如何避免“动态”事件订阅中的匿名方法？

Question

How could I refactor the method

private void ListenToPropertyChangedEvent(INotifyPropertyChanged source,
                                          string propertyName)
{
    source.PropertyChanged += (o, e) =>
    {
        if (e.PropertyName == propertyName)
            MyMagicMethod();
    };
}

if I wished to avoid using the anonymous method here?

Answer 1

Implement the closure that is implicitly created by the lambda explicitly:

private void ListenToPropertyChangedEvent(INotifyPropertyChanged source,
                                          string propertyName)
{
    var listener = new MyPropertyChangedListener(propertyName);
    source.PropertyChanged += listener.Handle;
}

class MyPropertyChangedListener
{
    private readonly string propertyName;

    public MyPropertyChangedListener(string propertyName)
    {
        this.propertyName = propertyName;
    }

    public void Handle(object sender, PropertyChangedEventArgs e)
    {
        if (e.PropertyName == this.propertyName)
        {
            // do something
        }
    }
}

Answer 2

You can handle this by having a single event handler for all instances that uses a dictionary of instances that you are following:

private Dictionary<INotifyPropertyChanged, List<string>> sourceMap =
    new Dictionary<INotifyPropertyChanged, List<string>>();

private void ListenToPropertyChangedEvent(INotifyPropertyChanged source,
                                            string propertyName)
{
    if (sourceMap.ContainsKey(source))
        sourceMap[source].Add(propertyName);
    else
    {
        source.PropertyChanged += source_PropertyChanged;
        sourceMap[source] = new List<string> { propertyName };
    }
}

void source_PropertyChanged(object sender, PropertyChangedEventArgs e)
{
    var source = sender as INotifyPropertyChanged;
    var list = sourceMap[source];
    if (list.Contains(e.PropertyName))
        MyMagicMethod();
}

This version doesn't have any error checking or removal but it demonstrates the technique. It is particularly valuable if you listen for multiple properties from the same source . This is because it only adds a single handler to the PropertyChanged event per instance.

Answer 3

I'm not sure exactly what youre trying to achieve or why you dont want to use anonymous methods, but you could do something more generic:

    private PropertyChangedEventHandler GetHandler
      (Func<PropertyChangedEventArgs, bool> test, Action toInvoke)
    {
        return new PropertyChangedEventHandler(
            (o, e) => 
            {
               if (test(e))
                toInvoke(); 
            }); 
    }

Then you can use it like so:

  source.PropertyChanged += GetHandler                
            (
                p => p.PropertyName == propertyName, MyMagicMethod
            ); 

That way your if test and the target method group can be swapped about easily. Your event handler is also strongly typed rather than anonymous.