Oftentimes it can become necessary to access platform specific APIs, e.g., adding advertisement services or a leaderboard functionality which are only available for Android/iOS/desktop. This can be achieved by allowing a specific implementation to be defined through a common API interface.
Take the following example, which tries to use a very simple leaderboard API that is only available on Android. For other targets we simply want to log invocations or provide mock return values.
The common interface
The first step is to create an abstraction of the API in form of an interface which is put into the core project:
public interface Leaderboard {
public void submitScore(String user, int score);
}
Next we create specific implementations for each platform and put these into their respective projects.
The Android implementation
On Android, we would like our code to call the Google Play API, which provides the method LeaderboardsClient#submitScore(String leaderboardId, long score);.
Thus, in the Android project, we need to implement our interface Leaderboard and call the platform-specific code as follows:
/** Android implementation, can access PlayGames directly **/
public class AndroidLeaderboard implements Leaderboard {
public void submitScore(String user, int score) {
// Ignore the user name, because Google Play reports the score for the currently signed-in player
// See https://developers.google.com/games/services/android/signin for more information on this
PlayGames.getLeaderboardsClient(activity).submitScore(getString(R.string.leaderboard_id), score);
}
}
The desktop implementation
The following code would go into the desktop project:
/** Desktop implementation, we simply log invocations **/
public class DesktopLeaderboard implements Leaderboard {
public void submitScore(String user, int score) {
Gdx.app.log("DesktopLeaderboard", "would have submitted score for user " + user + ": " + score);
}
}
The GWT implementation
The following code would go into the HTML5 project:
/** Html5 implementation, same as DesktopLeaderboard **/
public class Html5Leaderboard implements Leaderboard {
public void submitScore(String user, int score) {
Gdx.app.log("Html5Leaderboard", "would have submitted score for user " + user + ": " + score);
}
}
Obtaining the platform-specific implementation in core
Next, our ApplicationListener gets a constructor to which we can pass the concrete Leaderboard implementation:
public class MyGame implements ApplicationListener {
private final Leaderboard leaderboard;
public MyGame(Leaderboard leaderboardImpl) {
this.leaderboard = leaderboardImpl;
}
// rest omitted for clarity
}
In each starter class we then simply instantiate MyGame, passing the corresponding Leaderboard implementation as an argument, e.g., on the desktop:
public static void main(String[] argv) {
Lwjgl3ApplicationConfiguration config = new Lwjgl3ApplicationConfiguration();
new Lwjgl3Application(new MyGame(new DesktopLeaderboard()), config);
}
Alternatively, we can obtain the platform-specific implementation via reflection:
if (Gdx.app.getType() == ApplicationType.Desktop || Gdx.app.getType() == ApplicationType.HeadlessDesktop) {
try {
this.leaderboard = (Leaderboard) ClassReflection.newInstance(ClassReflection.forName("com.mygame.desktop.DesktopLeaderboard"));
} catch (ReflectionException e) {
e.printStackTrace();
}
}