WPF-Prism CanExecute方法未被调用

Question

我正在使用两个TextBox(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,但只有在应用程序启动时才会触发,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的代码:

    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”您的财产。意味着每次您的属性更改时，都会调用CanExecute-Method。好消息是您摆脱RaiseCanExecuteChanged了Propertysetter。如果要观察更多属性，也可以链式调用该方法：

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

此外，如果只希望根据布尔属性的状态调用DelegateCommand，则可以使用 .ObservesCanExecute(()=> BoolProp)

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

您this.CanLoginExecute不再需要。