Why RecyclerView.notifyItemChanged() will create a new ViewHolder and use both the old ViewHolder and new one?

Recently I use RecyclerView and add a custom header view (another type of item view) and try to updated it when data has changed. Something strange happens. The adapter creates a new HeaderViewHolder and uses both the new HeaderViewHolder and the old one.

Here's the sample.

MainActivity.java

public class MainActivity extends ActionBarActivity {

  private RecyclerView mRecyclerView;

  private MyAdapter mAdapter;

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

    mRecyclerView = (RecyclerView) findViewById(R.id.list);
    LinearLayoutManager llm = new LinearLayoutManager(this);
    llm.setSmoothScrollbarEnabled(true);
    mRecyclerView.setLayoutManager(llm);
    mRecyclerView.setAdapter(mAdapter = new MyAdapter(this, genItemList()));
  }

  @Override public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.menu_main, menu);
    return true;
  }

  @Override public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();

    //noinspection SimplifiableIfStatement
    if (id == R.id.action_settings) {
      return true;
    }

    return super.onOptionsItemSelected(item);
  }

  public void addItems(View view) {
    mAdapter.addItemList(genItemList());
  }

  private List<Item> genItemList() {
    List<Item> list = new ArrayList<>(50);
    for (int i = 0; i < 50; i++) {
      Item item = new Item();
      item.text1 = "AAAAAAAAAAAAAAAAAAAAAAAA";
      item.text2 = "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa";
      list.add(item);
    }
    return list;
  }

  public void updateHeader(View view) {
    mAdapter.updateHeader("Updated header");
  }
}

MyAdapter.java

public class MyAdapter extends RecyclerView.Adapter {

  private static final String TAG = "MyAdapter";

  private static final int TYPE_HEADER = 0;
  private static final int TYPE_ITEM   = 1;

  private LayoutInflater mInflater;

  private List<Item> mItemList;

  private Header mHeader;

  public MyAdapter(Context context, List<Item> items) {
    mInflater = LayoutInflater.from(context);
    mItemList = items != null ? items : new ArrayList<Item>();
    mHeader = new Header();
    mHeader.text = "header";
  }

  @Override public RecyclerView.ViewHolder onCreateViewHolder(ViewGroup viewGroup, int type) {
    switch (type) {
    case TYPE_HEADER:
      Log.d(TAG, "create header view holder");
      View headerView = mInflater.inflate(android.R.layout.simple_list_item_1, viewGroup, false);
      return new HeaderViewHolder(headerView);
    case TYPE_ITEM:
      View itemView = mInflater.inflate(R.layout.layout_item, viewGroup, false);
      return new MyViewHolder(itemView);
    }
    return null;
  }

  @Override public void onBindViewHolder(RecyclerView.ViewHolder viewHolder, int position) {
    if (viewHolder instanceof HeaderViewHolder) {
      Log.d(TAG, "bind header view holder");
      TextView textView = (TextView) viewHolder.itemView.findViewById(android.R.id.text1);
      textView.setText(mHeader.text);
      Log.d(TAG, "position: " + position + " holder: " + viewHolder + " text: " + mHeader.text);
    } else if (viewHolder instanceof MyViewHolder) {
      Item item = mItemList.get(position - 1)
      ((MyViewHolder) viewHolder).setText1(item.text1);
      ((MyViewHolder) viewHolder).setText2(item.text2);
    }
  }

  @Override public int getItemCount() {
    return mItemList == null ? 0 : mItemList.size() + 1; // plus header
  }

  @Override public int getItemViewType(int position) {
    return position == 0 ? TYPE_HEADER : TYPE_ITEM;
  }

  public void addItemList(List<Item> list) {
    if (list != null) {
      mItemList.addAll(list);
      notifyDataSetChanged();
    }
  }

  public void updateHeader(String text) {
    mHeader.text = text;
    notifyItemChanged(0);
    // notifyDataSetChanged();
  }

  static class HeaderViewHolder extends RecyclerView.ViewHolder {

    public HeaderViewHolder(View itemView) {
      super(itemView);
    }
  }

  static class MyViewHolder extends RecyclerView.ViewHolder {

    TextView mTextView1;
    TextView mTextView2;

    public MyViewHolder(View itemView) {
      super(itemView);

      mTextView1 = (TextView) itemView.findViewById(R.id.text1);
      mTextView2 = (TextView) itemView.findViewById(R.id.text2);
    }

    public void setText1(String text) {
      mTextView1.setText(text);
    }

    public void setText2(String text) {
      mTextView2.setText(text);
    }
  }
}

Header.java

public class Header {

  public String text;
}

Item.java

public class Item {

  public String text1;
  public String text2;
}

activity_main.xml

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
              xmlns:tools="http://schemas.android.com/tools"
              android:layout_width="match_parent"
              android:layout_height="match_parent"
              android:orientation="vertical"
              tools:context=".MainActivity">

  <android.support.v7.widget.RecyclerView
      android:id="@+id/list"
      android:layout_width="match_parent"
      android:layout_height="0dp"
      android:layout_weight="1" />

  <LinearLayout
      style="?android:buttonBarStyle"
      android:layout_width="match_parent"
      android:layout_height="56dp"
      android:orientation="horizontal">

    <Button
        android:id="@+id/add"
        style="?android:buttonBarButtonStyle"
        android:layout_width="0dp"
        android:layout_height="match_parent"
        android:layout_weight="1"
        android:onClick="addItems"
        android:text="Add items" />

    <Button
        android:id="@+id/update"
        style="?android:buttonBarButtonStyle"
        android:layout_width="0dp"
        android:layout_height="match_parent"
        android:layout_weight="1"
        android:onClick="updateHeader"
        android:text="update header" />
  </LinearLayout>

</LinearLayout>

layout_item.xml

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
              android:layout_width="match_parent"
              android:layout_height="match_parent"
              android:orientation="vertical">

  <TextView
      android:id="@+id/text1"
      android:layout_width="match_parent"
      android:layout_height="wrap_content"
      android:ellipsize="marquee"
      android:maxLines="1"
      android:textAppearance="?android:textAppearanceLarge"
      android:textColor="@android:color/black" />

  <TextView
      android:id="@+id/text2"
      android:layout_width="match_parent"
      android:layout_height="wrap_content"
      android:ellipsize="marquee"
      android:maxLines="1"
      android:textAppearance="?android:textAppearanceSmall"
      android:textColor="@android:color/black" />

</LinearLayout>

And then, here's the logcat output when I clicked 3 times "update header":

06-05 19:57:50.368  20400-20400/com.imaygou.recyclerupdateitemdemo D/MyAdapter﹕ create header view holder
06-05 19:57:50.369  20400-20400/com.imaygou.recyclerupdateitemdemo D/MyAdapter﹕ bind header view holder
06-05 19:57:50.370  20400-20400/com.imaygou.recyclerupdateitemdemo D/MyAdapter﹕ position: 0 holder: ViewHolder{3f742717 position=0 id=-1, oldPos=-1, pLpos:-1 no parent} text: header
06-05 19:57:54.030  20400-20400/com.imaygou.recyclerupdateitemdemo D/MyAdapter﹕ create header view holder
06-05 19:57:54.031  20400-20400/com.imaygou.recyclerupdateitemdemo D/MyAdapter﹕ bind header view holder
06-05 19:57:54.031  20400-20400/com.imaygou.recyclerupdateitemdemo D/MyAdapter﹕ position: 0 holder: ViewHolder{3ac01621 position=0 id=-1, oldPos=-1, pLpos:-1 no parent} text: Updated header
06-05 19:57:56.938  20400-20400/com.imaygou.recyclerupdateitemdemo D/MyAdapter﹕ bind header view holder
06-05 19:57:56.938  20400-20400/com.imaygou.recyclerupdateitemdemo D/MyAdapter﹕ position: 0 holder: ViewHolder{3f742717 position=0 id=-1, oldPos=-1, pLpos:-1 no parent} text: Updated header
06-05 19:57:59.613  20400-20400/com.imaygou.recyclerupdateitemdemo D/MyAdapter﹕ bind header view holder
06-05 19:57:59.613  20400-20400/com.imaygou.recyclerupdateitemdemo D/MyAdapter﹕ position: 0 holder: ViewHolder{3ac01621 position=0 id=-1, oldPos=-1, pLpos:-1 no parent} text: Updated header

If I use notifyDataSetChanged() instead of notifyItemChanged(0), everything works fine. No one more ViewHolder. But why?

Why it'll create a new ViewHolder and use both of them?

What's the best practise about using notifyItemChanged(int)?

RecyclerView use both of ViewHolder for smooth animation from an old state to a new. This is default behaviour of RecyclerView.ItemAnimator.

You can disable animation by passing an empty item animator to RecyclerView:

listView.setItemAnimator(null);

To avoid ViewHolder's recreating and without disabling smooth animation after call recyclerView.notifyItemChanged() you can set DefaultItemAnimator and override canReuseUpdatedViewHolder() method.

Kotlin:

val itemAnimator: DefaultItemAnimator = object : DefaultItemAnimator() {
      override fun canReuseUpdatedViewHolder(viewHolder: RecyclerView.ViewHolder): Boolean {
            return true
      }
}
recyclerView.itemAnimator = itemAnimator

((SimpleItemAnimator) myRecyclerView.getItemAnimator()).setSupportsChangeAnimations(false);

Here are a few issues with your implementation:

getItemCount expects the count of all items in the recyclerview including the header so you should return mItemList.size() + 1
the position field in onBindViewHolder() refers to the position of an element in the whole recyclerview including the header. so to bind a non-header item you will do something like item = mItemList.get(position - 1) -- this won't fail because getItemViewType returns a number greater than 0 for TYPE_ITEMs

By doing so, notifyItemChanged should behave as expected

You can also use notifyItemChanged with a payload: https://developer.android.com/reference/android/support/v7/widget/RecyclerView.Adapter.html#notifyitemchanged. It will reuse the same ViewHolder instance: "if the item is already represented by a ViewHolder and it will be rebound to the same ViewHolder"

Why RecyclerView.notifyItemChanged() will create a new ViewHolder and use both the old ViewHolder and new one?

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