c# - ejemplo - xamarin forms pages

Cómo establecer el foco desde ViewModel en formularios Xamarin

Question

Quiero establecer el foco en un control de SearchBox después de hacer algunas operaciones asincrónicas, y me gustaría hacerlo desde mi ViewModel .

¿Cómo podría hacer esto posible?

EDITAR

Código de ViewModel:

 private bool _searchBarFocused;
public bool SearchBarFocused
{
get { return _searchBarFocused; }
set
{
_searchBarFocused = value;
base.OnPropertyChanged("SearchBarFocused");
}
}
public async Task InitializeData()
{
// Other operations...
SearchBarFocused = true;
}

Ver código de código detrás:

 protected override void OnAppearing()
{
base.OnAppearing();
(this.BindingContext as MyViewModel).InitializeData();
}

Código SearchBar XAML:

 <SearchBar SearchCommand="{Binding SearchItemsCommand}">
<SearchBar.Triggers>
<DataTrigger TargetType="SearchBar"
Binding="{Binding SearchBarFocused, Mode=TwoWay}" Value="True">
<Trigger.EnterActions>
<triggers:SearchBarFocusTriggerAction Focused="True" />
</Trigger.EnterActions>
<Trigger.ExitActions>
<triggers:SearchBarFocusTriggerAction Focused="False" />
</Trigger.ExitActions>
</DataTrigger>
</SearchBar.Triggers>
</SearchBar>

Código de acción desencadenante:

public class SearchBarFocusTriggerAction : TriggerAction<SearchBar>
{
public bool Focused { get; set; }
protected override void Invoke(SearchBar searchBar)
{
if (Focused)
searchBar.Focus();
else
searchBar.Unfocus();
}
}

Answer 1

Una de las opciones es usar disparadores (forma XAML):

 <SearchBar x:Name="searchBar"
Text=""
Placeholder="type something">
<SearchBar.Triggers>
<DataTrigger TargetType="SearchBar"
Binding="{Binding ViewModelIsSearchBarFocused}"
Value="True">
<Trigger.EnterActions>
<local:FocusTriggerAction Focused="True" />
</Trigger.EnterActions>
<Trigger.ExitActions>
<local:FocusTriggerAction Focused="False" />
</Trigger.ExitActions>
</DataTrigger>
<EventTrigger Event="Unfocused">
<local:UnfocusedTriggerAction />
</EventTrigger>
</SearchBar.Triggers>
</SearchBar>

public class FocusTriggerAction : TriggerAction<SearchBar>
{
public bool Focused { get; set; }
protected override async void Invoke (SearchBar searchBar)
{
await Task.Delay(1000);
if (Focused)
{
searchBar.Focus();
}
else
{
searchBar.UnFocus();
}
}
}
public class UnfocusedTriggerAction : TriggerAction<SearchBar>
{
protected override void Invoke (SearchBar searchBar)
{
YourViewModel.ViewModelIsSearchBarFocused = false;
}
}

Lea más aquí: https://developer.xamarin.com/guides/cross-platform/xamarin-forms/working-with/triggers/

Answer 2

Este es un ejemplo de cómo lo hago, antes de esto solía hacer uso de MessagingCenter

en xaml, necesitas dar un x: Name al obj que quieres enfocar.

<!-- PICKER''s DEFINITION -->
<DatePicker
x:Name="Datepicker"
Date="{Binding SelectedDate, Mode=TwoWay}"
IsEnabled="true"
IsVisible="false">
</DatePicker>

entonces tiene que hacer referencia a ese control en su parámetro de comando en un botón o, por ejemplo, en este caso utilizo un elemento de la barra de herramientas.

<!-- MENU TOOLBAR -->
<ContentPage.ToolbarItems>
<ToolbarItem
Command="{Binding ShowCalendarCommand}"
Icon="Calendar"
CommandParameter="{x:Reference Datepicker}" />
</ContentPage.ToolbarItems>

luego en tu comando vm:

#region toolbar commands
public ICommand ShowCalendarCommand => new RelayCommand<Object>(ShowCalendar);
#endregion
private void ShowCalendar(Object obj)
{
var calendar = (DatePicker)obj;
calendar.Focus();
// MessagingCenter.Send(this, "Calendar");
}