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
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 {
@NonNull
private 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} */
@Override
public void onInserted(int position, int count) {
mAdapter.notifyItemRangeInserted(position, count);
}
/** {@inheritDoc} */
@Override
public void onRemoved(int position, int count) {
mAdapter.notifyItemRangeRemoved(position, count);
}
/** {@inheritDoc} */
@Override
public void onMoved(int fromPosition, int toPosition) {
mAdapter.notifyItemMoved(fromPosition, toPosition);
}
/** {@inheritDoc} */
@Override
public 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 {
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 {
/**
* The list of observers. An observer can be in the list at most
* once and will never be null.
*/
protected final ArrayList mObservers = new ArrayList();
/**
* 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.
*
* 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 frozen
setLayoutFrozen(false);
setAdapterInternal(adapter, true, removeAndRecycleExistingViews);
processDataSetCompletelyChanged(true);
requestLayout();
}
/**
* Set a new adapter to provide child views on demand.
*
* 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 frozen
setLayoutFrozen(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.
*
* 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.
*
* Components registering observers with an adapter are responsible for
* {@link #unregisterAdapterDataObserver(RecyclerView.AdapterDataObserver)
* unregistering} those observers when finished.
*
* @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() {
}
@Override
public void onChanged() {
assertNotInLayoutOrScroll(null);
mState.mStructureChanged = true;
processDataSetCompletelyChanged(true);
if (!mAdapterHelper.hasPendingUpdates()) {
requestLayout();
}
}
@Override
public void onItemRangeChanged(int positionStart, int itemCount, Object payload) {
assertNotInLayoutOrScroll(null);
if (mAdapterHelper.onItemRangeChanged(positionStart, itemCount, payload)) {
triggerUpdateProcessor();
}
}
@Override
public void onItemRangeInserted(int positionStart, int itemCount) {
assertNotInLayoutOrScroll(null);
if (mAdapterHelper.onItemRangeInserted(positionStart, itemCount)) {
triggerUpdateProcessor();
}
}
@Override
public void onItemRangeRemoved(int positionStart, int itemCount) {
assertNotInLayoutOrScroll(null);
if (mAdapterHelper.onItemRangeRemoved(positionStart, itemCount)) {
triggerUpdateProcessor();
}
}
@Override
public 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;
}
@Override
public void onChanged() {
adapterDataObserver.onChanged();
}
@Override
public void onItemRangeChanged(int positionStart, int itemCount) {
adapterDataObserver.onItemRangeChanged(positionStart + headerCount, itemCount);
}
@Override
public void onItemRangeChanged(int positionStart, int itemCount, @Nullable Object payload) {
adapterDataObserver.onItemRangeChanged(positionStart + headerCount, itemCount, payload);
}
@Override
public void onItemRangeInserted(int positionStart, int itemCount) {
adapterDataObserver.onItemRangeInserted(positionStart + headerCount, itemCount);
}
@Override
public void onItemRangeRemoved(int positionStart, int itemCount) {
adapterDataObserver.onItemRangeRemoved(positionStart + headerCount, itemCount);
}
@Override
public 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。