Why RecyclerView.notifyItemChanged() will create a new ViewHolder and use both the old ViewHolder and new one?
AndroidAndroid LayoutAndroid Problem Overview
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)?
Android Solutions
Solution 1 - Android
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);
Solution 2 - Android
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
Solution 3 - Android
((SimpleItemAnimator) myRecyclerView.getItemAnimator()).setSupportsChangeAnimations(false);
Solution 4 - Android
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
Solution 5 - Android
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"
Solution 6 - Android
More cleaner solution (is not bug in animator, but this is a feature of layout manager):
mRecyclerView.setLayoutManager(new GridLayoutManager(this, 5, LinearLayoutManager.VERTICAL, false){
@Override
public boolean supportsPredictiveItemAnimations() {
return false;//super.supportsPredictiveItemAnimations();
}
});