RecyclerView#Adapter使用中的两个陷阱

其实就是Adapter中可以被覆写的两个方法

1、onDetachedFromRecyclerView

看下方法说明

        /**
         * Called by RecyclerView when it stops observing this Adapter.
         *
         * @param recyclerView The RecyclerView instance which stopped observing this adapter.
         * @see #onAttachedToRecyclerView(RecyclerView)
         */
        public void onDetachedFromRecyclerView(@NonNull RecyclerView recyclerView) {
        }

在RecyclerView不再观察这个Adapter时被调用。
与之对应的是onAttachedToRecyclerView：

        /**
         * Called by RecyclerView when it starts observing this Adapter.
         * 

         * Keep in mind that same adapter may be observed by multiple RecyclerViews.
         *
         * @param recyclerView The RecyclerView instance which started observing this adapter.
         * @see #onDetachedFromRecyclerView(RecyclerView)
         */
        public void onAttachedToRecyclerView(@NonNull RecyclerView recyclerView) {
        }

在RecyclerView开始观察这个Adapter时，被调用。

通常我们的理解是这样的：
页面进入时，显示RecyclerView，调用onAttachedToRecyclerView，做一些注册工作；
页面退出时，销毁RecyclerView，调用onDetachedFromRecyclerView，做一些解注册和其他资源回收的操作。

而实际上，这两个方法的调用时机是：

    public void setAdapter(Adapter adapter) {
        // bail out if layout is frozen
        ...
        setAdapterInternal(adapter, false, true);
        ...
    }

    private void setAdapterInternal(Adapter adapter, boolean compatibleWithPrevious,
            boolean removeAndRecycleViews) {
        if (mAdapter != null) {
            mAdapter.unregisterAdapterDataObserver(mObserver);
            mAdapter.onDetachedFromRecyclerView(this);//这里onDetachedFromRecyclerView
        }
        if (!compatibleWithPrevious || removeAndRecycleViews) {
            removeAndRecycleViews();
        }
        mAdapterHelper.reset();
        final Adapter oldAdapter = mAdapter;
        mAdapter = adapter;
        if (adapter != null) {
            adapter.registerAdapterDataObserver(mObserver);
            adapter.onAttachedToRecyclerView(this);//这里onAttachedToRecyclerView
        }
        if (mLayout != null) {
            mLayout.onAdapterChanged(oldAdapter, mAdapter);
        }
        mRecycler.onAdapterChanged(oldAdapter, mAdapter, compatibleWithPrevious);
        mState.mStructureChanged = true;
    }

可以看到，在调用setAdapter方法时，新设置的Adapter会调用onAttachedToRecyclerView，原有的Adapter会调用onDetachedFromRecyclerView。
所以如果覆写了onDetachedFromRecyclerView，为了确保被调用，需要在页面退出时，手动调用setAdapter(null)。

2、onViewDetachedFromWindow

方法说明：

        /**
         * Called when a view created by this adapter has been detached from its window.
         *
         * 
Becoming detached from the window is not necessarily a permanent condition;
         * the consumer of an Adapter's views may choose to cache views offscreen while they
         * are not visible, attaching and detaching them as appropriate.
         *
         * @param holder Holder of the view being detached
         */
        public void onViewDetachedFromWindow(@NonNull VH holder) {
        }

简言之，就是当itemView被从window上detach时调用。看起来很美好，与之对应的方法是：

        /**
         * Called when a view created by this adapter has been attached to a window.
         *
         * 
This can be used as a reasonable signal that the view is about to be seen
         * by the user. If the adapter previously freed any resources in
         * {@link #onViewDetachedFromWindow(RecyclerView.ViewHolder) onViewDetachedFromWindow}
         * those resources should be restored here.
         *
         * @param holder Holder of the view being attached
         */
        public void onViewAttachedToWindow(@NonNull VH holder) {
        }

然后我们也如第一个方法般调用了：
onViewAttachedToWindow中做一些注册工作；
onViewDetachedFromWindow中做一些解注册和释放资源的工作。

在RecyclerView正常滚动时，这两个方法都会被调用。然而页面退出时，onViewDetachedFromWindow并不会被调用！

追根溯源，会发现症结在LinearLayoutManager中：

    @Override
    public void onDetachedFromWindow(RecyclerView view, RecyclerView.Recycler recycler) {
        super.onDetachedFromWindow(view, recycler);
        if (mRecycleChildrenOnDetach) {
            removeAndRecycleAllViews(recycler);
            recycler.clear();
        }
    }

默认mRecycleChildrenOnDetach=false。我们需要调用setRecycleChildrenOnDetach(true)才能实现在页面退出时，依然调用onViewDetachedFromWindow方法。

整合RecyclerView

可以设计一个RecyclerView的基类，在基类中做如下处理：

public class BaseRecyclerView extends RecyclerView implements LifecycleObserver {
    public BaseRecyclerView(@NonNull Context context) {
        super(context);
        init(context);
    }

    public BaseRecyclerView(@NonNull Context context, @Nullable AttributeSet attrs) {
        super(context, attrs);
        init(context);
    }

    public BaseRecyclerView(@NonNull Context context, @Nullable AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
        init(context);
    }

    private void init(Context context) {
        if (context instanceof LifecycleOwner) {
            ((LifecycleOwner) context).getLifecycle().addObserver(this);
        }
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
    public void onDestory() {
        /*
         确保Adapter#onDetachedFromRecyclerView被调用
         */
        setAdapter(null);
    }

    @Override
    public void setLayoutManager(LayoutManager layoutManager) {
        super.setLayoutManager(layoutManager);
        if (layoutManager instanceof LinearLayoutManager) {
            /*
            确保Adapter#onViewDetachedFromWindow被调用
             */
            ((LinearLayoutManager) layoutManager).setRecycleChildrenOnDetach(true);
        }
    }
}