小白也能看懂源码系列

第一步

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<T>{

    protected final ArrayList<T> mObservers = new ArrayList<T>();

    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<AdapterDataObserver>{

        //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<VH extends ViewHolder> {
        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~~~