如何在用戶控件中使用ViewModelLocator？

Question

我被要求將使用MVVM模式的獨立WPF應用程序轉換為用戶控件。 這個應用程序包含一個主窗口和一些其他窗口。 但是，嘗試這樣做時會遇到一些錯誤，這些錯誤都指向我的App.xaml類和聲明為轉換器的其他資源：

Library project file cannot specify ApplicationDefintion element. The project file contains a property value that is not valid. The name "ViewModelLocator" does not exist in the namespace "clr-namespace:MapperX.ViewModels" 。

因此，看來錯誤圍繞着我的ViewModelLocator。

當前，項目目錄結構的設置如下：

頂層-> ViewModels文件夾-> ViewModelLocator

App.xaml的設置如下：

<Application x:Class="MapperX.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:local="clr-namespace:MapperX"
             xmlns:vm="clr-namespace:MapperX.ViewModels"
             StartupUri="MainWindow.xaml">
    <Application.Resources>
        <vm:ViewModelLocator x:Key="ViewModelLocator" />
    </Application.Resources>
</Application>

這是ViewModelLocator類的片段：

namespace MapperX.ViewModels
{
    /// <summary>
    /// This class instantiates all the viewmodels
    /// </summary>
    public class ViewModelLocator
    {
        WpfMap map = new WpfMap();

        private MainViewModel _mainViewModel;
        public MainViewModel MainViewModel
        {
            get
            {
                if (_mainViewModel == null)
                {
                    _mainViewModel = new MainViewModel(map)
                }
                return _mainViewModel;
            }
        }

        private LayersViewModel _layersViewModel;
        public LayersViewModel LayersViewModel
        {
            get
            {
                if (_layersViewModel == null)
                {
                    _layersViewModel = new LayersViewModel(map)
                }
                return _layersViewModel;
            }
        }
    }
}

然后像這樣設置視圖.xaml的DataContext ： DataContext="{Binding Path=MainViewModel, Source={StaticResource ViewModelLocator}}"

在沒有App.xaml的情況下仍然能夠使用ViewModelLocator的正確方法是什么？

Answer 1

為什么不將ViewModelLocator放在頂級UserControl ？

<UserControl.Resources>
   <vm:ViewModelLocator x:Key="ViewModelLocator" />
</UserControl.Resources>

Answer 2

您可以創建一個附加屬性來設置控件庫中視圖的DataContext ，而不是依賴資源：

namespace ControlsAndResources
{
    public class View
    {
        private static readonly ViewModelLocator s_viewModelLocator = new ViewModelLocator();

        public static readonly DependencyProperty ViewModelProperty = DependencyProperty.RegisterAttached("ViewModel", typeof(string), 
            typeof(ViewModelLocator), new PropertyMetadata(new PropertyChangedCallback(OnChanged)));

        public static void SetViewModel(UserControl view, string value) => view.SetValue(ViewModelProperty, value);

        public static string GetViewModel(UserControl view) => (string)view.GetValue(ViewModelProperty);

        private static void OnChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            UserControl view = (UserControl)d;
            string viewModel = e.NewValue as string;
            switch (viewModel)
            {
                case "MainViewModel":
                    view.DataContext = s_viewModelLocator.MainViewModel;
                    break;
                case "LayersViewModel":
                    view.DataContext = s_viewModelLocator.LayersViewModel;
                    break;
                default:
                    view.DataContext = null;
                    break;
            }
        }
    }
}

用法：

<UserControl xmlns:local="clr-namespace:ControlsAndResources" ... 
             local:View.ViewModel="MainViewModel">