不像 ListView，RecyclerView 是没有提供 setEmptyView() 方法的，为了开发方便，就实现了一个。

ListView.setEmptyView() 的原理是什么？

在实现 RecyclerView.setEmptyView() 之前，先看一下 ListView.setEmptyView() 的原理是什么。

从 ListView.setEmptyView() 方法看起，下面是方法的实现

/**
 * Sets the view to show if the adapter is empty
 */
@android.view.RemotableViewMethod
public void setEmptyView(View emptyView) {
    mEmptyView = emptyView;

    // If not explicitly specified this view is important for accessibility.
    if (emptyView != null
            && emptyView.getImportantForAccessibility() == IMPORTANT_FOR_ACCESSIBILITY_AUTO) {
        emptyView.setImportantForAccessibility(IMPORTANT_FOR_ACCESSIBILITY_YES);
    }

    final T adapter = getAdapter();
    final boolean empty = ((adapter == null) || adapter.isEmpty());
    updateEmptyStatus(empty);
}

注意到方法体的最后两句

final boolean empty = ((adapter == null) || adapter.isEmpty());
updateEmptyStatus(empty);

这两句代码的作用是判断 Adapter 和数据源是否为空，然后以此做相应的操作。是看一下 updateEmptyStatus(empty) 的具体实现

/**
 * Update the status of the list based on the empty parameter.  If empty is true and
 * we have an empty view, display it.  In all the other cases, make sure that the listview
 * is VISIBLE and that the empty view is GONE (if it's not null).
 */
private void updateEmptyStatus(boolean empty) {
    if (isInFilterMode()) {
        empty = false;
    }

    if (empty) {
        if (mEmptyView != null) {
            mEmptyView.setVisibility(View.VISIBLE);
            setVisibility(View.GONE);
        } else {
            // If the caller just removed our empty view, make sure the list view is visible
            setVisibility(View.VISIBLE);
        }

        // We are now GONE, so pending layouts will not be dispatched.
        // Force one here to make sure that the state of the list matches
        // the state of the adapter.
        if (mDataChanged) {           
            this.onLayout(false, mLeft, mTop, mRight, mBottom); 
        }
    } else {
        if (mEmptyView != null) mEmptyView.setVisibility(View.GONE);
        setVisibility(View.VISIBLE);
    }
}

可以看到，如果数据为空，就把 EmptyView 显示，ListView 本身隐藏起来。

在看一下 updateEmptyStatus(boolean empty) 有再哪里调用，发现会在 checkFocus() 方法里调用，而这个 checkFocus() 方法会在 AdapterDataSetObserver 里调用到

class AdapterDataSetObserver extends DataSetObserver {

    private Parcelable mInstanceState = null;

    @Override
    public void onChanged() {
        mDataChanged = true;
        mOldItemCount = mItemCount;
        mItemCount = getAdapter().getCount();

        // Detect the case where a cursor that was previously invalidated has
        // been repopulated with new data.
        if (AdapterView.this.getAdapter().hasStableIds() && mInstanceState != null
                && mOldItemCount == 0 && mItemCount > 0) {
            AdapterView.this.onRestoreInstanceState(mInstanceState);
            mInstanceState = null;
        } else {
            rememberSyncState();
        }
        checkFocus();
        requestLayout();
    }

    @Override
    public void onInvalidated() {
        mDataChanged = true;

        if (AdapterView.this.getAdapter().hasStableIds()) {
            // Remember the current state for the case where our hosting activity is being
            // stopped and later restarted
            mInstanceState = AdapterView.this.onSaveInstanceState();
        }

        // Data is invalid so we should reset our state
        mOldItemCount = mItemCount;
        mItemCount = 0;
        mSelectedPosition = INVALID_POSITION;
        mSelectedRowId = INVALID_ROW_ID;
        mNextSelectedPosition = INVALID_POSITION;
        mNextSelectedRowId = INVALID_ROW_ID;
        mNeedSync = false;

        checkFocus();
        requestLayout();
    }

    public void clearSavedState() {
        mInstanceState = null;
    }
}

从这个类来看说明数据变化的时候，会通知 AdapterDataSetObserver 调用响应的方法。

在 BaseAdapter 中就存在 DataSetObserver，在调用 notifyDataSetChanged() 就会调用 mDataSetObservable.notifyChanged();

public abstract class BaseAdapter implements ListAdapter, SpinnerAdapter {
  private final DataSetObservable mDataSetObservable = new DataSetObservable();

  public boolean hasStableIds() {
      return false;
  }
  
  public void registerDataSetObserver(DataSetObserver observer) {
      mDataSetObservable.registerObserver(observer);
  }

  public void unregisterDataSetObserver(DataSetObserver observer) {
      mDataSetObservable.unregisterObserver(observer);
  }
  
  /**
   * Notifies the attached observers that the underlying data has been changed
   * and any View reflecting the data set should refresh itself.
   */
  public void notifyDataSetChanged() {
      mDataSetObservable.notifyChanged();
  }
  
  // 其他代码省略

总结一下：

Adapter 通过 DataSetObservable 的回调方法通知 ListView 显示或隐藏 EmptyView

RecyclerView.setEmptyView() 的原理

RecyclerView 内部有一个类：AdapterDataObserver，从代码注释来看

Observer base class for watching changes to an {@link Adapter}. See {@link Adapter#registerAdapterDataObserver(AdapterDataObserver)}.

AdapterDataObserver 是用来监听 Adapter 的数据源变化的，其实就是一个观察者模式的实现。

这个类提供了几个方法

public static abstract class AdapterDataObserver {
    public void onChanged() {
        // Do nothing
    }

    public void onItemRangeChanged(int positionStart, int itemCount) {
        // do nothing
    }

    public void onItemRangeChanged(int positionStart, int itemCount, 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
    }
}

和数据变化有关的方法有几个：onChanged()、onItemRangeInserted() 和 onItemRangeRemoved()。在这三个方法里把 EmptyView 显示出来就可以实现 setEmptyView() 的特性。

代码实现

import android.content.Context;
import android.support.annotation.Nullable;
import android.support.v7.widget.RecyclerView;
import android.util.AttributeSet;
import android.view.View;


public class EmptyRecyclerView extends RecyclerView {

    private View mEmptyView;

    public EmptyRecyclerView(Context context) {
        super(context);
    }

    public EmptyRecyclerView(Context context, @Nullable AttributeSet attrs) {
        super(context, attrs);
    }

    public EmptyRecyclerView(Context context, @Nullable AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
    }

    public void setEmptyView(View emptyView) {
        mEmptyView = emptyView;
    }

    private AdapterDataObserver mObserver = new AdapterDataObserver() {

        @Override
        public void onItemRangeInserted(int positionStart, int itemCount) {
            checkIfEmpty();
        }

        @Override
        public void onItemRangeRemoved(int positionStart, int itemCount) {
            checkIfEmpty();
        }

        @Override
        public void onChanged() {
            checkIfEmpty();
        }
    };

    @Override
    public void setAdapter(Adapter adapter) {
        super.setAdapter(adapter);
        adapter.registerAdapterDataObserver(mObserver);
        checkIfEmpty();
    }

    private void checkIfEmpty() {
        if (mEmptyView != null && getAdapter() != null) {
            boolean emptyViewVisible = getAdapter().getItemCount() == 0;

            // 显示为空视图
            mEmptyView.setVisibility(emptyViewVisible ? View.VISIBLE : View.GONE);
            // RecyclerView本身隐藏
            this.setVisibility(emptyViewVisible ? View.GONE : View.VISIBLE);
        }
    }
}

使用

布局文件。添加了两个按钮、一个 EmptyView 和一个 EmptyRecyclerView

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/activity_recycler_view_empty_view"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    tools:context="com.okada.RecyclerViewEmptyViewActivity">

    <LinearLayout
        android:layout_width="match_parent"
        android:orientation="horizontal"
        android:layout_height="wrap_content">

        <Button
            android:layout_width="0dp"
            android:layout_weight="1"
            android:layout_height="wrap_content"
            android:id="@+id/btn_clearDataSource"
            android:text="clear dataSource"/>

        <Button
            android:layout_width="0dp"
            android:layout_weight="1"
            android:layout_height="wrap_content"
            android:id="@+id/btn_setDataSource"
            android:text="set dataSource"/>
    </LinearLayout>

    <TextView
        android:id="@+id/tv_empty"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:gravity="center"
        android:text="啥也没有"
        android:textSize="18sp"
        android:visibility="gone"/>

    <com.okada.EmptyRecyclerView
        android:id="@+id/recyclerView"
        android:layout_width="match_parent"
        android:layout_height="match_parent"/>

</LinearLayout>

然后就是使用了

public class RecyclerViewEmptyViewActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_recycler_view_empty_view);


        final MyAdapter myAdapter = new MyAdapter(generateDataSource());

        EmptyRecyclerView recyclerView = (EmptyRecyclerView) findViewById(R.id.recyclerView);
        recyclerView.setLayoutManager(new LinearLayoutManager(this));
        recyclerView.setEmptyView(findViewById(R.id.tv_empty));
        recyclerView.setAdapter(myAdapter);

        findViewById(R.id.btn_clearDataSource).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                myAdapter.clearDataSource();
            }
        });
        findViewById(R.id.btn_setDataSource).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                myAdapter.setDataSource(generateDataSource());
            }
        });
    }

    private List<Integer> generateDataSource() {
        List<Integer> dataSource= new ArrayList<>(20);
        for (int i = 0; i < 20; i++) {
            dataSource.add(i);
        }
        return dataSource;
    }
}