本文主要是介绍Android RecyclerView + Paging Library 添加头部刷新会自动滚动的问题分析及解决，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！
一、前言
最近在做一款应用，因为 api 涉及到分页的，所以选择用 RecyclerView + Paging 去做，能省去向下滚动时去处理加载下一页的数据的逻辑，Paging 会自动去加载下一页的内容，首页要做个 Banner, 所以把 Banner 作为 RecyclerView 的 头添加进去，可是添加后下面的数据加载完就会自动滚到下面去，如下图，一下拉刷新，加载完数据后就滚到下面去了
二、问题分析
当我去掉头部刷新，一切是正常的，
所以原因应该就是添加了 Header,导致 position 不准确， RecyclerView 不知道有 Header 的存在，一刷新插入、删除等操作是从 0 开始的，但 Header 在前面霸占着位置了（陈独秀同学在前面站着不坐下），所以插入、删除等操作不要动到 Header 的位置，使其操作正确的 position, 那怎么让其不去动到 Heasder 的位置呢？只有看看源码能不能找到解决的办法了， RecyclerView + Paging Library 用的 Adapter 用的是继承 PagedListAdapter 的，
PagedListAdapter 是使用了 DiffUtil 的，DiffUtil 里的插入、删除等操作是用到 AdapterListUpdateCallback 这个类
AdapterListUpdateCallback 这个类里插入、删除等操作调用的是 RecyclerView.Adapter mAdapter 的，
/*** ListUpdateCallback that dispatches update events to the given adapter.** @see DiffUtil.DiffResult#dispatchUpdatesTo(RecyclerView.Adapter)*/
public final class AdapterListUpdateCallback implements ListUpdateCallback {@NonNullprivate final RecyclerView.Adapter mAdapter;/*** Creates an AdapterListUpdateCallback that will dispatch update events to the given adapter.** @param adapter The Adapter to send updates to.*/public AdapterListUpdateCallback(@NonNull RecyclerView.Adapter adapter) {mAdapter = adapter;}/** {@inheritDoc} */@Overridepublic void onInserted(int position, int count) {mAdapter.notifyItemRangeInserted(position, count);}/** {@inheritDoc} */@Overridepublic void onRemoved(int position, int count) {mAdapter.notifyItemRangeRemoved(position, count);}/** {@inheritDoc} */@Overridepublic void onMoved(int fromPosition, int toPosition) {mAdapter.notifyItemMoved(fromPosition, toPosition);}/** {@inheritDoc} */@Overridepublic void onChanged(int position, int count, Object payload) {mAdapter.notifyItemRangeChanged(position, count, payload);}
}
RecyclerView.Adapter 里用的是 mObservable 的插入、删除等操作，
这个 mObservable 是 AdapterDataObservable 类型的，
AdapterDataObservable 是继承 Observable 的
static class AdapterDataObservable extends Observable<AdapterDataObserver> {public boolean hasObservers() {return !mObservers.isEmpty();}public void notifyChanged() {// since onChanged() is implemented by the app, it could do anything, including// removing itself from {@link mObservers} - and that could cause problems if// an iterator is used on the ArrayList {@link mObservers}.// to avoid such problems, just march thru the list in the reverse order.for (int i = mObservers.size() - 1; i >= 0; i--) {mObservers.get(i).onChanged();}}public void notifyItemRangeChanged(int positionStart, int itemCount) {notifyItemRangeChanged(positionStart, itemCount, null);}public void notifyItemRangeChanged(int positionStart, int itemCount,@Nullable Object payload) {// since onItemRangeChanged() is implemented by the app, it could do anything, including// removing itself from {@link mObservers} - and that could cause problems if// an iterator is used on the ArrayList {@link mObservers}.// to avoid such problems, just march thru the list in the reverse order.for (int i = mObservers.size() - 1; i >= 0; i--) {mObservers.get(i).onItemRangeChanged(positionStart, itemCount, payload);}}public void notifyItemRangeInserted(int positionStart, int itemCount) {// since onItemRangeInserted() is implemented by the app, it could do anything,// including removing itself from {@link mObservers} - and that could cause problems if// an iterator is used on the ArrayList {@link mObservers}.// to avoid such problems, just march thru the list in the reverse order.for (int i = mObservers.size() - 1; i >= 0; i--) {mObservers.get(i).onItemRangeInserted(positionStart, itemCount);}}public void notifyItemRangeRemoved(int positionStart, int itemCount) {// since onItemRangeRemoved() is implemented by the app, it could do anything, including// removing itself from {@link mObservers} - and that could cause problems if// an iterator is used on the ArrayList {@link mObservers}.// to avoid such problems, just march thru the list in the reverse order.for (int i = mObservers.size() - 1; i >= 0; i--) {mObservers.get(i).onItemRangeRemoved(positionStart, itemCount);}}public void notifyItemMoved(int fromPosition, int toPosition) {for (int i = mObservers.size() - 1; i >= 0; i--) {mObservers.get(i).onItemRangeMoved(fromPosition, toPosition, 1);}}
}而 Observable 是个典型的观察者模式被观察者的写法
/*** Provides methods for registering or unregistering arbitrary observers in an {@link ArrayList}.** This abstract class is intended to be subclassed and specialized to maintain* a registry of observers of specific types and dispatch notifications to them.** @param T The observer type.*/
public abstract class Observable<T> {/*** The list of observers. An observer can be in the list at most* once and will never be null.*/protected final ArrayList<T> mObservers = new ArrayList<T>();/*** Adds an observer to the list. The observer cannot be null and it must not already* be registered.* @param observer the observer to register* @throws IllegalArgumentException the observer is null* @throws IllegalStateException the observer is already registered*/public void registerObserver(T observer) {if (observer == null) {throw new IllegalArgumentException("The observer is null.");}synchronized(mObservers) {if (mObservers.contains(observer)) {throw new IllegalStateException("Observer " + observer + " is already registered.");}mObservers.add(observer);}}/*** Removes a previously registered observer. The observer must not be null and it* must already have been registered.* @param observer the observer to unregister* @throws IllegalArgumentException the observer is null* @throws IllegalStateException the observer is not yet registered*/public void unregisterObserver(T observer) {if (observer == null) {throw new IllegalArgumentException("The observer is null.");}synchronized(mObservers) {int index = mObservers.indexOf(observer);if (index == -1) {throw new IllegalStateException("Observer " + observer + " was not registered.");}mObservers.remove(index);}}/*** Remove all registered observers.*/public void unregisterAll() {synchronized(mObservers) {mObservers.clear();}}
}那么是谁及什么时候注册到这个 mObservers 呢？什么时候就是在 RecyclerView setAdapter 或 swapAdapter 的时候
/*** Swaps the current adapter with the provided one. It is similar to* {@link #setAdapter(Adapter)} but assumes existing adapter and the new adapter uses the same* {@link ViewHolder} and does not clear the RecycledViewPool.* <p>* Note that it still calls onAdapterChanged callbacks.** @param adapter The new adapter to set, or null to set no adapter.* @param removeAndRecycleExistingViews If set to true, RecyclerView will recycle all existing* Views. If adapters have stable ids and/or you want to* animate the disappearing views, you may prefer to set* this to false.* @see #setAdapter(Adapter)*/
public void swapAdapter(Adapter adapter, boolean removeAndRecycleExistingViews) {// bail out if layout is frozensetLayoutFrozen(false);setAdapterInternal(adapter, true, removeAndRecycleExistingViews);processDataSetCompletelyChanged(true);requestLayout();
}/*** Set a new adapter to provide child views on demand.* <p>* When adapter is changed, all existing views are recycled back to the pool. If the pool has* only one adapter, it will be cleared.** @param adapter The new adapter to set, or null to set no adapter.* @see #swapAdapter(Adapter, boolean)*/
public void setAdapter(Adapter adapter) {// bail out if layout is frozensetLayoutFrozen(false);setAdapterInternal(adapter, false, true);processDataSetCompletelyChanged(false);requestLayout();
}/*** Replaces the current adapter with the new one and triggers listeners.* @param adapter The new adapter* @param compatibleWithPrevious If true, the new adapter is using the same View Holders and* item types with the current adapter (helps us avoid cache* invalidation).* @param removeAndRecycleViews If true, we'll remove and recycle all existing views. If* compatibleWithPrevious is false, this parameter is ignored.*/
private void setAdapterInternal(Adapter adapter, boolean compatibleWithPrevious,boolean removeAndRecycleViews) {if (mAdapter != null) {mAdapter.unregisterAdapterDataObserver(mObserver);mAdapter.onDetachedFromRecyclerView(this);}if (!compatibleWithPrevious || removeAndRecycleViews) {removeAndRecycleViews();}mAdapterHelper.reset();final Adapter oldAdapter = mAdapter;mAdapter = adapter;if (adapter != null) {adapter.registerAdapterDataObserver(mObserver);adapter.onAttachedToRecyclerView(this);}if (mLayout != null) {mLayout.onAdapterChanged(oldAdapter, mAdapter);}mRecycler.onAdapterChanged(oldAdapter, mAdapter, compatibleWithPrevious);mState.mStructureChanged = true;
}
Adapter 的 registerAdapterDataObserver 方法
/*** Register a new observer to listen for data changes.** <p>The adapter may publish a variety of events describing specific changes.* Not all adapters may support all change types and some may fall back to a generic* {@link android.support.v7.widget.RecyclerView.AdapterDataObserver#onChanged()* "something changed"} event if more specific data is not available.</p>** <p>Components registering observers with an adapter are responsible for* {@link #unregisterAdapterDataObserver(RecyclerView.AdapterDataObserver)* unregistering} those observers when finished.</p>** @param observer Observer to register** @see #unregisterAdapterDataObserver(RecyclerView.AdapterDataObserver)*/
public void registerAdapterDataObserver(@NonNull AdapterDataObserver observer) {mObservable.registerObserver(observer);
}那 adapter.registerAdapterDataObserver(mObserver) 这句中的 mObserver 是谁？
原来 mObserver 是 RecyclerViewDataObserver 类型的
private class RecyclerViewDataObserver extends AdapterDataObserver {RecyclerViewDataObserver() {}@Overridepublic void onChanged() {assertNotInLayoutOrScroll(null);mState.mStructureChanged = true;processDataSetCompletelyChanged(true);if (!mAdapterHelper.hasPendingUpdates()) {requestLayout();}}@Overridepublic void onItemRangeChanged(int positionStart, int itemCount, Object payload) {assertNotInLayoutOrScroll(null);if (mAdapterHelper.onItemRangeChanged(positionStart, itemCount, payload)) {triggerUpdateProcessor();}}@Overridepublic void onItemRangeInserted(int positionStart, int itemCount) {assertNotInLayoutOrScroll(null);if (mAdapterHelper.onItemRangeInserted(positionStart, itemCount)) {triggerUpdateProcessor();}}@Overridepublic void onItemRangeRemoved(int positionStart, int itemCount) {assertNotInLayoutOrScroll(null);if (mAdapterHelper.onItemRangeRemoved(positionStart, itemCount)) {triggerUpdateProcessor();}}@Overridepublic void onItemRangeMoved(int fromPosition, int toPosition, int itemCount) {assertNotInLayoutOrScroll(null);if (mAdapterHelper.onItemRangeMoved(fromPosition, toPosition, itemCount)) {triggerUpdateProcessor();}}void triggerUpdateProcessor() {if (POST_UPDATES_ON_ANIMATION && mHasFixedSize && mIsAttached) {ViewCompat.postOnAnimation(RecyclerView.this, mUpdateChildViewsRunnable);} else {mAdapterUpdateDuringMeasure = true;requestLayout();}}
}RecyclerViewDataObserver 是继承 AdapterDataObserver 的
/*** Observer base class for watching changes to an {@link Adapter}.* See {@link Adapter#registerAdapterDataObserver(AdapterDataObserver)}.*/
public abstract static class AdapterDataObserver {public void onChanged() {// Do nothing}public void onItemRangeChanged(int positionStart, int itemCount) {// do nothing}public void onItemRangeChanged(int positionStart, int itemCount, @Nullable Object payload) {// fallback to onItemRangeChanged(positionStart, itemCount) if app// does not override this method.onItemRangeChanged(positionStart, itemCount);}public void onItemRangeInserted(int positionStart, int itemCount) {// do nothing}public void onItemRangeRemoved(int positionStart, int itemCount) {// do nothing}public void onItemRangeMoved(int fromPosition, int toPosition, int itemCount) {// do nothing}
}RecyclerViewDataObserver 里用的是 mAdapterHelper 的插入、删除等操作，mAdapterHelper 是 AdapterHelper 类型的
AdapterHelper 是包访问权限的
突破口在 Adapter 的 registerAdapterDataObserver 方法，这个方法是 public 的且不是 final 的，我们可以在自己的 Adapter 覆盖 registerAdapterDataObserver 这个方法，由于 RecyclerViewDataObserver 这个类是 private 的，但 AdapterDataObserver 是 public 的，而且 registerAdapterDataObserver 要的是 AdapterDataObserver 类型的参数，我们可以写个类继承 AdapterDataObserver, 把传进 registerAdapterDataObserver 的 observer 参数和 Header 的个数传进去，在对目标 AdapterDataObserver 操作之前进行偷梁换柱，把 Header 的个数加进去，分析到这里问题就已经可以解决了。
三、问题解决
写个 AdapterDataObserverProxy 类继承 RecyclerView.AdapterDataObserver
class AdapterDataObserverProxy extends RecyclerView.AdapterDataObserver {RecyclerView.AdapterDataObserver adapterDataObserver;int headerCount;public ArticleDataObserver(RecyclerView.AdapterDataObserver adapterDataObserver, int headerCount) {this.adapterDataObserver = adapterDataObserver;this.headerCount = headerCount;}@Overridepublic void onChanged() {adapterDataObserver.onChanged();}@Overridepublic void onItemRangeChanged(int positionStart, int itemCount) {adapterDataObserver.onItemRangeChanged(positionStart + headerCount, itemCount);}@Overridepublic void onItemRangeChanged(int positionStart, int itemCount, @Nullable Object payload) {adapterDataObserver.onItemRangeChanged(positionStart + headerCount, itemCount, payload);}@Overridepublic void onItemRangeInserted(int positionStart, int itemCount) {adapterDataObserver.onItemRangeInserted(positionStart + headerCount, itemCount);}@Overridepublic void onItemRangeRemoved(int positionStart, int itemCount) {adapterDataObserver.onItemRangeRemoved(positionStart + headerCount, itemCount);}@Overridepublic void onItemRangeMoved(int fromPosition, int toPosition, int itemCount) {super.onItemRangeMoved(fromPosition + headerCount, toPosition + headerCount, itemCount);}
}
因为 registerAdapterDataObserver 是针对 AdapterDataObserver 进行处理的，所以这里用 AdapterDataObserverProxy 去做代理，对真正的 AdapterDataObserver 操作前把 Header 的个数加进去，所以覆盖后的 registerAdapterDataObserver 就可以这样写，
@Override
public void registerAdapterDataObserver(@NonNull RecyclerView.AdapterDataObserver observer) {super.registerAdapterDataObserver(new AdapterDataObserverProxy(observer, getHeaderCount()));
}getHeaderCount() 是 Header 的个数，具体怎么写就看自己的情况了，当然覆盖后的 registerAdapterDataObserver 这样写的话就要在 RecyclerView setAdapter 之前先把 Header 添加到 Adapter 去，毕竟 registerAdapterDataObserver 是在 setAdapter 里调用的。
四、后记
RecyclerView + Paging Library 用于做分页虽然好，但还是有很多坑的，现在还有坑没填，用了 RecyclerView + Paging Library 想做个添加 item 和删除 item 的功能还没有头绪，PagedList 虽然继承 AbstractList，但 add 和 remove 方法没有去实现，调用会出现 UnsupportedOperationException，脑阔疼。
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