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Android RecyclerView

Topics covered

Android RecyclerView

RecyclerView.Adapter

View holder pattern

ListView differences

RecyclerView is one the two UI widgets introduced by the support library in Android L. In this post I will describe how we can use it and what’s the difference between this widget and the “classic” ListView.
This new widget is more flexible that the ListView but introduces some complexities. As we are used RecyclerView introduces a new Adapter that must be used to represent the underlying data in the widget. This new adapter is called RecyclerView.Adapter. To use this widget you have to add latest v7 support library.

Introduction

We know already that in the ListView to increase the performance we have to use the ViewHolder pattern. This is simply a java class that holds the references to the widget in the row layout of the ListView (for example TextView, ImageView and so on). Using this pattern we avoid to call several times findById method to get the UI widget reference making the ListView scrolling smoother. Even if this pattern was suggested as best-practice we could implement our Adapter without using this pattern.
RecyclerView enforces this pattern making it the core of this UI widget and we have to use it in our Adapter.

The Adapter: RecyclerView.Adapter

If we want to show the information in a ListView or in the new RecyclerView we have to use an Adapter. This component stands behind the UI widget and determines how the rows, in the ListView, have to be rendered and what information to show. Also in the RecyclerView we have to use an Adapter:
public class MyRecyclerAdapter extends RecyclerView.Adapter<MyRecyclerAdapter.MyHolder> {
    ....
}



where MyHolder is our implementation of ViewHolder pattern. We can suppose, we have a simple row layout in our RecyclerView:
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical" android:layout_width="match_parent"
    android:layout_height="match_parent">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/txt1"/>

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/txt2"/>

</LinearLayout>

so our ViewHolder patter implementation is:
public static class MyHolder extends RecyclerView.ViewHolder {
    protected TextView txt1;
    protected TextView txt2;

    private MyHolder(View v) {
        super(v);
        this.txt1 = (TextView) v.findViewById(R.id.txt1);
        this.txt2 = (TextView) v.findViewById(R.id.txt2);
    }
}

As you can notice, the lookup process (findViewById) is made in the view holder instead of in getView method.

Now we have to implement some important method in our adapter to make it working properly. There are two important methods we have to override:


  • onCreateViewHolder(ViewGroup viewGroup, int i)
  • onBindViewHolder(MyHolder myHolder, int i)

OnCreateViewHolder is called whenever a new instance of View Holder class must be instantiated, so this method becomes:
@Override
public MyHolder onCreateViewHolder(ViewGroup viewGroup, int i) {
    numCreated++;
    Log.d("RV", "OncreateViewHolder ["+numCreated+"]");
    View v = LayoutInflater.from(viewGroup.getContext()).inflate(R.layout.row_layout, null);
    MyHolder mh = new MyHolder(v);
    return mh;
}

as you can notice, the method returns an instance of our View holder implementation, while the second method is called when the SO binds the view with the data, so we set the UI widget content to the data values:
@Override
public void onBindViewHolder(MyHolder myHolder, int i) {
    Log.d("RV", "OnBindViewHolder");
    Item item = itemList.get(i);
    myHolder.txt1.setText(item.name);
    myHolder.txt2.setText(item.descr);
}

Notice that we don’t make a lookup but we simply use the UI widget reference stored in our view holder.

RecyclerView


Now we have our adapter we can create our RecyclerView:
RecyclerView rv = (RecyclerView) findViewById(R.id.my_recycler_view);
rv.setLayoutManager(new LinearLayoutManager(this));
MyRecyclerAdapter adapter = new MyRecyclerAdapter(createList());
rv.setAdapter(adapter);

LinearLayoutManager is the “main” layout manager used to dispose items inside the RecyclerView. We can extend or implement our layout manager.

Final considerations


Running the example we obtain:

android_recyclerview

The most interesting aspect is how many times the onCreateViewHolder is called compared to the number of items shown. If you look at the log you will find that the object created is 1/3 of the total number.

7 comments:

  1. Thanks for this. What replaced setOnItemClickListener() ? How does one detect clicks?

    ReplyDelete
  2. Experiment with Recycler View
    https://github.com/rahulrj/Swipe_RecyclerView

    ReplyDelete
  3. Hi, great article. What does the "SO" stand for when you write "while the second method is called when the SO binds the view with the data..."?

    ReplyDelete
  4. It's just a typo due to the fact that the author is italian. SO stands for "Sistema Operativo", the italian for Operating System.

    ReplyDelete
  5. Thx for spotting it. As said SO stands for OS! :)

    ReplyDelete
  6. Thank you all. Regards!

    ReplyDelete

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