What mechanisms can be used to cancel requests from presenter to view?

Dead View: The presenter does not hold a direct reference to the View but receives it through an interface at the moment of method call. If the View no longer exists, the call simply does not happen.

State flags: The View can store a flag indicating whether it is active. Before performing an action that requires the View, the presenter checks this flag.

RxJava unsubscription: If Observable/Flowable are used for interaction, you can manage subscriptions and unsubscribe when the View is destroyed.

// In the presenter
private var disposable: Disposable? = null

fun loadData() {
    disposable = model.getData()
        .subscribeOn(Schedulers.io())
        .observeOn(AndroidSchedulers.mainThread())
        .subscribe({ data ->
            view?.showData(data) // Null check
        }, { error ->
            view?.showError(error) // Null check
        })
}

fun detachView() {
    disposable?.dispose() // Unsubscribe when View is destroyed
    view = null // Nullify the View reference
}

Callback with View check: Pass an object as a callback that internally checks for the existence of the View before performing an action.

// In the presenter
fun processData(data: Data) {
    val uiCallback = object : UICallback {
        override fun onSuccess(result: Result) {
            view?.showResult(result) // Null check
        }
        override fun onError(error: Throwable) {
            view?.showError(error) // Null check
        }
    }
    model.doAsyncOperation(data, uiCallback)
}

// In the View
interface UICallback {
    fun onSuccess(result: Result)
    fun onError(error: Throwable)
}

Lifecycle-aware components: Use components aware of the lifecycle (e.g., LifecycleObserver in Android architecture components) for automatic cancellation of requests when the View's state changes.

// In the presenter, which is a LifecycleObserver
class MyPresenter(private val view: MyView) : LifecycleObserver {

    @OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
    fun onDestroy() {
        // Cancel operations when the View is destroyed
    }

    // ...
}

// In the View
lifecycle.addObserver(presenter)