Lint error “Do not treat position as fixed; only use immediately…”

问题:

I'm contributing to open source library and got lint error "Do not treat position as fixed; only use immediately and call holder.getAdapterPosition() to look it up later" for this code:

  @Override public void onBindViewHolder(RecyclerView.ViewHolder holder, int position) {     mAdapter.onBindViewHolder(holder, position);      if (!isFirstOnly || position > mLastPosition) {       for (Animator anim : getAnimators(holder.itemView)) {         anim.setDuration(mDuration).start();         anim.setInterpolator(mInterpolator);       }       mLastPosition = position;     } else {       ViewHelper.clear(holder.itemView);     }   } 

I've checked that it is because the position is saved for the future use. It is a question to library creator why they need this logic. But issue disappeared when I change the usage of the position to the usage holder.getAdapterPosition():

  @Override public void onBindViewHolder(RecyclerView.ViewHolder holder, int position) {     mAdapter.onBindViewHolder(holder, position);      if (!isFirstOnly || holder.getAdapterPosition() > mLastPosition) {       for (Animator anim : getAnimators(holder.itemView)) {         anim.setDuration(mDuration).start();         anim.setInterpolator(mInterpolator);       }       mLastPosition = holder.getAdapterPosition();     } else {       ViewHelper.clear(holder.itemView);     }   } 

I assume that conceptually it didn't change much but lint is satisfied now. Why?

回答1:

The documentation of RecyclerView.Adapter.onBindViewHolder() states:

Note that unlike ListView, RecyclerView will not call this method again if the position of the item changes in the data set unless the item itself is invalidated or the new position cannot be determined. For this reason, you should only use the position parameter while acquiring the related data item inside this method and should not keep a copy of it. If you need the position of an item later on (e.g. in a click listener), use getAdapterPosition() which will have the updated adapter position

So, technically items may be re-arranged and binding will not be necessary because items are not invalidated yet. The position variable received holds true only for the scope of bind function and will not always point to correct position in the data set. That's why the function getAdapterPosition() must be called everytime updated position is needed.

IMHO, mLastPosition = holder.getAdapterPosition(); is still potentially wrong. Because item may be re-arranged and mLastPosition still points to old position.

About why lint is silent, perhaps Lint rule is not that thorough. Its only checking whether position parameter is being copied or not.

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