Sometimes we need to execute a piece of code in a separate thread to avoid to block the main thread. It is mandatory if you want to perform some network access or tasks that may take potentially longer than 4 secs, without getting an ARN. A good practice is to do it for every task that takes longer than 100 ms . Otherwise we are blocking (for more than 100 ms) the main thread and it may cause a bad user experience (for instance freeze scrolling). Spawn threads in Java is a mechanism that is provided by Java itself. Sometimes this “worker threads” have to give callbacks to the “main thread.” For that also Java provides “handlers“ to deal with inter-threads communication. Android provides some more elaborate “classes” beside framework that make it easy deal with that like AsyncTask.
We can not keep an activity or fragment reference (hard reference) inside an AsyncTask without getting into memory leak issue and or a NullPointerException.
During recreation “Activity/Fragment” as when orientation screen occurs. The old Activity instance is destroyed and the new one is created instead, but unfortunately the AsyncTask keeps the previous one reference (the old instance activity) so GC (garbage collector) cannot free memory heap up since that old instance Activity is still referenced (this is known as leak memory). It also drives a bad performance because callbacks will take place in the old “Activity/Fragment” instance and not in the new one. It may cause a NullPointerException if the hierarchy of views has been destroyed and AsyncTask tries to update UI.
Workarounds like to cancel task and execute it again and update the new instance “Activity/Fragment” reference keeps away from leak memory. However, it means that we have to discard the previous task and execute it again. In case we can discard tasks, in most cases, tasks can not be canceled only the callbacks are canceled so that it can drive into a possible duplicated execution task. Anyway, in the best case, it drives into a bad performance. Why should it cancels a task just because device configuration changed (f.i. when I rotate the device).
There are several solutions ranging from blocking the rotation (no recommended because recreation not only happens when screen rotates) or Handling the Configuration Change yourself or keep AsyncTask reference during recreation and update listeners (the most used in the past) till more elaborate ones like using Loaders or RxAndroid that allows binding executions callbacks into the “Activity/Fragment” lifecycle. There are others like using EventBus that I do not recommend at all, or a little bit better could post an event in the message Queue Main Thread or keeping the AsyncTask inside a non-UI Fragment and set setRetainInstance to true.
The solution UseCaseLoader (based on Loaders/AsyncTaskLoaders):
AsyncTaskLoader is an abstract class, so implementation is required. One that framework provides it is the CursorLoader, to get data from contentProviders in an easy way. Here I create my Loader, a custom class called UseCaseLoader that execute commands Callable or Runnable objects that can match with the application usecase Interactors. In the example app, I have two usecases that correspond with GetNoteListUseCase and AddNewNoteUseCase, both to extend UseCaseLoader.
see the example at DemoLoader
Brief introduction to UseCaseLoader:
Loaders and AsynTaskLoaders are an abstract class so the implementation may vary slightly. This is a brief introdcution to Loaders as was implemented on UseCaseLoaders.
Android 3.0 (API level 11) come up with a new feature called Loaders.
Perform a sync task taking account of Activity/Fragment lifecycle.
That means you can safely restore callbacks for the current loader-task into the new Activity or Fragment instance, even if the task was in progress at the time the activity/fragment was recreated.
Loaders are managed by LoaderManager. You can have access through getSupportLoaderManager()/ getLoaderManager in your Activity or Fragment;
LoaderManager public interface:
intLoader starts new one or reconnect with the previous loader (with the same id) if loader has not been created before, onCreateLoader callback will be called immediately.
If Loader was previously created, it will be updated with new LoaderCallback implementation and will be triggered onLoadFinished() callback with previous data loaded, even if the task was executing recreation meanwhile Activity/Fragment took place.
Loaders keep state during activity/fragment recreation, so if previous data was loaded, no reload needed. If you want to force load You can force loader calling **restartLoader() **it discard old data and reload again.
Place initLoaders onActivityCreated() Fragment Or OnCreate() Activity.
Keep in mind that loader is in the activity or fragment scope.
You can destroy loaders calling destroyLoader(), alternatively, just cancel task invoke cancelLoader().