How do you implement callbacks in Android/Java?

Question

I have written an application in Xamarin which I'm trying to transfer to Android studio.

The Xamarin Application is split into modules each one containing , a number of objects which represent the model and do all the work, one Activity which acts as a controller and interprets messages from the views and sends them to the model, and a number of fragments which represent the views and are responsible for handling screen events and sending the message to the Activity/controller.

Each fragment is always instantiated as new when it has to be shown. Using this code:

    private void show_fragment(int fragment_id, string id) {
        Fragment fragment = FragmentFactory.Create(fragment_id);
        FragmentManager
            .BeginTransaction()
            .Replace(Resource.Id.content, fragment)
            .AddToBackStack(id)
            .CommitAllowingStateLoss();
    }

and when the fragment is attached, it keeps a reference to the activity it was attached to using this code:

    [Obsolete] //this is for older versions of android 
    public override void OnAttach(Activity activity) {
        base.OnAttach(activity);
        do_attach(activity);
    }
    public override void OnAttach(Context context) {
        base.OnAttach(context);
        do_attach(context);
    }

    void do_attach(Context context) {
        _context = context;
        if (context is IModuleInteraction) {
            _parent = (IModuleInteraction)( context as object );
        } else {
            throw new Exception("Fragment parent type is incorrect");
        }
    }

when the fragment requires something from the controller, it sends it a request along with a callback, for example let's say that the user selected a product from the list, this is what would happen

In the fragment

    _parent.product_selected(product_id, () => update_screen(), (message) => show_error(message) );

so in this case the controller will try to find the product, if it can find the product it will call update_screen() if it can't it will call show_error(message)

In android studio this all works correctly, my question is how can I pass callbacks to the controller?

Right now what I do is this

 Fragment fragment = getFragmentManager.findFragmentById(Resource.Id.content)
 if (fragment instanceOf ProductSelectFragment) {
     ProductSelectFragment psf = (ProductSelectFragment) fragment;
     if (error == null)
         psf.update_screen();
     else
         psf.show_error(error);
 }

is there a better way to do this?

thanks in advance for any help you can provide

p.s. I an using the base FragmentManager, not the SupportFragmentManager

Isn't this line callback passing? : `_parent.product_selected(product_id, () => update_screen(), (message) => show_error(message) );` — Mister Smith, Jul 14 '16 at 07:43
yes but this is in C#, using Actions and Funcs, what is the java equivalent? that's what I'm asking , maybe I have gone overboard with the explanation :) — Cruces, Jul 14 '16 at 07:44
[Here](http://stackoverflow.com/questions/1184418/javas-equivalents-of-func-and-action) you are :) — Mister Smith, Jul 14 '16 at 07:46
yup, that's exactly what I was searching for, seems I should have searched for "java equivalent of action and func" , thanks a lot, sorry for wasting your time — Cruces, Jul 14 '16 at 07:47
NP. Also note that Java sucks for async. Unless you can afford to switch to the Jack compiler, which has reduced Java 8 support, you might also consider using RxJava, Agera or Jdeferred for your controller. — Mister Smith, Jul 14 '16 at 07:50
I'm not using async/await, even in Xamarin, I've always been using AsyncTask implementations, I'll check the controller implementations you mentioned though, I've never heard of them before, so they must be good — Cruces, Jul 14 '16 at 07:54
*I've never heard of them before, so they must be good* -- That logic seems flawed... — OneCricketeer, Jul 14 '16 at 08:24

How do you implement callbacks in Android/Java?

0 Answers0