c# - DelegateCommand.CanExecute 方法行为不正确

Question

我将控件绑定(bind)到 DelegateCommand，但它的 CanExecute 部分无法正常工作。我正在使用 Prism 库。谁能告诉我为什么？

命令声明和实例化:

public PlayerManagementViewModel(DatabaseManager dbManager)
{
    _dbManager = dbManager;
    this.ResetUpToDateStatusCommand = new DelegateCommand(() => this.ResetXpUpToDateStatus());
    this.DeletePlayerCommand = new DelegateCommand(() => this.DeleteSelectedPlayer(), () => SelectedPlayer != null);
    this.RefreshPlayers();
}

public ICommand DeletePlayerCommand { get; private set; }

SelectedPlayer 定义:

public Player SelectedPlayer
{
    get { return _selectedPlayer; }
    set
    {
        SetProperty(ref this._selectedPlayer, value);
        this.OnPropertyChanged(() => this.FormattedPlayerStatus);
    }
}

奇怪的是，如果您查看 DeletePlayerCommand 实例上方的行，那行工作得很好。我没有从中得到任何 CanExecute 行为，但至少它有效。事实上，DeletePlayerCommand 命令永远不会触发，即使有断点，除非我完全删除构造函数的 CanExecute 部分。

任何人都可以向我解释为什么这是或我做错了什么吗？

Answer 1

如果 DeletePlayerCommand 的 CanExecute 函数是 () => SelectedPlayer != null，那么必须有一个 DelegateCommandBase.RaiseCanExecuteChanged Method当 SelectedProperty 值改变时调用:

Raises CanExecuteChanged on the UI thread so every command invoker can requery to check if the command can execute.

适当的 UI 元素(带有数据绑定(bind)命令)是命令调用程序。

总而言之，SelectedPlayer 属性的实现应该更新如下:

class PlayerManagementViewModel : BindableBase
{
    private Player _selectedPlayer;
    private readonly DelegateCommand _deletePlayerCommand;

    public PlayerManagementViewModel(...)
    {
        _deletePlayerCommand = new DelegateCommand(() => DeleteSelectedPlayer(), () => SelectedPlayer != null);
    }

    public ICommand DeletePlayerCommand
    {
        get { return _deletePlayerCommand; }
    }

    public Player SelectedPlayer
    {
        get { return _selectedPlayer; }
        set
        {
            SetProperty(ref _selectedPlayer, value);
            OnPropertyChanged(() => FormattedPlayerStatus);
            _deletePlayerCommand.RaiseCanExecuteChanged();
        }
    }
}