c# - 在带有 Prism mvvm View 模型的xamarin形式的xaml中包括xaml。无法解析INavigationService

Question

我想在所选页面中包含一个后退按钮。由于某种原因，我将其称为“面包屑”，请假定其名为“BackButton” :)。

问题是导航服务无法在ioc中获得通过。

以下是我遇到的以下代码:

主页XAML -注意local:BreadcrumbControl

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:prism="clr-namespace:Prism.Mvvm;assembly=Prism.Forms"
             prism:ViewModelLocator.AutowireViewModel="True"
             x:Class="Test.Views.MainPage"
             xmlns:local="clr-namespace:Test.Views">

    <StackLayout>
        <local:BreadcrumbControl x:Name="Breadcrumb" />
    </Stacklayout>
<ContentPage>

面包屑控件XAML

<?xml version="1.0" encoding="utf-8" ?>
<ContentView xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:prism="clr-namespace:Prism.Mvvm;assembly=Prism.Forms"
             prism:ViewModelLocator.AutowireViewModel="True"
             x:Class="Test.Views.BreadcrumbControl">

    <Button Text="Back" Command="{Binding NavigateCommand}" CommandParameter="NavigationPage/ManageFoodGroupsPage" />

</ContentView>

面包屑页面 View 模型

public class BreadcrumbControlViewModel : BindableBase
{
    INavigationService navigationService;

    public DelegateCommand<string> NavigateCommand { get; set; }

    // If i uncomment the navigationservice the following error occurs (*)
    public BreadcrumbControlViewModel(/*INavigationService navigationService*/)
    {
        this.navigationService = navigationService;
        NavigateCommand = new DelegateCommand<string>(Navigate);
    }

    private void Navigate(string name)
    {
        navigationService.GoBackAsync();
    }
}

(*)

An exception of type 'Microsoft.Practices.Unity.ResolutionFailedException' occurred in Microsoft.Practices.Unity.dll but was not handled in user code

Additional information: Resolution of the dependency failed, type = "Test.ViewModels.BreadcrumbControlViewModel", name = "(none)".

Exception occurred while: Resolving parameter "navigationService" of constructor Test.ViewModels.BreadcrumbControlViewModel(Prism.Navigation.INavigationService navigationService).

Exception is: NullReferenceException - Object reference not set to an instance of an object.

---

At the time of the exception, the container was:

Resolving Test.ViewModels.BreadcrumbControlViewModel,(none)

Resolving parameter "navigationService" of constructor Test.ViewModels.BreadcrumbControlViewModel(Prism.Navigation.INavigationService navigationService)

App.cs容器/解析器注册

ViewModelLocationProvider.Register<BreadcrumbControl, BreadcrumbControlViewModel>();

Container.RegisterTypeForNavigation<MainPage>();

如何在Xamarin Forms Prism MVVM应用程序中包含常见的后退按钮样式的东西(由 View 模型驱动)。

Answer 1

由于Xamarin.Forms(基于页面)中导航的性质，INavigationService仅适用于属于Page类型的ViewModel。它必须是一个页面，否则，您将无法导航。如果必须在此ContentView的VM中具有INavigationService，则必须在容器中注册INavigationService，但是它可能无法正常运行，因为它将在Application.Current.MainPage而不是正确的Page上运行。