用 LiveData实现RecyclerView的点击事件

本文用到了：dataBinding、android架构组件

在使用RecyclerView时，点击item事件，我们通常会写一个OnItemClickListener接口，然后回调监听。在这里我们用安卓架构组件的LiveData试一下吧~
官方 todo-mvvm-live 示例里用了一个 SingleLiveEvent （继承 MutableLiveData）：

/**
 * A lifecycle-aware observable that sends only new updates after subscription, used for events like
 * navigation and Snackbar messages.
 * 

 * This avoids a common problem with events: on configuration change (like rotation) an update
 * can be emitted if the observer is active. This LiveData only calls the observable if there's an
 * explicit call to setValue() or call().
 * 

 * Note that only one observer is going to be notified of changes.
 */
public class SingleLiveEvent<T> extends MutableLiveData<T> {

    private static final String TAG = "SingleLiveEvent";

    private final AtomicBoolean mPending = new AtomicBoolean(false);

    @MainThread
    public void observe(LifecycleOwner owner, final Observer observer) {

        if (hasActiveObservers()) {
            Log.w(TAG, "Multiple observers registered but only one will be notified of changes.");
        }

        // Observe the internal MutableLiveData
        super.observe(owner, new Observer() {
            @Override
            public void onChanged(@Nullable T t) {
                if (mPending.compareAndSet(true, false)) { // 如果 mPending = true, 则设置为 false
                    observer.onChanged(t);
                }
            }
        });
    }

    @MainThread
    public void setValue(@Nullable T t) { 
        // 当调用setValue() 方法时，mPending设为true，
        mPending.set(true);
        super.setValue(t);
    }

    /**
     * Used for cases where T is Void, to make calls cleaner.
     */
    @MainThread
    public void call() {
        setValue(null);
    }
}

在ViewModel类：

// 实例化 SingleLiveEvent ，会调用SingleLiveEvent类中的observe()方法
public SingleLiveEvent mOpenTaskEvent = new SingleLiveEvent<>();


// 返回LiveData
public SingleLiveEvent getOpenTaskEvent(){
        return mOpenTaskEvent;
    }

在Fragment:

// 订阅点击事件
mViewModel.getOpenTaskEvent().observe(this, new Observer() {
            @Override
            public void onChanged(@Nullable String s) {

                Log.i("onChanged", s);

            }
        });

item点击事件：
0：OnTaskItemListener接口

public interface OnTaskItemListener {
    void onItemClick(ToDo entity);
}

① 在xml中绑定listener，设置onClick：

 "listener"
            type="test.carrie.todomvvmtest.ui.OnTaskItemListener"/>


"match_parent"
        android:layout_height="48dp"
        android:gravity="center_vertical"
        android:orientation="horizontal"
        android:onClick="@{()->listener.onItemClick(obj)}"
        >

② 在Adapter中：

 public class OnItemClickListener implements OnTaskItemListener {

        @Override
        public void onItemClick(ToDo entity) {

            mainViewModel.getOpenTaskEvent().setValue(entity.title);  // 调用了 setValue() 方法，[SingleLiveEvent:] mPending=true， observe()中的 super.observe...onChanged(T) 响应，由于mPending为true，设置 observer.onChanged(t); 目的是为了让Fragment中的观察者收到订阅结果。

        }

整个响应顺序：

public SingleLiveEvent mOpenTaskEvent = new SingleLiveEvent<>();
SingleLiveEvent.observe

Adapter.onItemClick
SingleLiveEvent.setValue
SingleLiveEvent.super.onChanged
SingleLiveEvent.observer.onChanged(t);
Fragment.observe.onChanged

如果不用SingleLiveEvent，直接用MutableLiveData：

// ViewModel 
public MutableLiveData mMutableOpenTaskEvent = new MutableLiveData<>();

public MutableLiveData getOpenTaskEventMutable(){
     return mMutableOpenTaskEvent;
}


// Adapter
public void onItemClick(ToDo entity) {        
     mViewModel.getOpenTaskEventMutable().setValue(entity.title);
}


// Fragment
 mViewModel.getOpenTaskEventMutable().observe(this, new Observer() {
            @Override
            public void onChanged(@Nullable String s) {
                LogUtil.i("onChanged", s);


            }
        });