If you are using MVVM, and want to set SelectedItem
from the view model, things get tricky. There seems to be a bug in Xamarin that prevents us from using SelectedItem
with a two way binding. More info: Xamarin Forms ListView SelectedItem Binding Issue and https://xamarin.github.io/bugzilla-archives/58/58451/bug.html.
Luckily, we can easily write our own Picker
.
public class TwoWayPicker : Picker
{
public TwoWayPicker()
{
SelectedIndexChanged += (sender, e) => SelectedItem = ItemsSource[SelectedIndex];
}
public static new readonly BindableProperty SelectedItemProperty = BindableProperty.Create(
nameof(SelectedItem), typeof(object), typeof(TwoWayPicker), null, BindingMode.TwoWay, propertyChanged: OnSelectedItemChanged);
public new object SelectedItem
{
get => GetValue(SelectedItemProperty);
set => SetValue(SelectedItemProperty, value);
}
private static void OnSelectedItemChanged(BindableObject bindable, object oldValue, object newValue)
{
var control = (TwoWayPicker)bindable;
control.SetNewValue(newValue);
}
private void SetNewValue(object newValue)
{
if (newValue == null)
{
return;
}
for(int i = 0; i < ItemsSource.Count; i++)
{
if (ItemsSource[i].Equals(newValue))
{
SelectedIndex = i;
return;
}
}
}
}
Because is uses the same SelectedItem
property, it is a drop-in replacement for Picker
.
Note that if you want value equality rather than reference equality for the item class, you'll also need to override Equals
like this:
public override bool Equals(object obj)
{
var other = obj as YourClass;
if (other == null)
{
return false;
}
else
{
return other.SomeValue == SomeValue; // implement your own
}
}