RecyclerView

The recent release of Android support library (v7:21.0.+) has introduced two new user interface widget: RecyclerView and CardView that helps in building rich Android apps.
The 1. RecyclerView is more advanced and flexible and efficient version of ListView. RecyclerView ViewGroup is an container for larger data set of views that can be recycled and scrolled very efficiently.
In the other hand the 2. CardView widget, is an extension of existing FrameLayout class. A CardView is a ViewGroup.This helps to wrap other UI elements as Google style cards. CardView widgets can have shadows and rounded corners.
Add CardView Support Library
Android SDK doesn't includes the CardView class, and hence for using CardView in your project you first need to add the card view support library to your project. Android Studio users can add the following graddle dependency in your build.graddle file to add CardView to project.

dependencies {
       compile 'com.android.support:cardview-v7:21.0.+'
       compile 'com.android.support:recyclerview-v7:21.0.+'
}

Declare CardView Layout
In this example, we will add an ImageView and two TextViews as shown in the following screenshot.

<android.support.v7.widget.CardView xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/card_view"
    android:layout_width="match_parent"
    android:layout_height="380dp"
    android:layout_margin="8dp">

    <RelativeLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent">

        <ImageView
            android:id="@+id/thumbnail"
            android:layout_width="match_parent"
            android:layout_height="250dp"
            android:layout_alignParentTop="true"
            android:scaleType="centerCrop"
            android:src="@drawable/wallpaper" />

        <TextView
            android:id="@+id/title"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_below="@+id/thumbnail"
            android:maxLines="3"
            android:padding="8dp"
            android:text="@string/title"
            android:textColor="#222"
            android:textStyle="bold"
            android:textSize="22dp" />

        <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_below="@+id/title"
            android:maxLines="3"
            android:padding="8dp"
            android:text="@string/description"
            android:textColor="#666"
            android:textSize="14dp" />

    </RelativeLayout>
</android.support.v7.widget.CardView>

The output of the above code is as follows

Now let us use the following CardView properties to customize the appearance of the CardView :

• cardView:cardCornerRadius - To set the corner radius in your layouts.
• cardView:cardBackgroundColor - To set the background color of a card.

CardView - RecyclerView

Creating a RecyclerView

Step 1: Defining It in a Layout
Using a RecyclerView instance is slightly more complicated. However, defining it in a layout XML file is quite simple. You can define it in a layout as follows:

<android.support.v7.widget.RecyclerView
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:id="@+id/rv"
    />

To obtain a handle to it in your Activity, use the following snippet:

RecyclerView rv = (RecyclerView)findViewById(R.id.rv);

If you are sure that the size of the RecyclerView won't be changing, you can add the following to improve performance:

rv.setHasFixedSize(true);

Step 2: Using a LayoutManager
Unlike a ListView, a RecyclerView needs a LayoutManager to manage the positioning of its items. You could define your own LayoutManager by extending the RecyclerView.LayoutManager class. However, in most cases, you could simply use one of the predefined LayoutManager subclasses:

• LinearLayoutManager
• GridLayoutManager
• StaggeredGridLayoutManager

In this tutorial, I am going to use a LinearLayoutManager. This LayoutManager subclass will, by default, make your RecyclerView look like a ListView.

LinearLayoutManager llm = new LinearLayoutManager(context);
rv.setLayoutManager(llm);

Step 3: Defining the Data
Just like a ListView, a RecyclerView needs an adapter to access its data. But before we create an adapter, let us create data that we can work with. Create a simple class to represent a person and then write a method to initialize a List of Person objects:

class Person {
    String name;
    String age;
    int photoId;
 
    Person(String name, String age, int photoId) {
        this.name = name;
        this.age = age;
        this.photoId = photoId;
    }
}
 
private List persons;
 
// This method creates an ArrayList that has three Person objects
// Checkout the project associated with this tutorial on Github if
// you want to use the same images.
private void initializeData(){
    persons = new ArrayList<>();
    persons.add(new Person("Emma Wilson", "23 years old", R.drawable.emma));
    persons.add(new Person("Lavery Maiss", "25 years old", R.drawable.lavery));
    persons.add(new Person("Lillie Watts", "35 years old", R.drawable.lillie));
}

Step 4: Creating an Adapter

To create an adapter that a RecyclerView can use, you must extend RecyclerView.Adapter. This adapter follows the view holder design pattern, which means that it you to define a custom class that extends RecyclerView.ViewHolder. This pattern minimizes the number of calls to the costly findViewById method.

Earlier in this tutorial, we already defined the XML layout for a CardView that represents a person. We are going to reuse that layout now. Inside the constructor of our custom ViewHolder, initialize the views that belong to the items of our RecyclerView.

public class RVAdapter extends RecyclerView.Adapter{
 
    public static class PersonViewHolder extends RecyclerView.ViewHolder {      
        CardView cv;
        TextView personName;
        TextView personAge;
        ImageView personPhoto;
 
        PersonViewHolder(View itemView) {
            super(itemView);
            cv = (CardView)itemView.findViewById(R.id.cv);
            personName = (TextView)itemView.findViewById(R.id.person_name);
            personAge = (TextView)itemView.findViewById(R.id.person_age);
            personPhoto = (ImageView)itemView.findViewById(R.id.person_photo);
        }
    }
 
}

Next, add a constructor to the custom adapter so that it has a handle to the data that the RecyclerView displays. As our data is in the form of a List of Person objects, use the following code:

List persons;
 
RVAdapter(List persons){
    this.persons = persons;
}

RecyclerView.Adapter has three abstract methods that we must override. Let us start with the getItemCount method. This should return the number of items present in the data. As our data is in the form of a List, we only need to call the size method on the List object:

@Override
public int getItemCount() {
    return persons.size();
}

Next, override the onCreateViewHolder method. As its name suggests, this method is called when the custom ViewHolder needs to be initialized. We specify the layout that each item of the RecyclerView should use. This is done by inflating the layout using LayoutInflater, passing the output to the constructor of the custom ViewHolder.

@Override
public PersonViewHolder onCreateViewHolder(ViewGroup viewGroup, int i) {
    View v = LayoutInflater.from(viewGroup.getContext()).inflate(R.layout.item, viewGroup, false);
    PersonViewHolder pvh = new PersonViewHolder(v);
    return pvh;
}

Override the onBindViewHolder to specify the contents of each item of the RecyclerView. This method is very similar to the getView method of a ListView's adapter. In our example, here's where you have to set the values of the name, age, and photo fields of the CardView.

@Override
public void onBindViewHolder(PersonViewHolder personViewHolder, int i) {
    personViewHolder.personName.setText(persons.get(i).name);
    personViewHolder.personAge.setText(persons.get(i).age);
    personViewHolder.personPhoto.setImageResource(persons.get(i).photoId);
}

Finally, you need to override the onAttachedToRecyclerView method. For now, we can simply use the superclass's implementation of this method as shown below.

@Override
public void onAttachedToRecyclerView(RecyclerView recyclerView) {
    super.onAttachedToRecyclerView(recyclerView);
}

Step 5: Using the Adapter
Now that the adapter is ready, add the following code to your Activity to initialize and use the adapter by calling the adapter's constructor and the RecyclerView's setAdapter method:

RVAdapter adapter = new RVAdapter(persons);
rv.setAdapter(adapter);

Step 6: Compile and Run When you run the RecyclerView example on an Android device, you should see something similar to the following result.

v7 Support Libraries

• v7 appcompat library
• v7 cardview library
• v7 gridlayout library
• v7 mediarouter library
• v7 palette library
• v7 recyclerview library
• v7 Preference Support Library

Ref : Google Support Libs