Agfdhyk

I would like to be notified with a PropertyChanged event raised from a class MyClass implementing an interface IMyInterface. The instance of this class is retrieved with DependencyService.Get<IMyInterface>();

Now, this is the class (Android project), in which there is a property with a backing field:

class MyClass : IMyInterface {



    private bool _isEnabled;

    public bool IsEnabled

    {

        get { return _isEnabled; }

        private set { _isEnabled = value; OnPropertyChanged("IsEnabled"); }

    }



    public event PropertyChangedEventHandler PropertyChanged;



    protected void OnPropertyChanged(string name)

    {

        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(name));

    }



    public void OnSomethingHappens() { //This method is called when something particular happens

        /* Some code... */

        IsEnabled = true; //The point where the property is changed

        /* ...some other code */

    }

}

The interface (Forms project) looks like this:

public interface IMyInterface : INotifyPropertyChanged

{

    /* code relating the interface */

}

This interface is implementing INotifyPropertyChanged in order to have the possibility to listen to a property change (as seen in the class above). So far, so good.

The problem is when, in my Forms project, I register an event handler with this:

_myService = DependencyService.Get<IMyInterface>();

_myService.PropertyChanged += (sender, e) => { /* do stuff */ };

But when the property in my implementing class changes, PropertyChanged results null, and this means that the event is never fired.

I would like to add a little thing: recently, I experienced that accessing a non-static list property with DependencyService.Get<IMyInterface>().MyList was useless, since MyList resulted to be of size 0, even if this list was filled inside the implementing class. I solved this using a static property. Maybe this is related to the issue above.

I would like to understand what I am mistaking in order to have this PropertyChanged event working correctly.

Ok, I figured out how to solve this.
Basically, I created a property with a private static event backing field, as follows:

private static event PropertyChangedEventHandler _propertyChanged;

public PropertyChangedEventHandler PropertyChanged

{

    get { return _propertyChanged; }

    set { _propertyChanged = value; }

}

This way, I could have the following code working properly:

protected void OnPropertyChanged(string name)

{

    _propertyChanged?.Invoke(this, new PropertyChangedEventArgs(name));

}

Infact, _propertyChanged was not null anymore, since I subscribed to the event with the following code (formally identical to the one posted on my question, with the difference that I used the PropertyChanged property backed with the private static field _propertyChanged):

_myService = DependencyService.Get<IMyInterface>();

_myService.PropertyChanged += (sender, e) => { /* do stuff */ };

Now I think that this was necessary because I am using DependencyService, otherwise, in normal code, this issue should not come out.
Anyway, I am guessing if the idea of having a EventHandler property backed with a private static event field is a good code practice or just sounds weird. But it works, and it works pretty good.

But, in order to do all of this, I had to change this:

public interface IMyInterface : INotifyPropertyChanged

{

    /* code relating the interface */

}

to this:

public interface IMyInterface

{

    PropertyChangedEventHandler PropertyChanged { get; set; }

    /* code relating the interface */

}

I had to add that property in IMyInterface so that I could access it from my Forms project code.
Moreover, I no more implement INotifyPropertyChanged, because INotifyPropertyChanged asks me to implement the interface with a:

public event PropertyChangedEventHandler PropertyChanged

inside my implementing class, but I substituted it with

private static event PropertyChangedEventHandler _propertyChanged;

public PropertyChangedEventHandler PropertyChanged

{

    get { return _propertyChanged; }

    set { _propertyChanged = value; }

}

as I exposed above.