我创建了一个实现ICommand的基本视图模型。这些命令可以很好地绑定,按预期执行,甚至以正确的状态开始,但作为影响命令是否可以更改的属性,这些命令的 CanExecute 似乎没有更新。
在下面的代码中,我可以单击"运行"按钮,一切都按预期工作,除了当程序状态更改为"正在运行"时应将其禁用
。在我的基本视图模型中:
public class RelayCommand : ICommand
{
#region ICommand Member Variables
private Action<object> _execute;
private Predicate<object> _canExecute;
private event EventHandler _canExecuteChangedInternal;
#endregion // ICommand Member Variables
#region Constructors
public RelayCommand(Action<object> execute, Predicate<object> canExecute)
{
_execute = execute;
_canExecute = canExecute;
}
public RelayCommand(Action<object> execute) : this(execute, DefaultCanExecute)
{
}
#endregion // Constructors
#region ICommand Members
public event EventHandler CanExecuteChanged
{
add
{
CommandManager.RequerySuggested += value;
_canExecuteChangedInternal += value;
}
remove
{
CommandManager.RequerySuggested -= value;
_canExecuteChangedInternal -= value;
}
}
public bool CanExecute(object parameter)
{
return _canExecute != null && _canExecute(parameter);
}
public void Execute(object parameter)
{
_execute(parameter);
}
public void OnCanExecuteChanged()
{
EventHandler eventHandler = _canExecuteChangedInternal;
if (eventHandler != null)
{
eventHandler.Invoke(this, EventArgs.Empty);
}
}
private static bool DefaultCanExecute(object parameter)
{
return true;
}
#endregion // ICommand Members
在我的视图中模型:
RelayCommand _runCommand;
public RelayCommand RunCommand
{
get
{
if (_runCommand == null)
{
_runCommand = new RelayCommand(param => Run(), param => CanRun);
}
return _runCommand;
}
}
public bool CanRun
{
get
{
bool result = false;
if (Machine.ProgramStatus != ProgramStatus.Running && Machine.ProgramStatus != ProgramStatus.TestRunning)
{
result = true;
}
return result;
}
}
在我看来:
<Button Content="Run" Command="{Binding Path=RunCommand}" />
您必须调用 CanExecuteChanged EventHandler。因此,当 ProgramStatus 更改为正在运行时,您可以为要更新的命令调用 OnCanExecuteChanged
帮助程序方法(例如 RunCommand.OnCanExecuteChanged();
)。
如果 CanExecute 绑定到属性,则可以在属性的 setter 中使用 OnCanExecuteChanged 帮助程序方法。
private bool _myProperty;
public bool MyProperty
{
get { return _myProperty; }
set
{
_myProperty= value;
RunCommand.OnCanExecuteChanged();
}
}