[移动开发]关于RecyclerView的Adapter的notifyItemInserted（）的一些分析

public final void notifyItemInserted(int position) {

mObservable.notifyItemRangeInserted(position, 1);

}

那这个mObservable到底是什么东西呢？阅读Adapter里面的代码可知，它是里面的一个成员变量，继承于Observable

public static abstract class Adapter<VH extends ViewHolder> {

private final AdapterDataObservable mObservable = new AdapterDataObservable();

}

我们先来看Observable里面的代码，我们知道这是一个典型的观察者模式的实现。

如果对观察者模式还不了解的，建议先阅读这篇文章：观察者设计模式 Vs 事件委托（java）

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 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);

}

}

_/

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**

• 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();

}

}

}

接着我们来看这个mObservable是在什么时候初始化的呢，我们回到setAdapter（）这个方法

public void setAdapter(Adapter adapter) {

// bail out if layout is frozen

setLayoutFrozen(false);

setAdapterInternal(adapter, false, true);

requestLayout();

}

private void setAdapterInternal(Adapter adapter, boolean compatibleWithPrevious,

boolean removeAndRecycleViews) {

if (mAdapter != null) {

mAdapter.unregisterAdapterDataObserver(mObserver);

mAdapter.onDetachedFromRecyclerView(this);

}

if (!compatibleWithPrevious || removeAndRecycleViews) {

// end all running animations

if (mItemAnimator != null) {

mItemAnimator.endAnimations();

}

// Since animations are ended, mLayout.children should be equal to

// recyclerView.children. This may not be true if item animator’s end does not work as

// expected. (e.g. not release children instantly). It is safer to use mLayout’s child

// count.

if (mLayout != null) {

mLayout.removeAndRecycleAllViews(mRecycler);

mLayout.removeAndRecycleScrapInt(mRecycler);

}

// we should clear it here before adapters are swapped to ensure correct callbacks.

mRecycler.clear();

}

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;

markKnownViewsInvalid();

}

在setAdapterInternal里面主要逻辑就是判断apdter是否为空，不为空的话调用mAdapter.unregisterAdapterDataObserver(mObserver);反注销掉mObserver 接着再调用adapter.registerAdapterDataObserver(mObserver); adapter.onAttachedToRecyclerView(this); 重新注册mObserver和依附到recycleView中。

确定了AdapterDataObservable这个类是什么时候初始化以后，接着我们再回到AdapterDataObservable 这个类

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, 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);

}

}

}

阅读了这个类的源码以后，我们惊喜地发现，这个类里面的notify方法是不是跟adapter里面的notify方法很相似，让我们来看看这个类里面的notify方法具体做了什么？ 以insert为例。

public final void notifyItemInserted(int position) {

mObservable.notifyItemRangeInserted(position, 1);

}