8 ways to communicate between Fragment and Activity in Android apps

There’s always need for communication, right 😉 Suppose we have OnboardingActivity that has several OnboardingFragment. Each Fragment has a startButton telling that the onboarding flow has finished, and only the last Fragment shows this button.

Here are several ways you can do that. Any feedback or improvement suggestions are more than welcome.

EventBus

Nearly all articles I found propose this , but I personally don’t like this idea because components are loosely coupled, every component and broadcast can listen to event from a singleton, which makes it very hard to reason when the project scales

data class OnboardingFinishEvent()

class OnboardingActivity: AppCompatActivity() {override fun onStart() {super.onStart()

    EventBus.getDefault().register(this)  
}

override fun onStop() {  
    EventBus.getDefault().unregister(this)  
    super.onStop()  
}  
  
@Subscribe(threadMode = ThreadMode.MAIN)  
fun onOnboardingFinishEvent(event: OnboardingFinishEvent) {  
    // finish  
}  

}

class OnboardingFragment: Fragment() {override fun onViewCreated(view: View?, savedInstanceState: Bundle?) {super.onViewCreated(view, savedInstanceState)

    startButton.onClick {  
        EventBus.getDefault().post(OnboardingFinishEvent())  
    }  
}  

}

Read more about this approach

Otto

Otto from said to be an enhanced Guava-based event bus with emphasis on Android support. However it is deprecated

This project is deprecated in favor of RxJava and RxAndroid. These projects permit the same event-driven programming model as Otto, but they’re more capable and offer better control of threading.

RxJava

We can use simple PublishSubject to create our own

import io.reactivex.Observableimport io.reactivex.subjects.PublishSubject

// Use object so we have a singleton instanceobject RxBus {

private val publisher = PublishSubject.create<Any>()

fun publish(event: Any) {  
    publisher.onNext(event)  
}

// Listen should return an Observable and not the publisher  
// Using ofType we filter only events that match that class type  
fun <T> listen(eventType: Class<T>): Observable<T> = publisher.ofType(eventType)  

}

// OnboardingFragment.ktstartButton.onClick {RxBus.publish(OnboardingFinishEvent())}

// OnboardingActivity.ktoverride fun onCreate(savedInstanceState: Bundle?) {super.onCreate(savedInstanceState)

RxBus.listen(OnboardingFinishEvent::class.java).subscribe({  
    // finish  
})  

}

Interface

This is advised here . Basically you define an interface OnboardingFragmentDelegate that whoever conforms to that, can be informed by the Fragment of events. This is similar to Delegate pattern in iOS 😉

interface OnboardingFragmentDelegate {fun onboardingFragmentDidClickStartButton(fragment: OnboardingFragment)}

class OnboardingFragment: Fragment() {var delegate: OnboardingFragmentDelegate? = null

override fun onAttach(context: Context?) {  
    super.onAttach(context)

    if (context is OnboardingFragmentDelegate) {  
        delegate = context  
    }  
}

override fun onViewCreated(view: View?, savedInstanceState: Bundle?) {  
    super.onViewCreated(view, savedInstanceState)

    startButton.onClick {  
        delegate?.onboardingFragmentDidClickStartButton(this)  
    }  
}  

}

class OnboardingActivity: AppCompatActivity(), OnboardingFragmentDelegate {override fun onboardingFragmentDidClickStartButton(fragment: OnboardingFragment) {onboardingService.hasShown = truestartActivity<LoginActivity>()}}

ViewModel

We can learn from to to communication between Fragment and Activity, by using a shared ViewModel that is scoped to the activity. This is a bit overkill

class OnboardingSharedViewModel: ViewModel() {val finish = MutableLiveData<Unit>()}

class OnboardingActivity: AppCompatActivity(), OnboardingFragmentDelegate {override fun onCreate(savedInstanceState: Bundle?) {super.onCreate(savedInstanceState)val viewModel = ViewModelProviders.of(this).get(OnboardingSharedViewModel::class.java)

    viewModel.finish.observe(this, Observer {  
        startActivity<LoginActivity>()  
    })  
}  

}

Note that we need to call ViewModelProviders.of(activity) to get the same ViewModel with the activity

class OnboardingFragment: Fragment() {override fun onViewCreated(view: View?, savedInstanceState: Bundle?) {super.onViewCreated(view, savedInstanceState)

    val viewModel = ViewModelProviders.of(activity).get(OnboardingSharedViewModel::class.java)  
    startButton.onClick({  
        viewModel.finish.value = Unit  
    })  
}  

}

Lambda

Create a lambda in Fragment, then set it on onAttachFragment. It does not work for now as there is no OnboardingFragment in onAttachFragment 😢

class OnboardingFragment: Fragment() {var didClickStartButton: (() -> Unit)? = null

override fun onViewCreated(view: View?, savedInstanceState: Bundle?) {  
    super.onViewCreated(view, savedInstanceState)

    startButton.onClick {  
        didClickStartButton?.invoke()  
    }  
}  

}

class OnboardingActivity: AppCompatActivity() {override fun onAttachFragment(fragment: Fragment?) {super.onAttachFragment(fragment)

    (fragment as? OnboardingFragment).let {  
        it?.didClickStartButton = {  
            // finish  
        }  
    }  
}  

}

Listener in Bundle

Another proposed way is through listener in bundle, the below post gives more insight into this approach

Original story

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