在Xamarin.Forms中使用加載的觸發器

Question

在WPF或UWP中使用MVVM Light時，可以使用觸發事件“加載”，例如使用導航到頁面的觸發器。

像這樣：

<i:Interaction.Behaviors>
            <core:EventTriggerBehavior EventName="Loaded">
                <core:InvokeCommandAction Command="{Binding LoadedCommand}" />
            </core:EventTriggerBehavior>
</i:Interaction.Behaviors>

這是名稱空間：

xmlns:i="using:Microsoft.Xaml.Interactivity"
xmlns:core="using:Microsoft.Xaml.Interactions.Core"

但是如何在Xamarin.Forms執行此Xamarin.Forms ？ 一樣嗎 我找不到一個例子。

UPDATE

我的xaml頁面：

對EventToCommandBehavior的引用：

xmlns:behaviors="clr-namespace:XamarinTest.Behaviors"

<ContentPage.Behaviors>
        <behaviors:EventToCommandBehavior EventName="OnAppearing"
                                          Command="{Binding LoadedCommand}" />
</ContentPage.Behaviors>
<ContentPage.Content>
    <StackLayout>
        <Label Text="Welcome to Xamarin.Forms!"
               VerticalOptions="CenterAndExpand" 
               HorizontalOptions="CenterAndExpand" />
    </StackLayout>
</ContentPage.Content>

這是我的ViewModel：

public RelayCommand LoadedCommand { get; }

public UserMainViewModel()
{
    LoadedCommand = new RelayCommand(LoadData);
}

private async void LoadData()
{
    //...
}

xaml Page和ViewModel之間的綁定在Page1.xaml.cs中是這樣的：

public Page1()
{
    InitializeComponent();
    BindingContext = App.Locator.UserMainViewModel;
}

錯誤：

 at System.Reflection.MonoCMethod.InternalInvoke (System.Object obj, System.Object[] parameters) [0x00017] in <657aa8fea4454dc898a9e5f379c58734>:0 
  at System.Reflection.MonoCMethod.DoInvoke (System.Object obj, System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) [0x0007a] in <657aa8fea4454dc898a9e5f379c58734>:0 
  at System.Reflection.MonoCMethod.Invoke (System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) [0x00000] in <657aa8fea4454dc898a9e5f379c58734>:0 
  at System.Reflection.ConstructorInfo.Invoke (System.Object[] parameters) [0x00000] in <657aa8fea4454dc898a9e5f379c58734>:0 
  at MyProject.Services.NavigationService.NavigateTo (System.String pageKey, System.Object parameter) [0x000e8] in C:\Projects\MyProject\MyProject\MyProject\Services\NavigationService.cs:89 
  at MyProject.Services.NavigationService.NavigateTo (System.String pageKey) [0x00001] in C:\Projects\MyProject\MyProject\MyProject\Services\NavigationService.cs:42 
  at MyProject.ViewModels.LoginViewModel.Login () [0x00002] in C:\Projects\MyProject\MyProject\MyProject\ViewModels\LoginViewModel.cs:25 

Answer 1

您必須按照此處所述使用EventToCommandBehavior https://developer.xamarin.com/guides/xamarin-forms/application-fundamentals/behaviors/reusable/event-to-command-behavior/

並在頁面中實現以下xaml：OnAppearing僅存在於頁面上。 （未加載事件）

<ContentPage.Behaviors>
   <local:EventToCommandBehavior EventName="Appearing" Command="{Binding LoadedCommand}" />
<ContentPage.Behaviors>