Android | RecyclerView Using Support Library ListAdapter

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android-recyclerview-listadapter ListAdapter for RecyclerView has been introduced in Revision 27.1.0 Release of Support Library. ListAdapter uses DiffUtil under the hood to compute list diffs on a background thread. This will help RecyclerView animate changes automatically with less work on the UI thread.

Environment, Tools & Library

  • Android Studio 3.1.4
  • Android Support Library com.android.support:recyclerview-v7:28.0.0
  • Java 1.8+ for Lambda experssions support

Overview

We will build an app that displays a list of hard-coded instances of class Link in a RecyclerView. We will use ListAdapterinstead of the usual RecyclerView.Adapter as shown in a previous post Create a List with RecyclerView. To display items on RecyclerView you need to the following:

  • RecyclerView widget added to the activity layout.
  • A class extending DiffUtil.ItemCallback
  • A class extending ListAdapter.
  • A class extending RecyclerView.ViewHolder.
  • Layout for RecyclerView items.

Files we need for this app are shown in the image below.

android-recyclerview-listadapter_files.png

( 1 ) Create new Android Project

Create new android application keep default options. This app is targeting API 19 or later

target_api

( 2 ) Add the Support Library

  • Add the v7 Support Library to gradle build file

build.gradle

dependencies {
    implementation 'com.android.support:recyclerview-v7:28.0.0'
}

( 3 ) Add RecyclerView to Layout

  • Add RecyclerView to the activity layout.

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

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

</LinearLayout>

( 4 ) Extend DiffUtil.ItemCallback

DiffUtil is a utility class that can calculate the difference between two lists and output a list of update operations that converts the first list into the second one. DiffUtil.Callback is a Callback class used by DiffUtil while calculating the diff between two lists. DiffUtil.Callback serves two roles - list indexing, and item diffing. ItemCallback handles just the second of these, which allows separation of code that indexes into an array or List from the presentation-layer and content specific diffing code.

  • We need an instance of DiffUtil.ItemCallback to be passed into ListAdapter constructor.

ListAdapter(ItemCallback diffCallback)

LinkDiffCallback.java

package com.hmkcode.utils;

import android.support.v7.util.DiffUtil;
import com.hmkcode.models.Link;

public class LinkDiffCallback extends DiffUtil.ItemCallback<Link> {
    @Override
    public boolean areItemsTheSame(Link oldItem, Link newItem) {
        return oldItem.getUrl().equals(newItem.getUrl());
    }

    @Override
    public boolean areContentsTheSame(Link oldItem, Link newItem) {
        return oldItem.getUrl().equals(newItem.getUrl());
    }
}

( 5 ) Functional Interface

This interface is not needed for the implementation of the RecyclerView. It just serves as a bridge in passing click event from ListAdapter to MainActivity to update the list of links List<Link> links;.

It is annotated with @FunctionalInterface to take advantage of lambda expression.

package com.hmkcode.listeners;

import com.hmkcode.models.Link;

@FunctionalInterface
public interface MyItemClickListener {
    public void onClick(Link link);
}

( 6 ) Extending ListAdapter & RecyclerView.ViewHolder

  • MyListAdapter calss extends ListAdapter<Link, MyListAdapter.MyViewHolder>
  • Define constructor MyListAdapter(ItemCallback, MyItemClickListener)
  • Override onCreateViewHolder() to inflate layout item_layout & return object of type MyViewHolder.
  • Override onBindViewHolder() to bind returned MyViewHolder with item i.e. Link returned by getItem(position) .
  • Define inner class MyViewHolder that extends RecyclerView.ViewHolder
package com.hmkcode.adapters;

import android.support.annotation.NonNull;
import android.support.v7.recyclerview.extensions.ListAdapter;
import android.support.v7.util.DiffUtil;
import android.support.v7.widget.RecyclerView;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ImageView;
import android.widget.TextView;
import com.hmkcode.R;
import com.hmkcode.listeners.MyItemClickListener;
import com.hmkcode.models.Link;

public class MyListAdapter extends ListAdapter<Link, MyListAdapter.MyViewHolder> {

    MyItemClickListener myItemClickListener;
    public MyListAdapter(@NonNull DiffUtil.ItemCallback diffCallback,
                         MyItemClickListener myItemClickListener) {
        super(diffCallback);
        this.myItemClickListener = myItemClickListener;
    }

    @NonNull
    @Override
    public MyViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
        View itemLayoutView = LayoutInflater.from(parent.getContext())
                .inflate(R.layout.item_layout, null);

        return new MyViewHolder(itemLayoutView);
    }

    @Override
    public void onBindViewHolder(@NonNull MyViewHolder myViewHolder, int position) {
        myViewHolder.bind(getItem(position));
    }
    
    // inner class
    public class MyViewHolder extends RecyclerView.ViewHolder {

        private View itemLayoutView;
        private TextView itemTitle;
        private TextView itemUrl;
        private ImageView itemIcon;

        public MyViewHolder(View itemLayoutView) {
            super(itemLayoutView);
            this.itemLayoutView = itemLayoutView;
            this.itemTitle = itemLayoutView.findViewById(R.id.item_title);
            this.itemUrl = itemLayoutView.findViewById(R.id.item_url);
            this.itemIcon = itemLayoutView.findViewById(R.id.item_icon);
        }
        public void bind(Link link){
            this.itemIcon.setImageResource(link.getIcon());
            this.itemTitle.setText(link.getTitle());
            this.itemUrl.setText(link.getUrl());

            this.itemLayoutView.setOnClickListener(
                    v -> myItemClickListener.onClick(link));

        }

    }
}

( 7 ) MainActivity

  • Obtain a handle to the RecycleView object
  • Set layout manager using setLayoutManager()

You can use a standard layout managers (such as LinearLayoutManager or GridLayoutManager), or implement your own.

  • Create an adapter of type MyListAdapter
listAdapter = new MyListAdapter(new LinkDiffCallback(), link -> updateList(link));

First argument is instance of LinkDiffCallback Second argument is lambda expression which is a cooler way of passing argument of type MyItemClickListener. You can use anonymous class to accomplish the same result.

  • Attach an adapter for the data to be displayed setAdapter(listAdapter).
  • Call listAdapter.submitList(getListData());

MainActivity.java

package com.hmkcode.activities;

import ...

public class MainActivity extends AppCompatActivity {

    private RecyclerView recyclerView;
    private List<Link> links;
    private ListAdapter listAdapter;

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

        // layout manager
        RecyclerView.LayoutManager layoutManager = new LinearLayoutManager(this);
        recyclerView.setLayoutManager(layoutManager);

        recyclerView.setClickable(true);

        // list adapter
        links = getListData();

        listAdapter = new MyListAdapter(new LinkDiffCallback(),
                link -> updateList(link));

        recyclerView.setAdapter(listAdapter);

        listAdapter.submitList(getListData());

    }

    private void updateList(Link link){
        boolean removed = links.remove(link);
        listAdapter.submitList(new LinkedList(links));
    }

    //generate a list of Link
    private List<Link> getListData(){...}
}

Source Code @ GitHub

Exploring Kotlin

Less than 100 pages covering Kotlin syntax and features in straight and to the point explanation.