Lists and RecyclerView

When we work with lists in Android, we first create a model to hold the data we want to display:

public class Message {
    private DateTime date;
    private String from, message;

    public Message(String from, String message, DateTime date) {
        this.date = date;
        this.from = from;
        this.message = message;
    }

    public String getFrom() {
        return from;
    }

    public String getMessage() {
        return message;
    }

    public DateTime getDate() {
        return date;
    }
}

Then, we create a layout xml file used for each message in the list. The views you declare here will end up in each row of the list, and you will be populate each row with the right data. We place this file in res/layout/item_message.xml.

<?xml version="1.0" encoding="utf-8"?>

    <RelativeLayout
            android:layout_height="match_parent"
            android:layout_width="match_parent"
            android:padding="10dp">
            <TextView
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:id="@+id/tvFrom"
                android:text="From:"/>
            <TextView
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:id="@+id/tvMessage"
                android:text="Message body"/>

            <TextView
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:id="@+id/tvDate"
                android:text="Timestamp"/>
        </RelativeLayout>

For displaying data in lists, we use what is called a RecyclerView. It uses an Adapter to control the display of each item in the list by asking you to first create a view for each position, and then define a way to update these views for the data at each position. We use a helper object called a ViewHolder that stores references to all the views we need for each row, e.g. a TextView for displaying an item name and a button for taking some action. We create this object in onCreateViewHolder using a LayoutInflater to create the item layout, and then findViewById to obtain references to the subviews. In onBindViewHolder we use the ViewHolder to get the stored TextView for this particular row and update the text displayed to the message text for this position in the list.

ppublic class MessagesAdapter extends RecyclerView.Adapter<MessagesAdapter.MessagesViewHolder> {
    private LayoutInflater inflater;
    private List<Message> messages;

    public MessagesAdapter(LayoutInflater inflater, List<Message> messages) {
        this.inflater = inflater;
        this.messages = messages;
    }

    @Override
    public MessagesViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
        View itemMessage = inflater.inflate(R.layout.item_message, parent, false);
        MessagesViewHolder messagesViewHolder = new MessagesViewHolder(itemMessage, tapDelegate, null);
        messagesViewHolder.view = itemMessage;
        messagesViewHolder.from = (TextView) itemMessage.findViewById(R.id.tvFrom);
        messagesViewHolder.message = (TextView) itemMessage.findViewById(R.id.tvMessage);
        messagesViewHolder.date = (TextView) itemMessage.findViewById(R.id.tvDate);

        return messagesViewHolder;
    }

    @Override
    public void onBindViewHolder(MessagesViewHolder holder, int position) {
        Message message = messages.get(position);
        holder.from.setText(message.getFrom());
        holder.message.setText(message.getMessage());
        holder.date.setText(message.getDateTimeString());
        holder.position = position;
    }

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

    public static class MessagesViewHolder extends RecyclerView.ViewHolder {
        View view;
        TextView from, message, date;
        int position;

        public MessagesViewHolder(View itemView) {
            super(itemView);
            this.view = itemView;
        }
    }
}

The MessagesViewHolder caches the view elements for quicker access. The MessagesAdapter provides access to the data set, as well as provides the correct view elements. In our example this is seen by the itemMessage.findViewById() calls. We also need a LayoutManager, for the layout of child views. To display a list, the LinearLayoutManager is the chosen layout manager. It can display items either vertically or horizontally.

myRecyclerView.setLayoutManager(new LinearLayoutManager(this));

In the activity where we want to display a list, we get the RecyclerView from the activity layout using the id R.id.messageList and set it to use our adapter and a LayoutManager.

ArrayList<Message> messages = new ArrayList<Message>();
RecyclerView msgView = (RecyclerView) findViewById(R.id.messageList);
msgView.setLayoutManager(new LinearLayoutManager(this));
MessagesAdapter msgAdapter = new MessagesAdapter(this.getLayoutInflater(), messages, this, this);
msgView.setAdapter(msgAdapter);