Windows Phone MVVM：按钮命令可以执行和命令参数-Java 学习之路

我想将MVVM模式实现到注册页面，如下所示：

页面包含用户名，电子邮件和密码的文本框 .

我想使用 ICommand 和 DelegateCommand 模式将注册按钮绑定到命令 .

问题是我希望如果文本框为空则禁用该按钮，如果文本框有文本则启用 .

我的 Model 是：

public class User
    {
        public string UserName { get; set; }
        public string Email { get; set; }
        public string Password { get; set; }
    }

我的 ViewModel ：

public class UserViewModel:INotifyPropertyChanged
    {
        private User user;
        public UserViewModel()
        {
            user = new User();
        }
#region Properties
.
.
.
#endregion
public ICommand RegisterCommand
        {
            get
            {
                return new DelegateCommand(Register,CanRegister);
            }
        }

        private void Register(object parameter)
        {
            //TODO call the web service
        }

        private bool CanRegister(object parameter)
        {
            return (!string.IsNullOrEmpty(UserName) && !string.IsNullOrEmpty(Password));
        }
}

我的 DelegateCommand 实施：

public class DelegateCommand:ICommand
    {
        //Delegate to the action that the command executes
        private Action<object> _executeAction;
        //Delegate to the function that check if the command can be executed or not
        private Func<object, bool> _canExecute;

        public bool canExecuteCache;

        public DelegateCommand(Action<object> executeAction):this(executeAction,null)
        {

        }

        public DelegateCommand(Action<object> action, Func<object, bool> canExecute)
        {
            this._executeAction = action;
            this._canExecute = canExecute;
        }

        //interface method, called when CanExecuteChanged event handler is fired
        public bool CanExecute(object parameter)
        {
            //true by default (in case _canExecute is null)
            bool result = true;
            Func<object, bool> canExecuteHandler = this._canExecute;
            if (canExecuteHandler != null)
            {
                result = canExecuteHandler(parameter);
            }

            return result;
        }

        //Event handler that the controld subscribe to 
        public event EventHandler CanExecuteChanged;


        //interface method
        public void Execute(object parameter)
        {
            _executeAction(parameter);
        }


        //rause the CanExecuteChanged event handler manually
        public void RaiseCanExecuteChanged()
        {
            EventHandler handler = this.CanExecuteChanged;
            if (handler != null)
            {
                handler(this, EventArgs.Empty);
            }

        }



    }

和我的 View ：

<Grid x:Name="ContentPanel" Grid.Row="1" Margin="12,0,12,0">
            <Grid.RowDefinitions>
                <RowDefinition Height="Auto"/>
                <RowDefinition Height="Auto"/>
                <RowDefinition Height="Auto"/>
                <RowDefinition Height="Auto"/>
                <RowDefinition Height="Auto"/>
            </Grid.RowDefinitions>
            <TextBlock Grid.Row="0" Text="Username:"/>
            <TextBox Grid.Row="1" Name="txtUserName" Text="{Binding UserName, Mode=TwoWay}" HorizontalAlignment="Stretch"/>
            <TextBlock Grid.Row="2" Text="Password:" HorizontalAlignment="Stretch" />
            <TextBox Grid.Row="3" Name="txtPassword" Text="{Binding Password, Mode=TwoWay}"/>
            <Button Grid.Row="4" Content="Register" Command="{Binding RegisterCommand }"   />
        </Grid>

我想要实现的是使按钮被禁用，直到用户输入每个 TextBox 中的信息为止

如何才能做到这一点？

谢谢

1 回答

首先要做的事情是：每次访问该属性时返回一个新的 DelegateCommand 将限制您调用 RaiseCanExecuteChanged() 方法的能力，因为您将没有对绑定的相同命令的引用 .

因此，将ViewModel更改为：

public class UserViewModel : INotifyPropertyChanged
{
   private User user;
   public UserViewModel()
   {
      user = new User();
      RegisterCommand = new DelegateCommand(Register,CanRegister);
   }

   public DelegateCommand RegisterCommand {get; private set;}

   private void Register(object parameter)
   {
      //TODO call the web service
   }

   private bool CanRegister(object parameter)
   {
      return (!string.IsNullOrEmpty(UserName) && 
              !string.IsNullOrEmpty(Password));
   }
}

您可以将 RegisterCommand 属性设置为 private set 而不进行PropertyChanged调用，因为它将在绑定发生之前进行实例化，并且不需要更改 .

假设属性 UserName 和 Password 的形式触发 PropertyChanged 事件，您可以在 RegisterCommand 上更改时调用 RaiseCanExecuteChanged() 方法 .

例如 .

private string _userName;
public string UserName
{
    get { return _userName; }
    set
    {
        if(_userName == value)
            return;

        _userName = value;
        RaisePropertyChanged("UserName");

        RegisterCommand.RaiseCanExecuteChanged();
    }
}

这将强制CanExecute方法重新评估 .

回复于 2024-05-17T09:55:36+08:00

Windows Phone MVVM：按钮命令可以执行和命令参数

1 回答

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