棱镜和简单的注射器

Question

我正在尝试使用简单的注入和棱镜进行简单的 HelloWorld。 Git Source

应用程序启动时，出现此错误

Failed to assign to property 'Prism.Windows.Mvvm.ViewModelLocator.AutoWireViewModel'. [Line: 8 Position: 5]"

Exception thrown: 'System.MissingMethodException' in Prism.Windows.dll Exception thrown: 'Windows.UI.Xaml.Markup.XamlParseException' in HelloWorldPrism.exe WinRT information: Failed to assign to property 'Prism.Windows.Mvvm.ViewModelLocator.AutoWireViewModel'. [Line: 8 Position: 5] An exception of type 'Windows.UI.Xaml.Markup.XamlParseException' occurred in HelloWorldPrism.exe but was not handled in user code WinRT information: Failed to assign to property 'Prism.Windows.Mvvm.ViewModelLocator.AutoWireViewModel'. [Line: 8 Position: 5] Additional information: The text associated with this error code could not be found. Failed to assign to property 'Prism.Windows.Mvvm.ViewModelLocator.AutoWireViewModel'. [Line: 8 Position: 5]

e.StackTrace " at Windows.UI.Xaml.Application.LoadComponent(Object component, Uri resourceLocator, ComponentResourceLocation componentResourceLocation)\r\n at HelloWorldPrism.Views.MainView.InitializeComponent()\r\n at HelloWorldPrism.Views.MainView..ctor()" string

<Page
    x:Class="HelloWorldPrism.Views.MainView"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    xmlns:mvvm="using:Prism.Windows.Mvvm"
    mvvm:ViewModelLocator.AutoWireViewModel="True"
    mc:Ignorable="d"
    >

public MainViewModel(INavigationService navigationService)
{
    _navigationService = navigationService;
}

如果我添加一个无参数构造函数，它会正常工作。

public MainViewModel()
{
}

App.cs

protected override Task OnLaunchApplicationAsync(LaunchActivatedEventArgs args)
{
    Window.Current.Activate();
    return Task.FromResult(true);
}

protected override void CreateAndConfigureContainer()
{
    Logger.Log("Creating and Configuring Container", Category.Debug, Priority.Low);
    Container = CreateContainer();
}

protected override Container CreateContainer()
{
    return new Container();
}

protected override UIElement CreateShell(Frame rootFrame)
{
    var shell = Container.GetInstance<MainView>();
    shell.SetFrame(rootFrame);
    return shell;
}

protected override Type GetPageType(string pageToken)
{
    var type = Type.GetType(string.Format(CultureInfo.InvariantCulture, GetType().AssemblyQualifiedName.Replace(GetType().FullName, GetType().Namespace + ".Views.{0}View"), pageToken));
    if (type != null)
        return type;
    throw new ArgumentException(string.Format(CultureInfo.InvariantCulture, ResourceLoader.GetForCurrentView("/Prism.Windows/Resources/").GetString("DefaultPageTypeLookupErrorMessage"), pageToken, GetType().Namespace + ".Views"), nameof(pageToken));
}

protected override Task OnInitializeAsync(IActivatedEventArgs args)
{
    Container.RegisterSingleton(SessionStateService);
    Container.RegisterSingleton(DeviceGestureService);
    Container.RegisterSingleton(NavigationService);
    Container.RegisterSingleton(EventAggregator);
    return Task.CompletedTask;
}

protected override void ConfigureViewModelLocator()
{
    ServiceLocator.SetLocatorProvider(() => new 
    SimpleInjectorServiceLocatorAdapter(Container));
}

Answer 1

When application starts, this error come up

在 MainView.xaml 中，您将 AutoWireViewModel 属性定义为 true。一旦属性设置为 True，ViewModelLocator 将尝试根据特定约定实例化相应的 ViewModel。由于您的 View 和 ViewModel 的名称符合约定，当您将此属性设置为 true 时，Prism 将帮助您实例化相应的 ViewModel。

在 Prism.mvvm 命名空间内，ViewModelLocationProvider class 为 AutoWireViewModelChanged 附加的属性设置为 true 的视图定位视图模型。并且错误由以下代码行 ViewModelLocationProvider class:

抛出

/// <summary>
/// The default view model factory which provides the ViewModel type as a parameter.
/// </summary>
static Func<Type, object> _defaultViewModelFactory = type => Activator.CreateInstance(type);

System.MissingMethodException: 'No parameterless constructor defined for this object.'

所以是Activator.CreateInstance(Type)方法需要public构造器造成的，请看MissingMethodException.

If I add a parameterless constructor it works normal.

看来这是正确的解决方案。如果你只是不想要 ViewModel 的无参数构造函数，你可以尝试实例化它并自己将 DataContext 设置为 View。如果您怀疑这是 Prism 库的问题，也许您可以打开一个线程 here.

更新：

根据@rubWhosebug，OnInitializeAsync 方法缺少 ViewModelLocationProvider.SetDefaultViewModelFactory((viewMo‌delType) => Container.GetInstance(viewModelType));。

棱镜和简单的注射器

Prism and Simple Injector

prism

mvvm

simple-injector

uwp