Is it really hard to understand? What level of cache is the recyclerview caching mechanism?

0. Preface

RecyclerView Cache mechanism , It can be said that he is a regular visitor in the interview . More Than This , In use , If you understand this caching mechanism , Then we can make better use of its features for development .

that , We'll do it in a situational way , Explain RecyclerView Cache mechanism . Two common scenarios are ：

1. slide RecyclerView The cache mechanism under the

2.RecyclerView Caching mechanism for initial loading process

This article will explain slide RecyclerView Next Cache mechanism

1. Cache hierarchy

Background knowledge ： Responsible for recycling and reuse ViewHolder The class is Recycler, Several member variables of this class are mainly responsible for caching . Let's post some source code to see （ The following comments on the source code （ And my notes ）, Very important , Remember to look carefully ）

/** * A Recycler is responsible for managing scrapped or detached item views for reuse. * A "scrapped" view is a view that is still attached to its parent RecyclerView but that has been marked for removal or reuse. * * Typical use of a Recycler by a RecyclerView.LayoutManager will be to obtain views * for an adapter's data set representing the data at a given position or item ID. * If the view to be reused is considered "dirty" the adapter will be asked to rebind it. * If not, the view can be quickly reused by the LayoutManager with no further work. * Clean views that have not requested layout may be repositioned by a LayoutManager without remeasurement. */
public final class Recycler {
    
    final ArrayList<ViewHolder> mAttachedScrap = new ArrayList<>();//  Store in visible range  ViewHolder ( But in  onLayoutChildren  When , Will bring all  View  Will be cached here ),  Reuse from here  ViewHolder  If  position  perhaps  id  On the corresponding , You do not need to rebind the data .
    ArrayList<ViewHolder> mChangedScrap = null;//  Stored within the visible range and the data has changed  ViewHolder, Reuse from here  ViewHolder  You need to rebind the data .

    final ArrayList<ViewHolder> mCachedViews = new ArrayList<ViewHolder>(); //  Deposit  remove  Dropped  ViewHolder, Reuse from here  ViewHolder  If  position  perhaps  id  On the corresponding , You do not need to rebind the data .

    private int mRequestedCacheMax = DEFAULT_CACHE_SIZE; //  The default value is  2
    int mViewCacheMax = DEFAULT_CACHE_SIZE; //  The default value is  2

    RecycledViewPool mRecyclerPool; //  Deposit  remove  fall , And reset the data  ViewHolder, Reuse from here  ViewHolder  You need to rebind the data . //  The default size is  5 

    private ViewCacheExtension mViewCacheExtension; //  Custom caching 
    }

As for how many levels of cache are there , I don't think this problem is important . Some people say three floors , Some people say the fourth floor . Some people say three floors , Because I think the custom layer , No RecyclerView Realized , So it's not ; Some people think that Scrap Not a real cache , So it's not .

From the source code , I prefer the latter ,Scrap Not a layer of cache . Because in the source code ,mCachedViews go by the name of first-level. As for why Scrap Not the first floor , My understanding is that ： Because this floor is just detach 了 , did not remove, Therefore, there is no concept of cache size in this layer , As long as the rules are met, they will join in .

// Search the first-level cache
final int cacheSize = mCachedViews.size();

2. Scene analysis ： Sliding RecyclerView Caching mechanisms

adopt Android Studio Of Profiles Tools , We can see the calling process

The entrance is ouTouchEvent

By table , Briefly explain what the process above is doing ？

Through the above table , We know . The most important thing is scrollBy Called in fill The method of the . So let's see fill What are you doing ？ Sliding out View Where did you end up ？ Sliding in View How did you get it ？（ With this question , Let's read the source code together ！ Be sure to bring ）, The source code only leaves the core part

int fill(RecyclerView.Recycler recycler, LayoutState layoutState,
        RecyclerView.State state, boolean stopOnFocusable) {
    // max offset we should set is mFastScroll + available
    final int start = layoutState.mAvailable;
    // The judgment of this statement block is preferred , Judge whether the current state is rolling , If so , The trigger  recycleByLayoutState  Method 
    if (layoutState.mScrollingOffset != LayoutState.SCROLLING_OFFSET_NaN) {
        // TODO ugly bug fix. should not happen
        if (layoutState.mAvailable < 0) {
            layoutState.mScrollingOffset += layoutState.mAvailable;
        }
        //  analysis 1---- Recycling 
        recycleByLayoutState(recycler, layoutState);
        }
    while ((layoutState.mInfinite || remainingSpace > 0) && layoutState.hasMore(state)) {
        // analysis 2---- Reuse 
        layoutChunk(recycler, state, layoutState, layoutChunkResult);
    }
}

---

//  analysis 1---- Recycling  
//  Through step-by-step tracking , We found that the last call was  removeAndRecycleViewAt() 
public void removeAndRecycleViewAt(int index, @NonNull Recycler recycler) {
    final View view = getChildAt(index);
    // analysis 1-1
    removeViewAt(index);
    // analysis 1-2
    recycler.recycleView(view);
}
//  analysis 1-1
//  from  RecyclerView  Remove a  View 
public void removeViewAt(int index) {
    final View child = getChildAt(index);
    if (child != null) {
        mChildHelper.removeViewAt(index);
    }
}
// analysis 1-2 
// recycler.recycleView(view)  The final call is  recycleViewHolderInternal(holder)  To recycle  VH (ViewHolder)
void recycleViewHolderInternal(ViewHolder holder) {
    if (forceRecycle || holder.isRecyclable()) {
        // Judge whether it meets the requirements of putting in  mCachedViews 
        if (mViewCacheMax > 0 && !holder.hasAnyOfTheFlags(ViewHolder.FLAG_INVALID| ViewHolder.FLAG_REMOVED| ViewHolder.FLAG_UPDATE| ViewHolder.FLAG_ADAPTER_POSITION_UNKNOWN)){
            //  Judge  mCachedViews  Whether is full 
            if (cachedViewSize >= mViewCacheMax && cachedViewSize > 0) {
                //  If full, subscript 0（ That is, the first to join ） remove , Also add it to  RecyclerPool  in 
                recycleCachedViewAt(0);
                cachedViewSize--;
                }  
            mCachedViews.add(targetCacheIndex, holder);
            cached = true;
            }
        // If the above conditions are not met , Save directly into  RecyclerPool  in     
        if (!cached) {
            addViewHolderToRecycledViewPool(holder, true);
            recycled = true;
         } 
     }
}

---

// analysis 2
void layoutChunk(RecyclerView.Recycler recycler, RecyclerView.State state,
        LayoutState layoutState, LayoutChunkResult result) {
    // analysis 2-1
    View view = layoutState.next(recycler);
    if (layoutState.mScrapList == null) {
        if (mShouldReverseLayout == (layoutState.mLayoutDirection
                == LayoutState.LAYOUT_START)) {
            // Add to  RecyclerView  On 
            addView(view);
        } else {
            addView(view, 0);
        }
    }
}
// analysis 2-1
//layoutState.next(recycler)  The last thing to call is  tryGetViewHolderForPositionByDeadline()  This method is exactly the same   Reuse   The core approach 
ViewHolder tryGetViewHolderForPositionByDeadline(int position,
        boolean dryRun, long deadlineNs) {
    // 0) If there is a changed scrap, try to find from there
    //  for example ： We call  notifyItemChanged  When the method is used 
    if (mState.isPreLayout()) {
        //  If it is  changed  Of  ViewHolder  So let's start with  mChangedScrap  In looking for 
        holder = getChangedScrapViewForPosition(position);
        fromScrapOrHiddenOrCache = holder != null;
    }
    // 1) Find by position from scrap/hidden list/cache
    if (holder == null) {
        // If you don't find it up there (holder == null), Then try to pass from  pos  stay  mAttachedScrap/ mHiddenViews / mCachedViews  In order to get 
        holder = getScrapOrHiddenOrCachedHolderForPosition(position, dryRun);
    }
    if (holder == null) {
        // 2) Find from scrap/cache via stable ids, if exists
        if (mAdapter.hasStableIds()) {
            // If you don't find it up there (holder == null), Then try to pass from  id  stay  mAttachedScrap/ mCachedViews  In order to get 
            holder = getScrapOrCachedViewForId(mAdapter.getItemId(offsetPosition),
        }
        if (holder == null && mViewCacheExtension != null) {
            // This is obtained from the user-defined cache , Ignore 
        }
        // If you don't find anything on it (holder == null), Then try in  RecycledViewPool  In order to get 
        if (holder == null) { // fallback to pool
            holder = getRecycledViewPool().getRecycledView(type);
            if (holder != null) {
                // Here is , To empty the data 
                holder.resetInternal();
            }
        }
        // If in  Scrap / Hidden / Cache / RecycledViewPool  None of them , Then you can only create one .
        if (holder == null) {
            holder = mAdapter.createViewHolder(RecyclerView.this, type);
        }
    }
    return holder;
}

3. summary

Make a summary , Before analyzing the source code , We asked three questions , Let's see what the answer is

Q： So let's see  fill  What are you doing ？

A： In fact, it's analysis 1（ Recycling  ViewHolder ） +  analysis  2 (  Reuse  ViewHolder )

---

Q： Sliding out  View  Where did you end up ？

A： First try to recycle to  mCachedViews  in , unsuccessful , Then recycle to  RecycledViewPool  in .

---

Q： Sliding in  View  How did you get it ？

A： If it is  isPreLayout  First, from  mChangedScrap  Trying to get .

 Not obtained , Again from  mAttachedScrap / mHiddenViews / mCachedViews （ adopt  position ）  Trying to get 

 Not obtained , Again from  mAttachedScrap / mCachedViews （ adopt  id） Trying to get 

 Not obtained , Again from   Attempt to get... From custom cache 

 Not obtained , Again from  RecycledViewPool  Trying to get 

 Not obtained , Create a new  ViewHolder

Recommended reading ：

Really ！ Interview with ALI 4 round ,RecyclerView Seven times ！！！

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