LifecycleObserver which allows easy cancelling of coroutines

CoroutineLifecycleObserver.kt

/*
 * Copyright 2018 Google LLC
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

class CoroutineLifecycleObserver : LifecycleObserver {
    var job: Job = Job()
        private set
    
    @OnLifecycleEvent(Lifecycle.Event.ON_START)
    fun start() {
        // If the job is complete (happens after being previously stopped)
        // lets create a new one
        if (job.isCompleted) {
            job = Job()
        }
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_STOP)
    fun cancel() {
        // If the job is active (running) cancel it
        if (job.isActive) {
            job.cancel()
        }
    }
}

class DetailFragment : Fragment() {
    private val coroutineLifecycle = CoroutineLifecycleObserver()
    
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        // Add observer so any jobs are automatically cancelled
        lifecycle.addObserver(coroutineLifecycle)
    }
    
    private fun runTask() {
        // Set the coroutine parent from LifecycleObserver so that it will
        // be automatically cancelled in onStop
        launch(parent = coroutineLifecycle.job) { 
            // do something long-running
        }
      
        launch(parent = coroutineLifecycle.job) { 
            // do something else long-running
        }
        
        // both of these will be cancelled in onStop (if still running)
    }
}

+1

Great stuff!! Although, this only works if you're launching coroutines in a fragment or Activity. How could this be applied if you launch your coroutines in another component that's lifecycle agnostic, e.g presenter?

@michelonwordi It works with anything which exposes a LifecycleOwner, so your presenter could easily expose one of those. You then just add this observer to it.

This looks like a great use case for inclusion in a base activity... given it supports business rules but doesn't in itself implement any. Thanks!

My impression here is that we should call runTask in onStart. Is that correct?

In this case I need to add arch dependency to my project?

There is a new approach: "Structured concurrency"
https://medium.com/@elizarov/structured-concurrency-722d765aa952