CommandManager.InvalidateRequerySuggested() 尝试验证所有命令，这完全无效（在您的情况下很慢） - 在每次更改时，您都要求 每个命令 重新检查其 CanExecute()！
您需要该命令来了解其 CanExecute 依赖于哪些对象和属性，并建议仅在它们更改时重新查询。那样的话，如果你改变一个对象的属性，只有依赖它的命令才会改变它们的状态。
这就是我解决问题的方法，但首先是一个预告片：
// in ViewModel's constructor - add a code to public ICommand:
this.DoStuffWithParameterCommand = new DelegateCommand<object>(
parameter =>
{
//do work with parameter (remember to check against null)
},
parameter =>
{
//can this command execute? return true or false
}
)
.ListenOn(whichObject, n => n.ObjectProperty /*type safe!*/, this.Dispatcher /*we need to pass UI dispatcher here*/)
.ListenOn(anotherObject, n => n.AnotherObjectProperty, this.Dispatcher); // chain calling!
该命令正在侦听来自对象的NotifyPropertyChanged 事件，这些事件会影响它是否可以执行，并且仅在需要重新查询时才调用检查。
现在，有很多代码（我们内部框架的一部分）来执行此操作：
我使用 Prism 的 DelegateCommand，看起来像这样：
/// <summary>
/// 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.
/// </summary>
public class DelegateCommand : ICommand
{
#region Constructors
/// <summary>
/// Constructor
/// </summary>
public DelegateCommand(Action executeMethod)
: this(executeMethod, null, false)
{
}
/// <summary>
/// Constructor
/// </summary>
public DelegateCommand(Action executeMethod, Func<bool> canExecuteMethod)
: this(executeMethod, canExecuteMethod, false)
{
}
/// <summary>
/// Constructor
/// </summary>
public DelegateCommand(Action executeMethod, Func<bool> canExecuteMethod, bool isAutomaticRequeryDisabled)
{
if (executeMethod == null)
{
throw new ArgumentNullException("executeMethod");
}
_executeMethod = executeMethod;
_canExecuteMethod = canExecuteMethod;
_isAutomaticRequeryDisabled = isAutomaticRequeryDisabled;
this.RaiseCanExecuteChanged();
}
#endregion
#region Public Methods
/// <summary>
/// Method to determine if the command can be executed
/// </summary>
public bool CanExecute()
{
if (_canExecuteMethod != null)
{
return _canExecuteMethod();
}
return true;
}
/// <summary>
/// Execution of the command
/// </summary>
public void Execute()
{
if (_executeMethod != null)
{
_executeMethod();
}
}
/// <summary>
/// Property to enable or disable CommandManager's automatic requery on this command
/// </summary>
public bool IsAutomaticRequeryDisabled
{
get
{
return _isAutomaticRequeryDisabled;
}
set
{
if (_isAutomaticRequeryDisabled != value)
{
if (value)
{
CommandManagerHelper.RemoveHandlersFromRequerySuggested(_canExecuteChangedHandlers);
}
else
{
CommandManagerHelper.AddHandlersToRequerySuggested(_canExecuteChangedHandlers);
}
_isAutomaticRequeryDisabled = value;
}
}
}
/// <summary>
/// Raises the CanExecuteChaged event
/// </summary>
public void RaiseCanExecuteChanged()
{
OnCanExecuteChanged();
}
/// <summary>
/// Protected virtual method to raise CanExecuteChanged event
/// </summary>
protected virtual void OnCanExecuteChanged()
{
CommandManagerHelper.CallWeakReferenceHandlers(_canExecuteChangedHandlers);
}
#endregion
#region ICommand Members
/// <summary>
/// ICommand.CanExecuteChanged implementation
/// </summary>
public event EventHandler CanExecuteChanged
{
add
{
if (!_isAutomaticRequeryDisabled)
{
CommandManager.RequerySuggested += value;
}
CommandManagerHelper.AddWeakReferenceHandler(ref _canExecuteChangedHandlers, value, 2);
}
remove
{
if (!_isAutomaticRequeryDisabled)
{
CommandManager.RequerySuggested -= value;
}
CommandManagerHelper.RemoveWeakReferenceHandler(_canExecuteChangedHandlers, value);
}
}
bool ICommand.CanExecute(object parameter)
{
return CanExecute();
}
void ICommand.Execute(object parameter)
{
Execute();
}
#endregion
#region Data
private readonly Action _executeMethod = null;
private readonly Func<bool> _canExecuteMethod = null;
private bool _isAutomaticRequeryDisabled = false;
private List<WeakReference> _canExecuteChangedHandlers;
#endregion
}
/// <summary>
/// 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.
/// </summary>
/// <typeparam name="T">Type of the parameter passed to the delegates</typeparam>
public class DelegateCommand<T> : ICommand
{
#region Constructors
/// <summary>
/// Constructor
/// </summary>
public DelegateCommand(Action<T> executeMethod)
: this(executeMethod, null, false)
{
}
/// <summary>
/// Constructor
/// </summary>
public DelegateCommand(Action<T> executeMethod, Func<T, bool> canExecuteMethod)
: this(executeMethod, canExecuteMethod, false)
{
}
/// <summary>
/// Constructor
/// </summary>
public DelegateCommand(Action<T> executeMethod, Func<T, bool> canExecuteMethod, bool isAutomaticRequeryDisabled)
{
if (executeMethod == null)
{
throw new ArgumentNullException("executeMethod");
}
_executeMethod = executeMethod;
_canExecuteMethod = canExecuteMethod;
_isAutomaticRequeryDisabled = isAutomaticRequeryDisabled;
}
#endregion
#region Public Methods
/// <summary>
/// Method to determine if the command can be executed
/// </summary>
public bool CanExecute(T parameter)
{
if (_canExecuteMethod != null)
{
return _canExecuteMethod(parameter);
}
return true;
}
/// <summary>
/// Execution of the command
/// </summary>
public void Execute(T parameter)
{
if (_executeMethod != null)
{
_executeMethod(parameter);
}
}
/// <summary>
/// Raises the CanExecuteChaged event
/// </summary>
public void RaiseCanExecuteChanged()
{
OnCanExecuteChanged();
}
/// <summary>
/// Protected virtual method to raise CanExecuteChanged event
/// </summary>
protected virtual void OnCanExecuteChanged()
{
CommandManagerHelper.CallWeakReferenceHandlers(_canExecuteChangedHandlers);
}
/// <summary>
/// Property to enable or disable CommandManager's automatic requery on this command
/// </summary>
public bool IsAutomaticRequeryDisabled
{
get
{
return _isAutomaticRequeryDisabled;
}
set
{
if (_isAutomaticRequeryDisabled != value)
{
if (value)
{
CommandManagerHelper.RemoveHandlersFromRequerySuggested(_canExecuteChangedHandlers);
}
else
{
CommandManagerHelper.AddHandlersToRequerySuggested(_canExecuteChangedHandlers);
}
_isAutomaticRequeryDisabled = value;
}
}
}
#endregion
#region ICommand Members
/// <summary>
/// ICommand.CanExecuteChanged implementation
/// </summary>
public event EventHandler CanExecuteChanged
{
add
{
if (!_isAutomaticRequeryDisabled)
{
CommandManager.RequerySuggested += value;
}
CommandManagerHelper.AddWeakReferenceHandler(ref _canExecuteChangedHandlers, value, 2);
}
remove
{
if (!_isAutomaticRequeryDisabled)
{
CommandManager.RequerySuggested -= value;
}
CommandManagerHelper.RemoveWeakReferenceHandler(_canExecuteChangedHandlers, value);
}
}
bool ICommand.CanExecute(object parameter)
{
// if T is of value type and the parameter is not
// set yet, then return false if CanExecute delegate
// exists, else return true
if (parameter == null &&
typeof(T).IsValueType)
{
return (_canExecuteMethod == null);
}
return CanExecute((T)parameter);
}
void ICommand.Execute(object parameter)
{
Execute((T)parameter);
}
#endregion
#region Data
private readonly Action<T> _executeMethod = null;
private readonly Func<T, bool> _canExecuteMethod = null;
private bool _isAutomaticRequeryDisabled = false;
private List<WeakReference> _canExecuteChangedHandlers;
#endregion
}
/// <summary>
/// This class contains methods for the CommandManager that help avoid memory leaks by
/// using weak references.
/// </summary>
internal class CommandManagerHelper
{
internal static void CallWeakReferenceHandlers(List<WeakReference> handlers)
{
if (handlers != null)
{
// Take a snapshot of the handlers before we call out to them since the handlers
// could cause the array to me modified while we are reading it.
EventHandler[] callees = new EventHandler[handlers.Count];
int count = 0;
for (int i = handlers.Count - 1; i >= 0; i--)
{
WeakReference reference = handlers[i];
EventHandler handler = reference.Target as EventHandler;
if (handler == null)
{
// Clean up old handlers that have been collected
handlers.RemoveAt(i);
}
else
{
callees[count] = handler;
count++;
}
}
// Call the handlers that we snapshotted
for (int i = 0; i < count; i++)
{
EventHandler handler = callees[i];
handler(null, EventArgs.Empty);
}
}
}
internal static void AddHandlersToRequerySuggested(List<WeakReference> handlers)
{
if (handlers != null)
{
foreach (WeakReference handlerRef in handlers)
{
EventHandler handler = handlerRef.Target as EventHandler;
if (handler != null)
{
CommandManager.RequerySuggested += handler;
}
}
}
}
internal static void RemoveHandlersFromRequerySuggested(List<WeakReference> handlers)
{
if (handlers != null)
{
foreach (WeakReference handlerRef in handlers)
{
EventHandler handler = handlerRef.Target as EventHandler;
if (handler != null)
{
CommandManager.RequerySuggested -= handler;
}
}
}
}
internal static void AddWeakReferenceHandler(ref List<WeakReference> handlers, EventHandler handler)
{
AddWeakReferenceHandler(ref handlers, handler, -1);
}
internal static void AddWeakReferenceHandler(ref List<WeakReference> handlers, EventHandler handler, int defaultListSize)
{
if (handlers == null)
{
handlers = (defaultListSize > 0 ? new List<WeakReference>(defaultListSize) : new List<WeakReference>());
}
handlers.Add(new WeakReference(handler));
}
internal static void RemoveWeakReferenceHandler(List<WeakReference> handlers, EventHandler handler)
{
if (handlers != null)
{
for (int i = handlers.Count - 1; i >= 0; i--)
{
WeakReference reference = handlers[i];
EventHandler existingHandler = reference.Target as EventHandler;
if ((existingHandler == null) || (existingHandler == handler))
{
// Clean up old handlers that have been collected
// in addition to the handler that is to be removed.
handlers.RemoveAt(i);
}
}
}
}
}
然后我编写了一个ListenOn 扩展方法，将命令“绑定”到一个属性，并调用它的RaiseCanExecuteChanged：
public static class DelegateCommandExtensions
{
/// <summary>
/// Makes DelegateCommnand listen on PropertyChanged events of some object,
/// so that DelegateCommnand can update its IsEnabled property.
/// </summary>
public static DelegateCommand ListenOn<ObservedType, PropertyType>
(this DelegateCommand delegateCommand,
ObservedType observedObject,
Expression<Func<ObservedType, PropertyType>> propertyExpression,
Dispatcher dispatcher)
where ObservedType : INotifyPropertyChanged
{
//string propertyName = observedObject.GetPropertyName(propertyExpression);
string propertyName = NotifyPropertyChangedBaseExtensions.GetPropertyName(propertyExpression);
observedObject.PropertyChanged += (sender, e) =>
{
if (e.PropertyName == propertyName)
{
if (dispatcher != null)
{
ThreadTools.RunInDispatcher(dispatcher, delegateCommand.RaiseCanExecuteChanged);
}
else
{
delegateCommand.RaiseCanExecuteChanged();
}
}
};
return delegateCommand; //chain calling
}
/// <summary>
/// Makes DelegateCommnand listen on PropertyChanged events of some object,
/// so that DelegateCommnand can update its IsEnabled property.
/// </summary>
public static DelegateCommand<T> ListenOn<T, ObservedType, PropertyType>
(this DelegateCommand<T> delegateCommand,
ObservedType observedObject,
Expression<Func<ObservedType, PropertyType>> propertyExpression,
Dispatcher dispatcher)
where ObservedType : INotifyPropertyChanged
{
//string propertyName = observedObject.GetPropertyName(propertyExpression);
string propertyName = NotifyPropertyChangedBaseExtensions.GetPropertyName(propertyExpression);
observedObject.PropertyChanged += (object sender, PropertyChangedEventArgs e) =>
{
if (e.PropertyName == propertyName)
{
if (dispatcher != null)
{
ThreadTools.RunInDispatcher(dispatcher, delegateCommand.RaiseCanExecuteChanged);
}
else
{
delegateCommand.RaiseCanExecuteChanged();
}
}
};
return delegateCommand; //chain calling
}
}
然后您需要将以下扩展名添加到NotifyPropertyChanged
/// <summary>
/// <see cref="http://dotnet.dzone.com/news/silverlightwpf-implementing"/>
/// </summary>
public static class NotifyPropertyChangedBaseExtensions
{
/// <summary>
/// Raises PropertyChanged event.
/// To use: call the extension method with this: this.OnPropertyChanged(n => n.Title);
/// </summary>
/// <typeparam name="T">Property owner</typeparam>
/// <typeparam name="TProperty">Type of property</typeparam>
/// <param name="observableBase"></param>
/// <param name="expression">Property expression like 'n => n.Property'</param>
public static void OnPropertyChanged<T, TProperty>(this T observableBase, Expression<Func<T, TProperty>> expression) where T : INotifyPropertyChangedWithRaise
{
observableBase.OnPropertyChanged(GetPropertyName<T, TProperty>(expression));
}
public static string GetPropertyName<T, TProperty>(Expression<Func<T, TProperty>> expression) where T : INotifyPropertyChanged
{
if (expression == null)
throw new ArgumentNullException("expression");
var lambda = expression as LambdaExpression;
MemberExpression memberExpression;
if (lambda.Body is UnaryExpression)
{
var unaryExpression = lambda.Body as UnaryExpression;
memberExpression = unaryExpression.Operand as MemberExpression;
}
else
{
memberExpression = lambda.Body as MemberExpression;
}
if (memberExpression == null)
throw new ArgumentException("Please provide a lambda expression like 'n => n.PropertyName'");
MemberInfo memberInfo = memberExpression.Member;
if (String.IsNullOrEmpty(memberInfo.Name))
throw new ArgumentException("'expression' did not provide a property name.");
return memberInfo.Name;
}
}
INotifyPropertyChangedWithRaise 在哪里（它建立了引发 NotifyPropertyChanged 事件的标准接口）：
public interface INotifyPropertyChangedWithRaise : INotifyPropertyChanged
{
void OnPropertyChanged(string propertyName);
}
最后一块拼图是这样的：
public class ThreadTools
{
public static void RunInDispatcher(Dispatcher dispatcher, Action action)
{
RunInDispatcher(dispatcher, DispatcherPriority.Normal, action);
}
public static void RunInDispatcher(Dispatcher dispatcher, DispatcherPriority priority, Action action)
{
if (action == null) { return; }
if (dispatcher.CheckAccess())
{
// we are already on thread associated with the dispatcher -> just call action
try
{
action();
}
catch (Exception ex)
{
//Log error here!
}
}
else
{
// we are on different thread, invoke action on dispatcher's thread
dispatcher.BeginInvoke(
priority,
(Action)(
() =>
{
try
{
action();
}
catch (Exception ex)
{
//Log error here!
}
})
);
}
}
}