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c# – 如何在Xamarin Forms中从ViewModel设置焦点

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我想在做一些异步操作后将焦点设置在SearchBox控件中,我想从我的ViewModel中做到这一点.

我怎么能这样做？

编辑

ViewModel代码：

private bool _searchBarFocused;

   public bool SearchBarFocused

   {

       get { return _searchBarFocused; }

       set

       {

           _searchBarFocused = value;

           base.OnPropertyChanged("SearchBarFocused");

       }

   }

   public async Task InitializeData()

   {

       // Other operations...

       SearchBarFocused = true;

   }

查看代码隐藏代码：

protected override void OnAppearing()

   {

       base.OnAppearing();

       (this.BindingContext as MyViewModel).InitializeData();

   }

SearchBar XAML代码：

<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>

触发动作代码：

public class SearchBarFocusTriggerAction : TriggerAction<SearchBar>

{

   public bool Focused { get; set; }

   protected override void Invoke(SearchBar searchBar)

   {

       if (Focused)

           searchBar.Focus();

       else

           searchBar.Unfocus();

   }

}

其中一个选项是使用触发器(XAML方式)：

<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;

   }

}