Xamarin Android for Android Developers

In the previous post we completed all the basic tasks building and publishing an app for Android using Xamarin Android. There are some other techniques that android developers tend to use that are slightly different when using C#

Click Handers in Recyclerviews

In kotlin you will often see click handlers attached to RecyclerView items by using setOnClickListener like this

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override fun onBindViewHolder(holder: RecyclerViewHolder, position: Int) {
  val id = presenter.getTrackId(position)
  val label = presenter.getTrackLabel(position)
  val sublabel = presenter.getTrackSubLabel(position)
  holder.bind(label, sublabel)
  holder.rowLabelContainer.setOnClickListener {
    presenter.trackSelected(id, position)
  }
}

override fun trackSelected(trackId: Long, position: Int) {
    if (trackId < 0) {
        return
    }
    if (inMultiSelectMode) {
        toggleSelection(position)
    } else {
        view?.navigateToTrack(trackId)
    }
}

Although you can (and possibly should) call setOnClickListener(null) to deregister the event handler most examples dont do this.

There are Xamarin.Android wrappers that mean I could just call setOnClickListener however while putting together the rest of the app I had decided I preferred to use C# EventHandlers so I needed a mechanism to use them, in C# I wrote this

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public override void OnBindViewHolder(RecyclerView.ViewHolder holder, int position)
{
  RecyclerViewHolder vh = holder as RecyclerViewHolder;
  // unsubscribe if it was subscribed before
  vh.Container.Click -= Container_Click;

  vh.Label.Text = ViewModel.GetLabelForListItem(Items[position]);
  vh.CheckBox.Checked = Items[position].Selected;

  vh.Container.Tag = position.ToString();
  vh.Container.Click += Container_Click;
}

private void Container_Click(object sender, EventArgs e)
{
  int position = Convert.ToInt32(((View)sender).Tag.ToString());
  Items[position].Selected = !Items[position].Selected;
  NotifyItemChanged(position);
  ViewModel.SelectionChanged(position);
}

class RecyclerViewHolder : RecyclerView.ViewHolder
{
  public View Container { get; private set; }
  public TextView Label { get; private set; }
  public AppCompatCheckBox CheckBox { get; private set; }

  public RecyclerViewHolder(View itemView) : base(itemView)
  {
    Container = itemView.FindViewById<View>(Resource.Id.purge_row_label_container);
    Label = itemView.FindViewById<TextView>(Resource.Id.purge_row_label);
    CheckBox = itemView.FindViewById<AppCompatCheckBox>(Resource.Id.purge_row_check);
  }
}

The ViewHolder.Container is just a simple Android View and I wanted to use its Click C# EventHandler, I would use standard C# += and -= syntax to add and remove the handler.

Inside the event handler Container_Click I needed to be able to find the id of the item that was being represented. To do this I just set the Tag property on the ViewHolder.Container to be the position in the list and then in the evnt handler I could read the Tag property and convert it back into the position