The content in my symposium poster session on using object-oriented design to create cross-platform games and applications.

1. Cross-Platform
Software Design
Michael Henson
Professor
College of Engineering and Information Sciences
mhenson@devry.edu
/arachnojava
@arachnojava
+MichaelHenson /michaeljhenson
Arachnojava mhframework.blogspot.com

2. The Problems
• Professional developers need to:
– Reach the maximum number of customers.
– Keep current with new computing devices.
• Students need to:
– Quickly adapt to the technologies that will grab
the attention of potential employers.
– Increase the probability that a potential employer
will be able to evaluate their work.
• Wouldn’t it be nice if we could develop
just one product and make it available
for a variety of computing devices?
– PC, tablet, smart phone, game console, etc.

3. One Possible Solution
1. Identify parts that are same across platforms.
– We’ll call this the core functionality.
2. Encapsulate the parts that differ.
– We’ll call this the platform layer.
3. Create an interface to the platform layer that can
be repeated for all supported platforms.
4. Make sure the core only talks to the platform
layer interface and not the actual platform.
– Then we can swap platforms and the core never
needs to know!

4. A Bird’s-Eye
View of the
Idea

5. Proof of Concept: Snow vs. Man
• Very early prototype built with
MHFramework game engine which
implements this design.
• Play it on my laptop, tablet, or phone.
• Objective: Prevent the evil snowmen
from extinguishing your campfire.
• How to Play: Click (tap) on open spaces
to place defensive turrets.

6. Systems That Can
(or Will) Run My Game
Android
Phone
Android
Tablet
OUYA Game
Console
M.O.J.O. Game
Console
Linux
Mac OS
Windows

7. So…How Does It Work?
1. The main program runs in a platform-specific
context.
– For instance, an Activity on Android or a JFrame
on Windows.
2. The program requests the appropriate
platform layer depending on the context.
3. All calls to the platform go through a
common interface that handles
communication with whatever platform lies
beneath.

8. Main Program Code
Notice the major similarities (and minor differences) between
the PC and Android launchers for Snow vs. Man.
PC Version
public static void main(String[] args)
{
// Create the game's start screen.
MHScreen startingScreen = new SVMMainMenuScreen();
// Configure video settings.
MHVideoSettings displaySettings = new MHVideoSettings();
displaySettings.windowCaption = "Snow Vs. Man";
displaySettings.displayWidth = 800;
displaySettings.displayHeight = 480;
// Launch the game.
MHPCPlatform.run(new JFrame(),
startingScreen, displaySettings);
}
Android Version
protected void onCreate(Bundle savedInstanceState)
{
// Initialize the Android activity.
super.onCreate(savedInstanceState);
// Create the game's start screen.
MHScreen startingScreen = new SVMMainMenuScreen();
// Configure video settings.
MHVideoSettings displaySettings = new MHVideoSettings();
displaySettings.windowCaption = "Snow Vs. Man";
displaySettings.displayWidth = 800;
displaySettings.displayHeight = 480;
// Launch the game.
MHAndroidPlatform.run(this,
startingScreen, displaySettings);
}

9. A Common Interface For All
Platforms
Since every platform interface will have these same
functions, the game can call them without caring which
platform is performing the tasks behind the scenes.
public interface MHPlatformFactory
{
public MHBitmapImage createImage(int width, int height);
public MHBitmapImage createImage(String filename);
public MHColor createColor(int r, int g, int b, int a);
public MHSoundManager getSoundManager();
public MHFont createFont(String fontName);
public MHKeyCodes getKeyCodes();
public String getAssetsDirectory();
public MHTextFile openTextFile(String filename, Mode mode);
}

11. The Technical Stuff
for Nerds

12. The Guiding Object-Oriented
Design Principles
• Separate the aspects that vary from the aspects that stay
the same.
– The platform varies while the game code stays the same.
• Strive for loosely coupled designs between objects that
interact.
– The common interface makes it easy to swap one platform for
another without affecting the game code.
• The Principle of Least Knowledge: “Talk only to your
immediate friends.”
– Minimizes places where game depends on platform.
• The Hollywood Principle: “Don’t call us, we’ll call you.”
– The platform serves the game, never vice versa.

13. The Design Patterns That
Make This Possible
• Façade Pattern
– Makes the platform easier to use by hiding its complexity.
• Strategy Pattern
– The game engine only supports elements of the platform
on which it is running. Few wasted resources, no
extraneous conditional logic.
• Singleton Pattern
– Ensures single platform layer and provides access to it.
• Template Method Pattern
– Allows for the possibility that some functions may be the
same across platforms while others may differ.

14. • Adapter Pattern
– Each platform may have different ways of handling
graphics, input, audio, etc. Adapter makes them all
look the same from the game’s point of view.
• Observer Pattern
– Allows input handlers to use a generic set of events
independent of those generated by the platforms.
• Abstract Factory Pattern
– Handles the details of creating the correct objects
(graphics, sounds, etc.) for each platform.

15. Drawbacks To This Approach
• Platform-specific advantages may be lost.
– Example: A mouse cannot multi-touch, while touch
screens cannot right-click.
• The extra level of abstraction may degrade
runtime performance.
• The platform layer adds unavoidable
complexity to the software architecture.
Software development involves trade-offs at every step. We
must always consider the pros and cons of any solution.

16. References
Freeman, E. (2004). Head First design patterns. Sebastopol, CA:
O'Reilly.
Gamma, E. (1995). Design patterns: elements of reusable object-oriented
software. Reading, Mass.: Addison-Wesley.
Gregory, J. (2009). Game engine architecture. Wellesley, Mass.: A K
Peters.
Henson, M. (2013, July 24). Class structure of the platform layer.
Retrieved June 20, 2014, from
http://mhframework.blogspot.com/2013/07/class-structure-of-platform-
layer.html
Pressman, R. S. (2010). Software engineering: a practitioner's
approach (7th ed.). Boston, Mass.: McGraw-Hill.