Boilerpart code for asynchronous MVP design for Android, without 3rd party libraries.
How to implement a simple, decoupled, testable, asynchronous MVP design, without using any 3rd party libraries, that can also handle screen orientation.
The proposed solution for handling screen orientation, is a simple cache mechanism for the presenters:
public class Cache implements Serializable {
private static Cache ourInstance;
private Map<UUID, Presenter> cache;
public static Cache getInstance() {
if (ourInstance == null) {
ourInstance = new Cache();
}
return ourInstance;
}
private Cache() {
cache = new HashMap<>();
}
public void cachePresenterFor(UUID uuid, Presenter presenter) {
cache.put(uuid, presenter);
}
public Presenter restorePresenterFor(UUID uuid) {
Presenter presenter = cache.get(uuid);
cache.remove(presenter);
return presenter;
}
}How does it work?
During orientation change, the Android OS calls two callback methods, where the developer can save and restore the view state. These callbacks are: onSaveInstanceState and onRestoreInstanceState.
When onSaveInstanceState is called, the presenter is cached:
@Override
protected void onSaveInstanceState(Bundle outState) {
// Save presenter instance
outState.putString(PRESENTER, presenter.getUuid().toString());
cachePresenter(presenter);
}
@Override
public void cachePresenter(Presenter presenter) {
Cache.getInstance().cachePresenterFor(presenter.getUuid(), presenter);
}The reason each presenter instance has a UUID, is to handle the case of multiple instances of the same view. Each view has to have its own presenter, so cache will contain multiple instances of the same presenter class. When the view is restored, it has to find the appropriate presenter in the cache.
When onRestoreInstanceState is called, the view restores its presenter from the cache, and the presenter restores its reference to the view:
@Override
protected void onRestoreInstanceState(Bundle savedInstanceState) {
// Restore presenter instance
restorePresenter(UUID.fromString(savedInstanceState.getString(PRESENTER)));
}
@Override
public void restorePresenter(UUID uuid) {
presenter = (XPresenter) Cache.getInstance().restorePresenterFor(uuid);
presenter.setScreen(this);
}Asynchronous operations are handled in the presenters. Each presenter acts as a "bridge" between the view and the model (business logic). This "bridge" mechanism is implemented with AsyncTasks. Each AsyncTask orchestrates the flow of date between the view and the model, performing background operations and publishing the results on the UI.
@Override
public void requestAction1() {
new AsyncTask<Void, Void, String>() {
@Override
protected String doInBackground(Void... params) {
Util.simulateNetworkLatency(2000);
return model.requestAction1();
}
@Override
protected void onPostExecute(String s) {
super.onPostExecute(s);
view.postResult1(s);
}
}.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR);
}If you are looking for an implementation of this proposed solution, please take a look at the following repository: Asynchronous MVP design for Android, without 3rd party libraries.
See also a short video discussion about this solution here.
master - The master branch
freeze-version-1 - A backup of the master branch. More loosely coupled MVP design.
freeze-version-2 - A branch based on "freeze-version-1". Removed the AsyncTasks from the presenters. Moved asynchronous operations to views. More tightly coupled MVP design.
Copyright 2017 Ovidiu CONEAC
Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
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