Preferences behaviour differs on Android

Issue details

I notice that behaviour of Preferences is different on Android; a newly-written preference will not be readable until a flush has occurred. On other platforms, it can be read immediately, provided the same Preferences instance is used.

This is because of the use of the SharedPreferences Editor in the Android implementation. I am happy to make the change myself to bring the behaviour in-line with the other platforms; the reason this is an issue rather than a PR is that I would like input from contributors as to which behaviour is actually desired.

Minimal example

import com.badlogic.gdx.*;
import com.badlogic.gdx.backends.lwjgl.LwjglApplication;
import com.badlogic.gdx.graphics.GL20;

public class Barebones extends ApplicationAdapter {
	public void create () {
		Preferences preferences = Gdx.app.getPreferences(".pref");
                preferences.putString("flower", "tulip");
                Gdx.app.log("Flower", preferences.getString("flower", "daffodil"));
                // On Desktop and iOS this prints "tulip". On Android it prints "daffodil".
                preferences.flush();
                Gdx.app.log("Flower", preferences.getString("flower", "daffodil"));
                // This will now print "tulip" on all platforms.
	}

	public void render () {
		Gdx.gl.glClear(GL20.GL_COLOR_BUFFER_BIT);
	}

	public static void main (String[] args) throws Exception {
		new LwjglApplication(new Barebones());
	}
}

Version of LibGDX and/or relevant dependencies

Latest

Please select the affected platforms

• Android
• iOS (robovm)
• iOS (MOE)
• HTML/GWT
• Windows
• Linux
• MacOS

[obigu]

The documentation of the Preferences interface could definitely improve by stating that there is no guarantee changes will be available until flush is called. The current sentence:

Changes to a preferences instance will be cached in memory until {@link #flush()} is invoked.

is not accurate (at least it may be misleading) imo as it can be read as if changes will be available in memory until persisted to disk, which is not the case on the Android implementation.

[MrStahlfelge]

When doing this change, tt should also be added that you should only use one instance of a Preference and hold it in your game class. Users had problems with using multiple instances on iOS.