Android中RecyclerView源码解析一(Adapter与RecyclerView是怎么关联的？)

小白也能看懂源码系列

先看下面这个普通的抽象类：Observable它其实里面就一个ArrayList，只做了一个add与remove的操作而已。

Observable.java


public abstract class Observable{
protected final ArrayList mObservers = new ArrayList();
public void registerObserver(T observer) {
//④
mObservers.add(observer);
}
public void unregisterObserver(T observer) {
mObservers.remove(index);
}
public void unregisterAll(T observer) {
mObservers.clear();
}
}
第二步

直接上精简版的RecyclerView类

RecyclerView.java

public class RecyclerView extends ViewGroup implements ScrollingView, NestedScrollingChild2 {
Adapter mAdapter;
private final RecyclerViewDataObserver mObserver = new RecyclerViewDataObserver();
public void setAdapter(Adapter adapter) {
setAdapterInternal(adapter, false, true);
}
private void setAdapterInternal(Adapter adapter, boolean compatibleWithPrevious, boolean removeAndRecycleViews) {
if (mAdapter != null) {
mAdapter.unregisterAdapterDataObserver(mObserver);
}
final Adapter oldAdapter = mAdapter;
mAdapter = adapter;
if (adapter != null) {
//①注意参数哈，其实就是RecyclerView里的成员变量RecyclerViewDataObserver mObserver
adapter.registerAdapterDataObserver(mObserver);
}
}
//内部类：AdapterDataObservable
static class AdapterDataObservable extends Observable{
//③其实就是调用的super.registerObserver(RecyclerViewDataObserver mObserver)，super就是指第一步那个Observable类
//此时：RecyclerView.mAdapter.mObservable.mObservers.get(0) = mObserver
//mObservers指的是其父类的ArrayList而已
public void notifyChanged() {
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) {
for (int i = mObservers.size() - 1; i >= 0; i--) {
mObservers.get(i).onItemRangeChanged(positionStart, itemCount, payload);
}
}
……
}
//内部类：RecyclerViewDataObserver
private class RecyclerViewDataObserver extends AdapterDataObserver {
RecyclerViewDataObserver() {} @Override
public void onChanged() {
mAdapterHelper.hasPendingUpdates();
}
@Override
public void onItemRangeChanged(int positionStart, int itemCount, Object payload) {
mAdapterHelper.onItemRangeChanged(positionStart, itemCount, payload)
}
……
}
//内部类：Adapter
public abstract static class Adapter {
private final AdapterDataObservable mObservable = new AdapterDataObservable();
public abstract VH onCreateViewHolder(@NonNull ViewGroup parent, int viewType);
public abstract void onBindViewHolder(@NonNull VH holder, int position);
public abstract int getItemCount();
public int getItemViewType(int position) {
return 0;
}
public long getItemId(int position) {
return NO_ID;
}
public void onViewRecycled(@NonNull VH holder) {
//空实现
}
public void registerAdapterDataObserver(@NonNull AdapterDataObserver observer) {
//②
mObservable.registerObserver(observer);
}
public void unregisterAdapterDataObserver(@NonNull AdapterDataObserver observer) {
mObservable.unregisterObserver(observer);
}
//当用户主动调用这个方法的时候(通常是：yourAdapterWhichExtendsRecyclerViewAdapter.notifyDataSetChanged())
//此时：相当于调用的是mObservable(即: AdapterDataObservable)的notifyChanged()方法。
//而这个方法的作用是：把RecyclerView.mAdapter.mObservable.mObservers里list存放的RecyclerViewDataObserver些循环同时调用其onChanged方法。
public final void notifyDataSetChanged() {
mObservable.notifyChanged();
} ……
}
}
第三步

模拟debug方式一步一步运行。当调用RecyclerView#setAdapter()后的运行步骤如上①②③④所示：在setAdapter的执行完成后就实现了RecyclerView.mAdapter.mObservable.mObservers.get(0) = mObserver，而这个mObserver就是RecyclerView里的成员变量private final RecyclerViewDataObserver mObserver = new RecyclerViewDataObserver()

当用户手动调用adapter.notifyDataSetChanged()或adapter.notifyItemChanged(int position)之类的方法的时候其实就是调用了mObserver里的onChanged()或onItemRangeChanged(int positionStart, int itemCount)。而具体的RecyclerView在界面上体现出来的变化则是通过mAdapterHelper.hasPendingUpdates()和mAdapterHelper.onItemRangeChanged(positionStart, itemCount, payload)之类来实现的。具体的得进入到mAdapterHelper里再看。下回分解~~~