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Extending the RecyclerView Example

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The basic app described in A Basic RecyclerView Example actually doesn't do much – it simply scrolls and displays a fixed list of photograph items to facilitate browsing. In real-world applications, users expect to be able to interact with the app by tapping items in the display. Also, the underlying data source can change (or be changed by the app), and the contents of the display must remain consistent with these changes. In the following sections, you'll learn how to handle item-click events and update RecyclerView when the underlying data source changes.

Handling Item-Click Events

When a user touches an item in the RecyclerView, an item-click event is generated to notify the app as to which item was touched. This event is not generated by RecyclerView – instead, the item view (which is wrapped in the view holder) detects touches and reports these touches as click events.

To illustrate how to handle item-click events, the following steps explain how the basic photo-viewing app is modified to report which photograph had been touched by the user. When an item-click event occurs in the sample app, the following sequence takes place:

  1. The photograph's CardView detects the item-click event and notifies the adapter.

  2. The adapter forwards the event (with item position information) to the activity's item-click handler.

  3. The activity's item-click handler responds to the item-click event.

First, an event handler member called ItemClick is added to the PhotoAlbumAdapter class definition:

public event EventHandler<int> ItemClick;

Next, an item-click event handler method is added to MainActivity. This handler briefly displays a toast that indicates which photograph item was touched:

void OnItemClick (object sender, int position)
{
    int photoNum = position + 1;
    Toast.MakeText(this, "This is photo number " + photoNum, ToastLength.Short).Show();
}

Next, a line of code is needed to register the OnItemClick handler with PhotoAlbumAdapter. A good place to do this is immediately after PhotoAlbumAdapter is created:

mAdapter = new PhotoAlbumAdapter (mPhotoAlbum);
mAdapter.ItemClick += OnItemClick;

In this basic example, handler registration takes place in the main activity's OnCreate method, but a production app might register the handler in OnResume and unregister it in OnPause – see Activity Lifecycle for more information.

PhotoAlbumAdapter will now call OnItemClick when it receives an item-click event. The next step is to create a handler in the adapter that raises this ItemClick event. The following method, OnClick, is added immediately after the adapter's ItemCount method:

void OnClick (int position)
{
    if (ItemClick != null)
        ItemClick (this, position);
}

This OnClick method is the adapter's listener for item-click events from item views. Before this listener can be registered with an item view (via the item view's view holder), the PhotoViewHolder constructor must be modified to accept this method as an additional argument, and register OnClick with the item view Click event. Here's the modified PhotoViewHolder constructor:

public PhotoViewHolder (View itemView, Action<int> listener)
    : base (itemView)
{
    Image = itemView.FindViewById<ImageView> (Resource.Id.imageView);
    Caption = itemView.FindViewById<TextView> (Resource.Id.textView);

    itemView.Click += (sender, e) => listener (base.LayoutPosition);
}

The itemView parameter contains a reference to the CardView that was touched by the user. Note that the view holder base class knows the layout position of the item (CardView) that it represents (via the LayoutPosition property), and this position is passed to the adapter's OnClick method when an item-click event takes place. The adapter's OnCreateViewHolder method is modified to pass the adapter's OnClick method to the view-holder's constructor:

PhotoViewHolder vh = new PhotoViewHolder (itemView, OnClick);

Now when you build and run the sample photo-viewing app, tapping a photo in the display will cause a toast to appear that reports which photograph was touched:

Example Toast that appears when a photo Card is tapped

This example demonstrates just one approach for implementing event handlers with RecyclerView. Another approach that could be used here is to place events on the view holder and have the adapter subscribe to these events. If the sample photo app provided a photo editing capability, separate events would be required for the ImageView and the TextView within each CardView: touches on the TextView would launch an EditView dialog that lets the user edit the caption, and touches on the ImageView would launch a photo touchup tool that lets the user crop or rotate the photo. Depending on the needs of your app, you must design the best approach for handling and responding to touch events.

To demonstrate how RecyclerView can be updated when the data set changes, the sample photo-viewing app can be modified to randomly pick a photo in the data source and swap it with the first photo. First, a Random Pick button is added to the example photo app's Main.axml layout:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent">
    <Button
        android:id="@+id/randPickButton"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:gravity="center_horizontal"
        android:textAppearance="?android:attr/textAppearanceLarge"
        android:text="Random Pick" />
    <android.support.v7.widget.RecyclerView
        android:id="@+id/recyclerView"
        android:scrollbars="vertical"
        android:layout_width="fill_parent"
        android:layout_height="fill_parent" />
</LinearLayout>

Next, code is added at the end of the main activity's OnCreate method to locate the Random Pick button in the layout and attach a handler to it:

Button randomPickBtn = FindViewById<Button>(Resource.Id.randPickButton);

randomPickBtn.Click += delegate
{
    if (mPhotoAlbum != null)
    {
        // Randomly swap a photo with the first photo:
        int idx = mPhotoAlbum.RandomSwap();
    }
};

This handler calls the photo album's RandomSwap method when the Random Pick button is tapped. The RandomSwap method randomly swaps a photo with the first photo in the data source, then returns the index of the randomly-swapped photo. When you compile and run the sample app with this code, tapping the Random Pick button does not result in a display change because the RecyclerView is not aware of the change to the data source.

To keep RecyclerView updated after the data source changes, the Random Pick click handler must be modified to call the adapter's NotifyItemChanged method for each item in the collection that has changed (in this case, two items have changed: the first photo and the swapped photo). This causes RecyclerView to update its display so that it is consistent with the new state of the data source:

Button randomPickBtn = FindViewById<Button>(Resource.Id.randPickButton);

randomPickBtn.Click += delegate
{
    if (mPhotoAlbum != null)
    {
        int idx = mPhotoAlbum.RandomSwap();

        // First photo has changed:
        mAdapter.NotifyItemChanged(0);

        // Swapped photo has changed:
        mAdapter.NotifyItemChanged(idx);
    }
};

Now, when the Random Pick button is tapped, RecyclerView updates the display to show that a photo further down in the collection has been swapped with the first photo in the collection:

First screenshot before swap, second screenshot after swap

Of course, NotifyDataSetChanged could have been called instead of making the two calls to NotifyItemChanged, but doing so would force RecyclerView to refresh the entire collection even though only two items in the collection had changed. Calling NotifyItemChanged is significantly more efficient than calling NotifyDataSetChanged.