c# - 如果字典填充较晚，则绑定(bind)无法与字典一起使用

Question

我有一个 WPF 应用程序。我正在使用 MVVM 模式。我有一本字典:

 public abstract class ViewModelBase
 {
        public static Dictionary<string, Action> Permissions { get; set; }

        ...
 }

我想将它的值绑定(bind)到菜单项的可见性，如下所示:

<MenuItem x:Name="systemMenuItem" Header="System" Visibility="{Binding Permissions[systemMenuItem].CanBeShow, Converter={StaticResource BoolToVis}}">...</MenuItem>

为了填充这个字典，我需要窗口构建可视化树，因为字典的元素包含来自窗口 MenuItem 的信息。如果我将在 InitializeComponent 之前创建字典，我将得到一个异常，即没有值为 systemMenuItem 的键因为 VisualTreeHelper.GetChildrenCount返回零个元素。如果我会在 Loaded 上做事件我将获得正常填充的字典，但在这种情况下绑定(bind)不起作用。如何在向用户显示窗口并从 MenuItems 获取信息之前填写我的字典？在这种情况下，我怎样才能使我的绑定(bind)工作？该窗口是主要和启动。

Answer 1

作为一般规则使用字典进行绑定(bind)是一个坏主意，这可能是为什么 MS 从未将 Observable Dictionary 创建为 .Net 框架的一部分，你最好的办法是创建一个 Permission 类，然后拥有它们的 Observable Collection ，这将为您提供权限上的集合绑定(bind)和更改绑定(bind)

注意:这里使用 C#6，所以如果您使用的是早期版本，您可能需要对其进行调整

例子

Xaml

<Window x:Class="WpfApplication1.MainWindow"
        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:local="clr-namespace:WpfApplication1"
        mc:Ignorable="d"
        Title="MainWindow" Height="350" Width="525">
    <Window.Resources>
        <BooleanToVisibilityConverter x:Key="BooleanToVisibilityConverter"/>
    </Window.Resources>
    <Window.DataContext>
        <local:ViewModel/>
    </Window.DataContext>
    <DockPanel>
        <Menu DockPanel.Dock="Top" ItemsSource="{Binding Permissions}">
            <Menu.ItemContainerStyle>
                <Style TargetType="MenuItem">
                    <Setter Property="Header" Value="{Binding Name}"/>
                    <Setter Property="Visibility" Value="{Binding CanBeShow, Converter={StaticResource BooleanToVisibilityConverter}}"/>
                    <Setter Property="Command" Value="{Binding Action}"/>
                </Style>
            </Menu.ItemContainerStyle>
        </Menu>
        <Grid/>
    </DockPanel>
</Window>

查看模型

public class ViewModel
{
    public ViewModel()
    {
        //some dummy data
        Permissions.Add(new Permission()
        {
            Name = "Open",
            CanBeShow = true,
            Action = ApplicationCommands.Open
        });
        Permissions.Add(new Permission()
        {
            Name = "Save",
            CanBeShow = false,
            Action = ApplicationCommands.Save
        });
        Permissions.Add(new Permission()
        {
            Name = "Delete",
            CanBeShow = true,
            Action = ApplicationCommands.Delete
        });
    }

    public ObservableCollection<Permission> Permissions { get; } = new ObservableCollection<Permission>();
    //notice no set you want to change the content of the collection not the collection
}

public class Permission:INotifyPropertyChanged
{
    private string name;

    public string Name
    {
        get { return name; }
        set
        {
            name = value;
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(Name)));
        }
    }

    private bool canBeShow;

    public bool CanBeShow
    {
        get { return canBeShow; }
        set
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(CanBeShow)));
            canBeShow = value;
        }
    }


    private ICommand action;
    public ICommand Action
    {
        get { return action; }
        set {
            action = value;
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(Action)));
        }
    }

    public event PropertyChangedEventHandler PropertyChanged;
}

但是，如果您尝试对菜单操作实现某种安全性，
那么这样做的首选方法是覆盖 ICommand 以创建一个自定义命令，该命令将查找它自己的权限并将它们公开为属性而不靠近 View

public class PermissionCommand:INotifyPropertyChanged,ICommand
{
    public event EventHandler CanExecuteChanged;
    public event PropertyChangedEventHandler PropertyChanged;

    private string name;

    public string Name
    {
        get { return name; }
        set
        {
            name = value;
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(Name)));
        }
    }

    public bool CanBeShow
    {
        get
        {
             //Check permissions for CanBeShow here 
        }
    }

    public bool CanExecute(object parameter)
    {
        //Check permissions for Execution here 
    }

    public void Execute(object parameter)
    {
        // perform action here
    }
    //you will need to trigger the events when the permissions change
}