Question

我正在使用两个TextBoxes（用户名和密码）和一个Login按钮编写一个简单的登录UserControl。我希望只有在填写用户名和密码字段时才启用“登录”按钮。我使用的是Prism和MVVM。 LoginViewModel包含一个名为LoginCommand的属性，该属性绑定到Login按钮。我的ViewModel中有一个CanLoginExecute（）方法，但它只在应用程序出现时才会触发，然后再也不会触发。因此，从不启用“登录”按钮。我错过了什么？

这是我的xaml：

<TextBox x:Name="username"
    Text="{Binding Path=Username, UpdateSourceTrigger=PropertyChanged, ValidatesOnDataErrors=True}" />
<TextBox x:Name="password"
    Text="{Binding Path=Password, UpdateSourceTrigger=PropertyChanged, ValidatesOnDataErrors=True}" />
<Button Content="Login"
    cmnd:Click.Command="{Binding LoginCommand}" />

这是我的ViewModel

class LoginViewModel : IDataErrorInfo, INotifyPropertyChanged
{
    public LoginViewModel()
    {
        this.LoginCommand =
            new DelegateCommand<object>(
                this.LoginExecute, this.CanLoginExecute);
    }

    private Boolean CanLoginExecute(object dummyObject)
    {
        return (string.IsNullOrEmpty(Username) ||
                string.IsNullOrEmpty(Password)) ? false : true;
    }

    private void LoginExecute(object dummyObject)
    {
        if (CheckCredentials(Username, Password))
        {
            ....
        }
    }

    #region IDataErrorInfo Members

    public string Error
    {
        get { throw new NotImplementedException(); }
    }

    public string this[string columnName]
    {
        get
        {
            string result = null;
            if (columnName == "Username")
            {
                if (string.IsNullOrEmpty(Username))
                    result = "Please enter a username";
            }
            else if (columnName == "Password")
            {
                if (string.IsNullOrEmpty(Password))
                    result = "Please enter a password";
            }
            return result;
        }
    }

    #endregion // IDataErrorInfo Members

    #region INotifyPropertyChanged Members

    public event PropertyChangedEventHandler PropertyChanged;

    void OnPropertyChanged(string propertyName)
    {
        if (PropertyChanged != null)
            PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
    }

    #endregion // INotifyPropertyChanged Members

    #region Properties

    private String _username;
    public String Username
    {
        get { return _username; }
        set
        {
            if (value == _username)
                return;
            _username = value;
            this.OnPropertyChanged("Username");
        }
    }

    private String _password;
    public String Password
    {
        get { return _password; }
        set
        {
            if (value == _password)
                return;
            _password = value;
            this.OnPropertyChanged("Password");
        }
    }

    public ICommand LoginCommand { get; private set; }

    #endregion // Properties
}

Answer 1

绑定控件很可能永远不再要求 CanExecute 状态。每当检测到更改命令的 CanExecute 状态的条件时，您需要在DelegateCommand上调用 RaiseCanExecuteChanged 方法。这表示绑定控件更新 CanExecute 状态。

Answer 2

RaiseCanExecuteChanged代码：

    private void RaiseCanExecuteChanged()
    {
        DelegateCommand<object> command = LoginCommand as DelegateCommand<object>;
        command.RaiseCanExecuteChanged();
    }

    public const string UsernameProperty = "Username";
    private String _username;
    public String Username
    {
        get { return _username; }
        set
        {
            _username = value;
            this.NotifyPropertyChanged(UsernameProperty);
            RaiseCanExecuteChanged();
        }
    }

Answer 3

从Prism6开始，DelegateCommand可以“观察”你的属性。意味着每次你的财产改变时，都会调用CanExecute-Method。好处是你摆脱了Propertysetter中的RaiseCanExecuteChanged。如果要观察更多属性，也可以对该方法进行链式调用：

public LoginViewModel()
{
    this.LoginCommand =
        new DelegateCommand<object>(
            this.LoginExecute, this.CanLoginExecute).ObservesProperty(() => Username).ObservesProperty(() => Password);
}

此外，如果您只想根据布尔属性的状态调用DelegateCommand，可以使用.ObservesCanExecute(()=> BoolProp)

public LoginViewModel()
{
    this.LoginCommand =
        new DelegateCommand<object>(
            this.LoginExecute).ObservesCanExecute(()=> IsServerOnline).ObservesProperty(() => Username).ObservesProperty(() => Password);
}

您不再需要this.CanLoginExecute。

Answer 4

以下是Prism的一些解决方法（已通过Prism.Core 7.1.0.431测试）：

public class RelayCommand : DelegateCommand
{
    public RelayCommand(Action executeMethode) : base(executeMethode)
    {

    }

    public RelayCommand(Action executeMethode, Func<bool> canExecuteMethode) : base(executeMethode, canExecuteMethode)
    {

    }

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

WPF-Prism CanExecute方法未被调用

4 个答案: