RecyclerView is a ViewGroup that helps us to display a scrollable list of elements. It is one of the most used UI components in the Android apps and it involves having a data source, an adapter, and the RecyclerView. Other than displaying the content, RecyclerView is efficiently reusing the views that have scrolled off the screen by recycling them.

📃Components Overview

• Data source – list of objects that could be obtained from a local database, or as a result of a http request, or even it could be a list with some predefined values.
• RecyclerView is a scrolling list for list items and to have access at it we must add some dependencies in the gradle file
• Layout for one item of data – XML file
• Layout manager handles the organization of UI components in a view (there are 3 predefined ways to display the items)
• View holder has view information for displaying one item 
• Adapter connects data to the RecyclerView 
• Set the Adapter to the RecyclerView and we are done 🙂

LayoutManager

• All view groups have layout managers
• Positions item views inside a RecyclerView
• Reuses item views that are no longer visible to the user 
• Built-in layout managers include LinearLayoutManager, GridLayoutManager, and StaggeredGridLayoutManager
• For RecyclerView, it extends RecyclerView.LayoutManager

Adapter

• We are talking about applying the adapter design pattern as an intermediary between the source data we want to display and the view
• Adapter is a structural design pattern that allows objects with incompatible interfaces to collaborate
• Responsibile to create, updade, add and delete items from the list
• It suppose to implement the abstract class RecyclerView.Adapter

ViewHolder

• Used by the adapter to prepare one item view for each element from the datasource  
• The layout of the item is specified in an XML resource file, it’s like a 1:1 relation between the view and the element from the list of objects provided as a datasource
• Can have clickable elements and it is placed by the layout manager
• We must implement the abstract class RecyclerView.ViewHolder

👩🏻‍💻Implementation steps

1. Get the data by creating a new object class (data source)
2. Add the RecyclerView dependency to  app/build.gradle file and add RecyclerView to layout
3. Create XML layout for item
4. Define the LayoutManager in activity
5. Extend RecyclerView.ViewHolder in a separated class
6. Extend RecyclerView.Adapter in a separated class
7. In onCreate of activity, create a RecyclerView with adapter and layout manager

To cover the implementation steps we will display a list of emails. The datasource is hardcoded, right now we are focusing on learning how to use RecyclerView.

Step 1: Get the data by creating a new object class (data source)

	public class Email {
	private int id;
	private String fromName;
	private String subject;
	private String shortBody;
	public Email(int id, String fromName, String subject, String shortBody) {
	this.id = id;
	this.fromName = fromName;
	this.subject = subject;
	this.shortBody = shortBody;
	}
	public int getId() {
	return id;
	}
	public void setId(int id) {
	this.id = id;
	}
	public String getFromName() {
	return fromName;
	}
	public void setFromName(String fromName) {
	this.fromName = fromName;
	}
	public String getSubject() {
	return subject;
	}
	public void setSubject(String subject) {
	this.subject = subject;
	}
	public String getShortBody() {
	return shortBody;
	}
	public void setShortBody(String shortBody) {
	this.shortBody = shortBody;
	}
	@Override
	public String toString() {
	return "Email{" +
	"id='" + id + '\'' +
	", fromName='" + fromName + '\'' +
	", title='" + subject + '\'' +
	", shortBody='" + shortBody + '\'' +
	'}';
	}
	}

view raw Email.java hosted with ❤ by GitHub

	private void inbox() {
	emails = new ArrayList<>();
	Email email = null;
	for (int i = 0; i < 25; i++) {
	email = new Email(0, "Magda " + i, "Hello Android " + i, "This is an intro about Android");
	emails.add(email);
	}
	}

view raw MainActivity.java hosted with ❤ by GitHub

Step 2: Add the RecyclerView dependency to  app/build.gradle file and add RecyclerView to layout

implementation 'androidx.recyclerview:recyclerview:1.2.0-beta01'

view raw build.gradle hosted with ❤ by GitHub

	<?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:orientation="vertical">
	<androidx.recyclerview.widget.RecyclerView
	android:id="@+id/recyclerViewEmails"
	android:layout_width="match_parent"
	android:layout_height="match_parent" />
	</LinearLayout>

view raw activity_main.xml hosted with ❤ by GitHub

Step 3: Create XML layout for item

	<?xml version="1.0" encoding="utf-8"?>
	<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="wrap_content"
	android:orientation="vertical"
	android:id="@+id/linearLayoutEmail"
	android:padding="@dimen/small_padding">
	<TextView
	android:id="@+id/textViewFrom"
	android:layout_width="match_parent"
	android:layout_height="wrap_content"
	android:textStyle="bold"
	tools:text="Magda" />
	<TextView
	android:id="@+id/textViewSubject"
	android:layout_width="match_parent"
	android:layout_height="wrap_content"
	android:paddingTop="@dimen/extra_small_padding"
	android:textColor="@color/purple_700"
	tools:text="Android Fundamentals" />
	<TextView
	android:id="@+id/textViewBody"
	android:layout_width="match_parent"
	android:layout_height="wrap_content"
	android:paddingTop="@dimen/extra_small_padding"
	tools:text="this is a welcome email …." />
	</LinearLayout>

view raw email_item.xml hosted with ❤ by GitHub

Step 4: Define the LayoutManager in activity

	private void setEmailsLayoutManager() {
	recyclerViewEmails.setLayoutManager(new LinearLayoutManager(this));
	}

view raw MainActivity.java hosted with ❤ by GitHub

Step 5: Extend RecyclerView.ViewHolder in a separated class

	public class EmailViewHolder extends RecyclerView.ViewHolder{
	private final TextView textViewFrom, textViewSubject, textViewBody;
	private final LinearLayout linearLayoutEmail;
	public EmailViewHolder(@NonNull View itemView) {
	super(itemView);
	textViewFrom = itemView.findViewById(R.id.textViewFrom);
	textViewSubject = itemView.findViewById(R.id.textViewSubject);
	textViewBody = itemView.findViewById(R.id.textViewBody);
	linearLayoutEmail = itemView.findViewById(R.id.linearLayoutEmail);
	}
	public TextView getTextViewFrom() {
	return textViewFrom;
	}
	public TextView getTextViewSubject() {
	return textViewSubject;
	}
	public TextView getTextViewBody() {
	return textViewBody;
	}
	public LinearLayout getLinearLayoutEmail() {
	return linearLayoutEmail;
	}
	}

view raw EmailViewHolder.java hosted with ❤ by GitHub

Step 6: Extend RecyclerView.Adapter in a separated class

	public class EmailAdapter extends RecyclerView.Adapter<EmailViewHolder> {
	private List<Email> emails;
	private Context context;
	public EmailAdapter(Context context, List<Email> emails) {
	this.emails = emails;
	this.context = context;
	}
	// creates the items and add them to the RecyclerView, just the layout
	@NonNull
	@Override
	public EmailViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
	View itemView = LayoutInflater.from(context).inflate(R.layout.email_item, parent, false);
	return new EmailViewHolder(itemView);
	}
	// binds (displays) the content from the list of emails for each item
	@Override
	public void onBindViewHolder(@NonNull EmailViewHolder holder, int position) {
	Email currentEmail = emails.get(position);
	holder.getTextViewFrom().setText(currentEmail.getFromName());
	holder.getTextViewSubject().setText(currentEmail.getSubject());
	holder.getTextViewBody().setText(currentEmail.getShortBody());
	}
	// we tell to the Recycler View how many items to display
	@Override
	public int getItemCount() {
	return emails.size();
	}
	}

view raw EmailAdapter.java hosted with ❤ by GitHub

Step 7: In onCreate of activity, create a RecyclerView with adapter and layout manager

	private void setEmailsAdapter() {
	recyclerViewEmails.setAdapter(new EmailAdapter(this, emails));
	}
	private void displayEmailsList() {
	// data source – checked
	inbox();
	// layout manager – checked
	setEmailsLayoutManager();
	// adapter – checked
	setEmailsAdapter();
	}

view raw MainActivity.java hosted with ❤ by GitHub

🔔Notify the Adapter

To not impact the speed of rendering the UI elements for a RecyclerView make sure that you’re not calling notifyDataSetChanged(), setAdapter(Adapter), or swapAdapter(Adapter, boolean) for small updates. [official recommendation] The solution is to use SortedList or DiffUtil to create minimal updates when the datasource has changed.

	public class EmailDiffCallback extends DiffUtil.Callback {
	private List<Email> oldList;
	private List<Email> newList;
	public EmailDiffCallback(List<Email> oldList, List<Email> newList) {
	this.oldList = oldList;
	this.newList = newList;
	}
	@Override
	public int getOldListSize() {
	return oldList.size();
	}
	@Override
	public int getNewListSize() {
	return newList.size();
	}
	@Override
	public boolean areItemsTheSame(int oldItemPosition, int newItemPosition) {
	// add a unique ID property on Email and expose a getId() method
	return oldList.get(oldItemPosition).getId() == newList.get(newItemPosition).getId();
	}
	@Override
	public boolean areContentsTheSame(int oldItemPosition, int newItemPosition) {
	Email oldEmail = oldList.get(oldItemPosition);
	Email newEmail = newList.get(newItemPosition);
	if (oldEmail.getFromName() == newEmail.getFromName() && oldEmail.getSubject() == newEmail.getSubject() && oldEmail.getShortBody() == newEmail.getShortBody()) {
	return true;
	}
	return false;
	}
	}

view raw EmailDiffCallback.java hosted with ❤ by GitHub

	// NOT OK
	void onNewEmailsArrivedNotRecommended(List<Email> newEmails) {
	emailAdapter.setEmails(newEmails);
	emailAdapter.notifyDataSetChanged();
	}
	// OK
	void onNewDataArrivedFastRendering(List<Email> newEmails) {
	List<Email> oldEmails = emailAdapter.getEmails();
	DiffUtil.DiffResult result = DiffUtil.calculateDiff(new EmailDiffCallback(oldEmails, newEmails));
	emailAdapter.setEmails(newEmails);
	result.dispatchUpdatesTo(emailAdapter);
	}

view raw MainActivity.java hosted with ❤ by GitHub

✨Item decorators

We could set dividers between the items using DividerItemDecoration

	private void setItemDecorator() {
	RecyclerView.ItemDecoration itemDecoration = new
	DividerItemDecoration(this, DividerItemDecoration.VERTICAL);
	recyclerViewEmails.addItemDecoration(itemDecoration);
	}

view raw MainActivity.java hosted with ❤ by GitHub

💫Swipe to refresh

Step 1: Add a new dependency in the gradle file

implementation 'androidx.swiperefreshlayout:swiperefreshlayout:1.1.0'

view raw build.gradle hosted with ❤ by GitHub

Step 2: Wrap the RecyclerView in a SwipeRefreshLayout

	<?xml version="1.0" encoding="utf-8"?>
	<androidx.swiperefreshlayout.widget.SwipeRefreshLayout xmlns:android="http://schemas.android.com/apk/res/android"
	android:id="@+id/swipeContainer"
	android:layout_width="match_parent"
	android:layout_height="match_parent">
	<androidx.recyclerview.widget.RecyclerView
	android:id="@+id/recyclerViewEmails"
	android:layout_width="match_parent"
	android:layout_height="match_parent" />
	</androidx.swiperefreshlayout.widget.SwipeRefreshLayout>

view raw main_layout.xml hosted with ❤ by GitHub

Step 3: Update the code in the Adapter

	// Clean all elements of the recycler
	public void clear() {
	emails.clear();
	notifyDataSetChanged();
	}
	// Add a list of items
	public void addAll(List<Email> list) {
	emails.addAll(list);
	notifyDataSetChanged();
	}

view raw EmailAdapter.java hosted with ❤ by GitHub

Step 4: Setup SwipeRefreshLayout

	private void setupSwipeToRefresh() {
	swipeRefreshLayout.setOnRefreshListener(new SwipeRefreshLayout.OnRefreshListener() {
	@Override
	public void onRefresh() {
	// Make sure you call swipeRefreshLayout.setRefreshing(false)
	// once the network request has completed successfully.
	inbox();
	}
	});
	// Configure the refreshing colors
	swipeRefreshLayout.setColorSchemeResources(android.R.color.holo_blue_bright,
	android.R.color.holo_green_light,
	android.R.color.holo_orange_light,
	android.R.color.holo_red_light);
	}

view raw MainActivity.java hosted with ❤ by GitHub

You could check the full source code here.

📚Learn more

• Android Vitals – Slow Rendering
• Create dynamic lists with RecyclerView  
• RecyclerView ins and outs – Google I/O 2016
• Yigit Boyar: Pro RecyclerView

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Thank you for reading! 🙂

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