unScrap();
} else if (holder.wasReturnedFromScrap()) {
holder.clearReturnedFromScrapFlag();
}
recycleViewHolderInternal(holder);
}
holder = null;
} else {
fromScrap = true;
}
}
}
if (holder == null) {
final int offsetPosition = mAdapterHelper.findPositionOffset(position);
if (offsetPosition < 0 || offsetPosition >= mAdapter.getItemCount()) {
throw new IndexOutOfBoundsException("Inconsistency detected. Invalid item "
+ "position " + position + "(offset:" + offsetPosition + ")."
+ "state:" + mState.getItemCount());
}

final int type = mAdapter.getItemViewType(offsetPosition);
// 2) Find from scrap via stable ids, if exists
if (mAdapter.hasStableIds()) {
holder = getScrapViewForId(mAdapter.getItemId(offsetPosition), type, dryRun);
if (holder != null) {
// update position
holder.mPosition = offsetPosition;
fromScrap = true;
}
}
if (holder == null && mViewCacheExtension != null) {
// We are NOT sending the offsetPosition because LayoutManager does not
// know it.
final View view = mViewCacheExtension
.getViewForPositionAndType(this, position, type);
if (view != null) {
holder = getChildViewHolder(view);
if (holder == null) {
throw new IllegalArgumentException("getViewForPositionAndType returned"
+ " a view which does not have a ViewHolder");
} else if (holder.shouldIgnore()) {
throw new IllegalArgumentException("getViewForPositionAndType returned"
+ " a view that is ignored. You must call stopIgnoring before"
+ " returning this view.");
}
}
}
if (holder == null) { // fallback to recycler
// try recycler.
// Head to the shared pool.
if (DEBUG) {
Log.d(TAG, "getViewForPosition(" + position + ") fetching from shared "
+ "pool");
}
holder = getRecycledViewPool()
.getRecycledView(mAdapter.getItemViewType(offsetPosition));
if (holder != null) {
holder.resetInternal();
if (FORCE_INVALIDATE_DISPLAY_LIST) {
invalidateDisplayListInt(holder);
}
}
}
if (holder == null) {
holder = mAdapter.createViewHolder(RecyclerView.this,
mAdapter.getItemViewType(offsetPosition));
if (DEBUG) {
Log.d(TAG, "getViewForPosition created new ViewHolder");
}
}
}
boolean bound = false;
if (mState.isPreLayout() && holder.isBound()) {
// do not update unless we absolutely have to.
holder.mPreLayoutPosition = position;
} else if (!holder.isBound() || holder.needsUpdate() || holder.isInvalid()) {
if (DEBUG && holder.isRemoved()) {
throw new IllegalStateException("Removed holder should be bound and it should"
+ " come here only in pre-layout. Holder: " + holder);
}
final int offsetPosition = mAdapterHelper.findPositionOffset(position);
mAdapter.bindViewHolder(holder, offsetPosition);
attachAccessibilityDelegate(holder.itemView);
bound = true;
if (mState.isPreLayout()) {
holder.mPreLayoutPosition = position;
}
}

final ViewGroup.LayoutParams lp = holder.itemView.getLayoutParams();
final LayoutParams rvLayoutParams;
if (lp == null) {
rvLayoutParams = (LayoutParams) generateDefaultLayoutParams();
holder.itemView.setLayoutParams(rvLayoutParams);
} else if (!checkLayoutParams(lp)) {
rvLayoutParams = (LayoutParams) generateLayoutParams(lp);
holder.itemView.setLayoutParams(rvLayoutParams);
} else {
rvLayoutParams = (LayoutParams) lp;
}
rvLayoutParams.mViewHolder = holder;
rvLayoutParams.mPendingInvalidate = fromScrap && bound;
return holder.itemView;
}

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：

public static abstract class ViewHolder {
public final View itemView;
int mPosition = NO_POSITION;
int mOldPosition = NO_POSITION;
long mItemId = NO_ID;
int mItemViewType = INVALID_TYPE;
int mPreLayoutPosition = NO_POSITION;

// The item that this holder is shadowing during an item change event/animation
ViewHolder mShadowedHolder = null;
// The item that is shadowing this holder during an item change event/animation
ViewHolder mShadowingHolder = null;

/**
* This ViewHolder has been bound to a position; mPosition, mItemId and mItemViewType
* are all valid.
*/
static final int FLAG_BOUND = 1 << 0;

/**
* The data this ViewHolder's view reflects is stale and needs to be rebound
* by the adapter. mPosition and mItemId are consistent.
*/
static final int FLAG_UPDATE = 1 << 1;

/**
* This ViewHolder's data is invalid. The identity implied by mPosition and mItemId
* are not to be trusted and may no longer match the item view type.
* This ViewHolder must be fully rebound to different data.
*/
static final int FLAG_INVALID = 1 << 2;

/**
* This ViewHolder points at data that represents an item previously removed from the
* data set. Its view may still be used for things like outgoing animations.
*/
static final int FLAG_REMOVED = 1 << 3;

/**
* This ViewHolder should not be recycled. This flag is set via setIsRecyclable()
* and is intended to keep views around during animations.
*/
static final int FLAG_NOT_RECYCLABLE = 1 << 4;

/**
* This ViewHolder is returned from scrap which means we are expecting an addView call
* for this itemView. When returned from scrap, ViewHolder stays in the scrap list until
* the end of the layout pass and then recycled by RecyclerView if it is not added back to
* the RecyclerView.
*/
static final int FLAG_RETURNED_FROM_SCRAP = 1 << 5;

/**
* This ViewHolder's contents have changed. This flag is used as an indication that
* change animations may be used, if supported by the ItemAnimator.
*/
static final int FLAG_CHANGED = 1 << 6;

/**
* This ViewHolder is fully managed by the LayoutManager. We do not scrap, recycle or remove
* it unless LayoutManager is replaced.
* It is still fully visible to the LayoutManager.
*/
static final int FLAG_IGNORE = 1 << 7;

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转自：http://www.cnblogs.com/halzhang/p/4445145.html