使用 NInject 在 WPF 中注入没有无参数构造函数的视图模型 class

Question

我正在使用 NInject 解决我的第一个 WPF 应用程序的依赖关系。以下是我的代码片段。

我的App.xaml.cs是这样的。

public partial class App : Application
{
    private IKernel container;

    protected override void OnStartup(StartupEventArgs e)
    {
        base.OnStartup(e);
        ConfigureContainer();
        ComposeObjects();
    }

    private void ComposeObjects()
    {
        Current.MainWindow = this.container.Get<MainWindow>();
    }

    private void ConfigureContainer()
    {
        this.container = new StandardKernel();
        container.Bind<ISystemEvents>().To<MySystemEvents>();

    }
}

App.xaml 是这样的。

<Application x:Class="Tracker.App"
         xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
    <Application.Resources>

    </Application.Resources>
</Application>

MainWindow.xaml.

<Window x:Class="Tracker.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:viewmodel="clr-namespace:Tracker.ViewModel"
        Title="MainWindow" Height="150" Width="350">
    <Window.DataContext>
        <viewmodel:TrackerViewModel>
        </viewmodel:TrackerViewModel>
    </Window.DataContext>
    <Grid>
    </Grid>
</Window>

MainWindow.xaml.cs

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
    }
}

和视图模型

internal class TrackerViewModel : System.ComponentModel.INotifyPropertyChanged
{
    public TrackerViewModel(ISystemEvents systemEvents)
    {
        systemEvents.SessionSwitch += SystemEvents_SessionSwitch;
    }

    private void SystemEvents_SessionSwitch(object sender, SessionSwitchEventArgs e)
    {
    }
}

现在，当我启动应用程序时，我在 InitializeComponent() 方法中遇到异常 An unhandled exception of type 'System.NullReferenceException' occurred in PresentationFramework.dll。

我知道它是因为视图模型 class 没有无参数构造函数。但我无法理解为什么依赖注入器无法解决这个问题？我做错了什么吗？

如有任何帮助，我们将不胜感激。

Answer 1

trackerviewmodel 将由自动生成的 xaml 设计器代码实例化，而不是 ninject。我从未使用过 ninject，但我认为您需要配置容器以了解您的 viewModel，然后注入 Ninject 的 viewmodel 来解析它及其依赖项：

public class MainWindow : Window
{
    [Inject]
    public TrackerViewModel ViewModel { get; set; }

    public MainWindow()
    {
        InitializeComponent();
        DataContext = ViewModel;
    }
}

Answer 2

首先，推荐阅读Dependency Injection in .NET, especially the section about WPF. But even if you don't read it, there is a helpful example in the code download这本书

您已经确定需要从 App.xaml 文件中删除 StartupUri="MainWindow.xaml"。

但是，当使用 DI 时，您不能以声明方式连接 DataContext，否则它只能使用默认构造函数。

<Window x:Class="WpfWithNinject.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="150" Width="350">

</Window>

当涉及到 DI 时，WPF 中使用的模式有点令人困惑。主要问题是，如果您希望 ViewModel 能够控制自己的 windowing 环境，则 MainWindow 与其 ViewModel 之间存在循环依赖问题，因此您需要创建一个Abstract Factory 为了实例化 ViewModel 以满足依赖关系。

创建 ViewModel 工厂

internal interface ITrackerViewModelFactory
{
    TrackerViewModel Create(IWindow window);
}

internal class TrackerViewModelFactory : ITrackerViewModelFactory
{
    private readonly ISystemEvents systemEvents;

    public TrackerViewModelFactory(ISystemEvents systemEvents)
    {
        if (systemEvents == null)
        {
            throw new ArgumentNullException("systemEvents");
        }

        this.systemEvents = systemEvents;
    }

    public TrackerViewModel Create(IWindow window)
    {
        if (window == null)
        {
            throw new ArgumentNullException("window");
        }

        return new TrackerViewModel(this.systemEvents, window);
    }
}

TrackerViewModel 也需要进行一些修改，以便它可以将 IWindow 接受到其构造函数中。这允许 TrackerViewModel 控制自己的 windowing 环境，例如向用户显示模式对话框。

internal class TrackerViewModel : System.ComponentModel.INotifyPropertyChanged
{
    private readonly IWindow window;

    public TrackerViewModel(ISystemEvents systemEvents, IWindow window)
    {
        if (systemEvents == null)
        {
            throw new ArgumentNullException("systemEvents");
        }
        if (window == null)
        {
            throw new ArgumentNullException("window");
        }

        systemEvents.SessionSwitch += SystemEvents_SessionSwitch;
        this.window = window;
    }

    private void SystemEvents_SessionSwitch(object sender, SessionSwitchEventArgs e)
    {
    }

    public event System.ComponentModel.PropertyChangedEventHandler PropertyChanged;
}

调整 Window

您需要使用 windows、IWindow 的抽象类型和帮助管理每个 windows、WindowAdapter.

internal interface IWindow
{
    void Close();

    IWindow CreateChild(object viewModel);

    void Show();

    bool? ShowDialog();
}

internal class WindowAdapter : IWindow
{
    private readonly Window wpfWindow;

    public WindowAdapter(Window wpfWindow)
    {
        if (wpfWindow == null)
        {
            throw new ArgumentNullException("window");
        }

        this.wpfWindow = wpfWindow;
    }

    #region IWindow Members

    public virtual void Close()
    {
        this.wpfWindow.Close();
    }

    public virtual IWindow CreateChild(object viewModel)
    {
        var cw = new ContentWindow();
        cw.Owner = this.wpfWindow;
        cw.DataContext = viewModel;
        WindowAdapter.ConfigureBehavior(cw);

        return new WindowAdapter(cw);
    }

    public virtual void Show()
    {
        this.wpfWindow.Show();
    }

    public virtual bool? ShowDialog()
    {
        return this.wpfWindow.ShowDialog();
    }

    #endregion

    protected Window WpfWindow
    {
        get { return this.wpfWindow; }
    }

    private static void ConfigureBehavior(ContentWindow cw)
    {
        cw.WindowStartupLocation = WindowStartupLocation.CenterOwner;
        cw.CommandBindings.Add(new CommandBinding(PresentationCommands.Accept, (sender, e) => cw.DialogResult = true));
    }
}

public static class PresentationCommands
{
    private readonly static RoutedCommand accept = new RoutedCommand("Accept", typeof(PresentationCommands));

    public static RoutedCommand Accept
    {
        get { return PresentationCommands.accept; }
    }
}

然后我们有一个专门用于 MainWindow 的 window 适配器，它确保 DataContext 属性使用 ViewModel.[=36 正确初始化=]

internal class MainWindowAdapter : WindowAdapter
{
    private readonly ITrackerViewModelFactory vmFactory;
    private bool initialized;

    public MainWindowAdapter(Window wpfWindow, ITrackerViewModelFactory viewModelFactory)
        : base(wpfWindow)
    {
        if (viewModelFactory == null)
        {
            throw new ArgumentNullException("viewModelFactory");
        }

        this.vmFactory = viewModelFactory;
    }

    #region IWindow Members

    public override void Close()
    {
        this.EnsureInitialized();
        base.Close();
    }

    public override IWindow CreateChild(object viewModel)
    {
        this.EnsureInitialized();
        return base.CreateChild(viewModel);
    }

    public override void Show()
    {
        this.EnsureInitialized();
        base.Show();
    }

    public override bool? ShowDialog()
    {
        this.EnsureInitialized();
        return base.ShowDialog();
    }

    #endregion

    private void DeclareKeyBindings(TrackerViewModel vm)
    {
        //this.WpfWindow.InputBindings.Add(new KeyBinding(vm.RefreshCommand, new KeyGesture(Key.F5)));
        //this.WpfWindow.InputBindings.Add(new KeyBinding(vm.InsertProductCommand, new KeyGesture(Key.Insert)));
        //this.WpfWindow.InputBindings.Add(new KeyBinding(vm.EditProductCommand, new KeyGesture(Key.Enter)));
        //this.WpfWindow.InputBindings.Add(new KeyBinding(vm.DeleteProductCommand, new KeyGesture(Key.Delete)));
    }

    private void EnsureInitialized()
    {
        if (this.initialized)
        {
            return;
        }

        var vm = this.vmFactory.Create(this);
        this.WpfWindow.DataContext = vm;
        this.DeclareKeyBindings(vm);

        this.initialized = true;
    }
}

合成根

最后，您需要一种创建对象图的方法。您在正确的地方这样做，但是将其分成许多步骤并没有给自己带来任何好处。将容器作为应用程序级变量不一定是好事 - 它打开容器作为 service locator.

滥用

public partial class App : Application
{
    protected override void OnStartup(StartupEventArgs e)
    {
        base.OnStartup(e);

        // Begin Composition Root
        var container = new StandardKernel();

        // Register types
        container.Bind<ISystemEvents>().To<MySystemEvents>();
        container.Bind<ITrackerViewModelFactory>().To<TrackerViewModelFactory>();
        container.Bind<Window>().To<MainWindow>();
        container.Bind<IWindow>().To<MainWindowAdapter>();

        // Build the application object graph
        var window = container.Get<IWindow>();

        // Show the main window.
        window.Show();

        // End Composition Root
    }
}

我认为您遇到的主要问题是您需要确保在 MainWindow 上手动调用 Show()。

如果您真的想将注册分解为另一个步骤，您可以使用一个或多个 Ninject Modules。

using Ninject.Modules;
using System.Windows;

public class MyApplicationModule : NinjectModule
{
    public override void Load()
    {
        Bind<ISystemEvents>().To<MySystemEvents>();
        Bind<ITrackerViewModelFactory>().To<TrackerViewModelFactory>();
        Bind<Window>().To<MainWindow>();
        Bind<IWindow>().To<MainWindowAdapter>();
    }
}

然后 App.xaml.cs 文件将如下所示：

public partial class App : Application
{
    protected override void OnStartup(StartupEventArgs e)
    {
        base.OnStartup(e);

        // Begin Composition Root
        new StandardKernel(new MyApplicationModule()).Get<IWindow>().Show();

        // End Composition Root
    }
}

使用 NInject 在 WPF 中注入没有无参数构造函数的视图模型 class

injecting viewmodel class without parameterless constructor in WPF with NInject

c#

wpf

dependency-injection

ninject

mvvm

创建 ViewModel 工厂

调整 Window

合成根