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

Question

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)?

Answer 1

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); 

Answer 2

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