如何使用 Prism 6 正确触发 CanExecute

Question

我有模特

public class Irritant : BindableBase
{
    private short _id;
    private string _name;
    private string _description;

    public short Id
    {
        get { return _id; }
        set { SetProperty(ref _id, value); }
    }

    public string Name
    {
        get { return _name; }
        set { SetProperty(ref _name, value); }
    }

    public string Description
    {
        get { return _description; }
        set { SetProperty(ref _description, value); }
    }


    public Irritant()
    {
        Id = 0;
        Name = "";
        Description = "";       
    }
}

然后我的 ViewModel 有两个版本

public class IrritantViewModel : BindableBase
{
    private IrritantDb db = new IrritantDb();


    //Version 1 - The Model's property is coded in IrritantViewModel
    //private short _id;
    //private string _name = "Alen";
    //private string _description;

    //public short Id
    //{
    //    get { return _id; }
    //    set { SetProperty(ref _id, value); }
    //}

    //public string Name
    //{
    //    get { return _name; }
    //    set { SetProperty(ref _name, value); }
    //}

    //public string Description
    //{
    //    get { return _description; }
    //    set { SetProperty(ref _description, value); }
    //}


    //Version2 - I use the Irritant Model as property of IrritantViewModel
    private DateTime? _lastUpdated;
    private Irritant _entity;

    public Irritant Entity
    {
        get { return _entity; }
        set { SetProperty(ref _entity, value); }
    }

    public DateTime? LastUpdated
    {
        get { return _lastUpdated; }
        set { SetProperty(ref _lastUpdated, value); }
    }

    public DelegateCommand UpdateCommand { get; set; }

    public IrritantViewModel()
    {
        Entity = new Irritant();

        //Version1
        //UpdateCommand = new DelegateCommand(EditCommand, CanExecute).ObservesProperty(() => Name);

        //Version2
        UpdateCommand = new DelegateCommand(EditCommand, CanExecute).ObservesProperty(() => Entity.Name);
    }

    private bool CanExecute()
    {
        //Version1
        //switch (Name)
        //{
        //    case null:
        //        return false;
        //    case "":
        //        return false;
        //}


        //Version2
        switch (Entity.Name)
        {
            case null:
                return false;
            case "":
                return false;
        }

        return true;
    }

    private void EditCommand()
    {
        LastUpdated = DateTime.UtcNow;
    }
}

这是我的观点

public partial class IrritantView : UserControl
{
    public IrritantView()
    {
        InitializeComponent();
        DataContext = new IrritantViewModel();
    }
}

<Grid >
    <ScrollViewer>
        <StackPanel MinWidth="200">

        <TextBlock Text="Irritant"  />



        <!--Version 1-->
                <!--<TextBlock Text="Name" />
                <TextBox Text="{Binding Name, UpdateSourceTrigger=PropertyChanged}" />
                <TextBlock Text="Description" />
                <TextBox Text="{Binding Description, UpdateSourceTrigger=PropertyChanged}" />
                -->


        <!--Version 2-->

            <TextBlock Text="Name" />
            <TextBox Text="{Binding Entity.Name, UpdateSourceTrigger=PropertyChanged}" />
            <TextBlock Text="Description" />
            <TextBox Text="{Binding Entity.Description, UpdateSourceTrigger=PropertyChanged}" />



            <TextBlock Text="Last Updated" />
            <Label Content="{Binding LastUpdated, UpdateSourceTrigger=PropertyChanged}" />
            <Button Content="Save" 
                    Command="{Binding UpdateCommand}"
                    />
        </StackPanel>
    </ScrollViewer>
</Grid>

Version1 工作正常，当绑定到 Name (TextBox Text="{Binding Name, UpdateSourceTrigger=PropertyChanged}") 的 TextBox 为 null 或空时，保存按钮将禁用。

但是对于版本 2，保存按钮不会禁用。它仅在初始化期间调用 CanExecute 方法，删除 TextBox 中的文本不会禁用 Button。我做错了什么？

Answer 1

DelegateCommand 不会自动引发 CanExecuteChanged 事件，您必须在适当的时候通过调用 RaiseCanExecuteChanged 手动引发该事件。除了使用 DelegateCommand，您还可以使用 RelayCommand，它依赖 CommandManager.RequerySuggested 事件为您做类似的事情。

更改命令定义返回 ICommand:

public ICommand UpdateCommand { get; set; }

使用以下命令初始化命令：

UpdateCommand = new AutoCanExecuteCommand(new DelegateCommand(EditCommand, CanExecute));

使用以下 class 作为包装器：

public class AutoCanExecuteCommand : ICommand
{
    public ICommand WrappedCommand { get; private set; }

    public AutoCanExecuteCommand(ICommand wrappedCommand)
    {
        if (wrappedCommand == null) 
        {
            throw new ArgumentNullException("wrappedCommand");
        }

        WrappedCommand = wrappedCommand;
    }

    public void Execute(object parameter)
    {
        WrappedCommand.Execute(parameter);
    }

    public bool CanExecute(object parameter)
    {
        return WrappedCommand.CanExecute(parameter);
    }

    public event EventHandler CanExecuteChanged
    {
        add { CommandManager.RequerySuggested += value; }
        remove { CommandManager.RequerySuggested -= value; }
    }
}

Answer 2

出于多种原因，我不建议连接到 CommandManager。除了内存泄漏之外，它还会在您的应用程序中引入性能问题，因为您无法控制 CommandManager 调用 CanExecute 的时间或次数（发生在 UI 线程上）。相反，我会建议使用您的模型对象的 INPC，而不是如这个答案所示：

如何使用 Prism 6 正确触发 CanExecute

How to make the CanExecute trigger properly using Prism 6

wpf

prism

mvvm

delegatecommand