Using OnBackPressedCallback in Xamarin

Question

I'm trying to implement something similar to OnBackPressed() in a fragment in Xamarin, but the only solutions I found so far are for Java.

Here is one example in Java that does what I want:

OnBackPressedCallback callback = new OnBackPressedCallback(true) {
        @Override
        public void handleOnBackPressed() {
            Toast.makeText(mContext, "back pressed", Toast.LENGTH_SHORT).show();
            
            // And when you want to go back based on your condition
            if (yourCondition) {
                this.setEnabled(false);
                requireActivity().onBackPressed();
            }
        }
    };

How could I go about converting that piece of code into C#?

Please check the updated answer

FreakyAli
– FreakyAli

2022-09-06 10:08:28 +00:00
Commented Sep 6, 2022 at 10:08 — FreakyAli
– FreakyAli, Commented Sep 6, 2022 at 10:08

halfer · Accepted Answer · 2023-11-17 20:44:04Z

0

You cannot have anonymous classes in C#, so just create a class that inherits this abstract class and you're good:

public class HandleOnBackPressedCallback : OnBackPressedCallback
{
    public HandleOnBackPressedCallback(bool enabled) : base(enabled)
    {
    }

    public override void HandleOnBackPressed()
    {
        Toast.MakeText(context, "back pressed", ToastLength.Long).Show();

        // And when you want to go back based on your condition
        if (yourCondition)
        {
            this.setEnabled(false);
            requireActivity().onBackPressed();
        }
    }
}

Update

Create an interface for handling your event:

public interface IOnBackPressedHandler
{
    bool OnBackPressed();
}

In your activity (below is just an example) setup your interface callback as follows:

public class FragActivity : AppCompatActivity
{

    public override void OnBackPressed()
    {
        Fragment fragment = SupportFragmentManager.FindFragmentById(Resource.Id.yourFragmentId);
        if (!(fragment is IOnBackPressedHandler) || !((IOnBackPressedHandler)fragment).OnBackPressed()) {
            base.OnBackPressed();
        }
    }

}

Finally, your fragment would look something like that:

public class SomeFragment : Fragment, IOnBackPressedHandler
{
    public bool OnBackPressed()
    {
        //Callbacks will be received here.
        return true;
    }
}

You can also make this method a part of your base Fragment mark it as virtual and then have all classes inherit this to keep overriding it.

edited Nov 17, 2023 at 20:44

halfer

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answered Sep 6, 2022 at 9:08

FreakyAli

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4 Comments

Librapulpfiction Over a year ago

Thanks for the conversion. How do I actually make the HandleOnBackPressed() method run though? I don't want to call the method directly, what I want is for that method to be called when the user pressed the back button in a fragment. Here is the code I wrote in the fragment: HandleOnBackPressedCallBack callback = new HandleOnBackPressedCallback(true); Activity.OnBackPressedDispatcher.AddCallback(this, callback);

FreakyAli Over a year ago

So what you want is that every time your back button is pressed in a fragment you want a callback in your fragment?

Librapulpfiction Over a year ago

I'm not exactly sure how callbacks work. What I'm trying to do is be able to use a specific method when the OnBackPressed() event fires in a specific fragment. So, for example, fragment A would execute method A when OnBackPressed() fires, while fragment B would execute method B when OnBackPressed() fires.

FreakyAli Over a year ago

I see updating my answer...

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