在Xamarin中传递的RelayCommand参数

我是Xamarin cross-platform新手，虽然我确实有一些WPF和MVVM经验，但我仍然使用下面的ICommand实现了解参数化RelayCommand调用的问题。 有人可以解释如何正确地从我的View传递和接收CommandParameter到我绑定的RelayCommand因为这似乎与正常的WPF版本的RelayCommand ：

  /// 
 /// A command whose sole purpose is to relay its functionality /// to other objects by invoking delegates. /// The default return value for the CanExecute method is 'true'. ///  needs to be called whenever ///  is expected to return a different value. /// 
 public class RelayCommand : ICommand { private readonly Action _execute; private readonly Func _canExecute; /// 
 /// Raised when RaiseCanExecuteChanged is called. /// 
 public event EventHandler CanExecuteChanged; /// 
 /// Creates a new command that can always execute. /// 
 /// The execution logic. public RelayCommand(Action execute) : this(execute, null) { } /// 
 /// Creates a new command. /// 
 /// The execution logic. /// The execution status logic. public RelayCommand(Action execute, Func canExecute) { if (execute == null) throw new ArgumentNullException("execute"); _execute = execute; _canExecute = canExecute; } /// 
 /// Determines whether this  can execute in its current state. /// 
 ///  /// Data used by the command. If the command does not require data to be passed, this object can be set to null. ///  /// true if this command can be executed; otherwise, false. public bool CanExecute(object parameter) { return _canExecute == null ? true : _canExecute(); } /// 
 /// Executes the  on the current command target. /// 
 ///  /// Data used by the command. If the command does not require data to be passed, this object can be set to null. ///  public void Execute(object parameter) { _execute(); } /// 
 /// Method used to raise the  event /// to indicate that the return value of the  /// method has changed. /// 
 public void RaiseCanExecuteChanged() { var handler = CanExecuteChanged; if (handler != null) { handler(this, EventArgs.Empty); } } } 

在WPF之前，我曾经有类似的东西：

在ViewModel端：

  OpenMenuItemCommand = new RelayCommand(OpenMenuItem); ... public void OpenMenuItem(object sender, ItemTappedEventArgs args) { } 

所以我的参数将来自args 。

Answer 1

我相信你会让事件和命令混乱。 两者之间的一些区别是您需要订阅事件并且必须发生事件。 命令可以被任何人调用，也可以被阻止。

因此，为了让您的示例正常工作，您应该修改代码以允许RelayCommand使用参数执行操作。 此参数将定义参数的类型。 我会使用类似MVVMLight的东西，它包含一个Generic RelayCommand，这样你就不必编写自己的。 完成后，您应该能够将代码更改为这样。

  OpenMenuItemCommand = new RelayCommand(OpenMenuItem); ... public void OpenMenuItem(MenuItem item) { } 

我写了一篇小博客文章 ，其中包含一个完整的工作项目，如果你想看到一个有效的例子。

Answer 2

Xamarin的中继或代理命令

这就是我实现它的方式，我希望它对某些人有所帮助

 public class DelegateCommand : ICommand { /// 
 /// The _execute /// 
 private readonly Action _execute; /// 
 /// The _can execute /// 
 private readonly Func _canExecute; /// 
 /// Initializes a new instance of the  class. /// 
 /// The execute. /// The can execute. /// execute public DelegateCommand(Action execute, Func canExecute) { _execute = execute ?? throw new ArgumentNullException("execute"); if (canExecute != null) { this._canExecute = canExecute; } } /// 
 /// Initializes a new instance of the DelegateCommand class that /// can always execute. /// 
 /// The execution logic. /// If the execute argument is null. public DelegateCommand(Action execute) : this(execute, null) { } /// 
 /// Occurs when changes occur that affect whether the command should execute. /// 
 public event EventHandler CanExecuteChanged; /// 
 /// Raises the can execute changed. /// 
 public void RaiseCanExecuteChanged() { CanExecuteChanged?.Invoke(this, EventArgs.Empty); } /// 
 /// Defines the method that determines whether the command can execute in its current state. /// 
 /// Data used by the command. If the command does not require data to be passed, this object can be set to null. /// true if this command can be executed; otherwise, false. public bool CanExecute(object parameter) { return _canExecute == null || _canExecute.Invoke(); } /// 
 /// Defines the method to be called when the command is invoked. /// 
 /// Data used by the command. If the command does not require data to be passed, this object can be set to null. public virtual void Execute(object parameter) { if (CanExecute(parameter)) { _execute.Invoke(); } } } /// 
 /// This class allows delegating the commanding logic to methods passed as parameters, /// and enables a View to bind commands to objects that are not part of the element tree. /// 
 /// Type of the parameter passed to the delegates public class DelegateCommand : ICommand { /// 
 /// The execute /// 
 private readonly Action _execute; /// 
 /// The can execute /// 
 private readonly Predicate _canExecute; /// 
 /// Initializes a new instance of the  class. /// 
 /// The execute action. /// execute public DelegateCommand(Action execute) : this(execute, null) { } /// 
 /// Initializes a new instance of the  class. /// 
 /// The execute. /// The can execute predicate. /// execute public DelegateCommand(Action execute, Predicate canExecute) { _execute = execute ?? throw new ArgumentNullException("execute"); if (canExecute != null) { _canExecute = canExecute; } } /// 
 /// Occurs when changes occur that affect whether the command should execute. /// 
 public event EventHandler CanExecuteChanged; /// 
 /// Raise  event. /// 
 public void RaiseCanExecuteChanged() { CanExecuteChanged?.Invoke(this, EventArgs.Empty); } /// 
 /// Determines whether this instance can execute the specified parameter. /// 
 /// The parameter. /// true if this instance can execute the specified parameter; otherwise, false. public bool CanExecute(object parameter) { return _canExecute == null || _canExecute.Invoke((T)parameter); } /// 
 /// Executes the specified parameter. /// 
 /// The parameter. public virtual void Execute(object parameter) { if (CanExecute(parameter)) { _execute((T)parameter); } } } 

在你的视图中

在您的视图模型中

 public ICommand LoginCommand { get; } LoginCommand = new DelegateCommand