Question

If I have a member, constructor and method like so

//The injected object exposes a public property of type bool which will raise
//a NotifyPropertyChangedEvent
IInjectedObject _injectedObject;

public someClass(IInjectedObject injectedObject)
{
    _injectedObject = injectedObject;
}

public void DoSomething()
{

}

Is there a way to call the method in my class when the property on the injected object changes?

Answer 1

You state that your interface inherits from INotifyPropertyChanged, so it can reasonably be expected to notify listeners of property changes through the “PropertyChanged” event. Assuming something like this:

public interface IInjectedObject : INotifyPropertyChanged
{
    bool MyImportantProperty { get; }
}

Then, your dependant object must listen for the INotifyPropertyChanged.PropertyChanged event:

public class MyDependantClass
{
    public MyDependantClass(IInjectedObject injectedObject)
    {
        MyInjectedObject = injectedObject;
    }

    // We wrap the private field in a protected property,
    // to capture the event subscriptions
    private IInjectedObject _myInjectedObject;
    protected IInjectedObject MyInjectedObject
    {
        get { return _myInjectedObject; }
        set
        {
            // unsubscribe from the old property's event
            if(_myInjectedObject!= null)
                _myInjectedObject.PropertyChanged -= OnPropertyChanged;

            _myInjectedObject= value;

            // subscribe to the new property's event
            if(_myInjectedObject!= null)
               _myInjectedObject.PropertyChanged += OnPropertyChanged;
        }
    }

    private void OnPropertyChanged(object sender, PropertyChangedEventArgs args)
    {
        if(args.PropertyName == "MyImportantProperty")
        {
            //react to the changed property here!
        }
    }
}