Is this a good way to do a game loop for an iPhone game?

Question

Hi all,

I'm new to iPhone dev, but trying to build a 2D game. I was following a book, but the game loop it created basically said:

function gameLoop
    update()
    render()
    sleep(1/30th second)
    gameLoop

The reasoning was that this would run at 30fps. However, this seemed a little mental, because if my frame took 1/30th second, then it would run at 15fps (since it'll spend as much time sleeping as updating).

So, I did some digging and found the CADisplayLink class which would sync calls to my gameLoop function to the refresh rate (or a fraction of it). I can't find many samples of it, so I'm posting here for a code review :-) It seems to work as expected, and it includes passing the elapsed (frame) time into the Update method so my logic can be framerate-independant (however I can't actually find in the docs what CADisplayLink would do if my frame took more than its allowed time to run - I'm hoping it just does its best to catch up, and doesn't crash!).

//
//  GameAppDelegate.m
//
//  Created by Danny Tuppeny on 10/03/2010.
//  Copyright Danny Tuppeny 2010. All rights reserved.
//

#import "GameAppDelegate.h"
#import "GameViewController.h"
#import "GameStates/gsSplash.h"

@implementation GameAppDelegate

@synthesize window;
@synthesize viewController;

- (void) applicationDidFinishLaunching:(UIApplication *)application
{
 // Create an instance of the first GameState (Splash Screen)
 [self doStateChange:[gsSplash class]];

 // Set up the game loop
 displayLink = [CADisplayLink displayLinkWithTarget:self selector:@selector(gameLoop)];
 [displayLink setFrameInterval:2];
 [displayLink addToRunLoop:[NSRunLoop currentRunLoop] forMode:NSDefaultRunLoopMode];
}

- (void) gameLoop
{
 // Calculate how long has passed since the previous frame
 CFTimeInterval currentFrameTime = [displayLink timestamp];
 CFTimeInterval elapsed = 0;

 // For the first frame, we want to pass 0 (since we haven't elapsed any time), so only
 // calculate this in the case where we're not the first frame
 if (lastFrameTime != 0)
 {
  elapsed = currentFrameTime - lastFrameTime;
 }

 // Keep track of this frames time (so we can calculate this next time)
 lastFrameTime = currentFrameTime;

 NSLog([NSString stringWithFormat:@"%f", elapsed]);

 // Call update, passing the elapsed time in
 [((GameState*)viewController.view) Update:elapsed];
}

- (void) doStateChange:(Class)state
{
 // Remove the previous GameState
 if (viewController.view != nil)
 {
  [viewController.view removeFromSuperview];
  [viewController.view release];
 }

 // Create the new GameState
 viewController.view = [[state alloc] initWithFrame:CGRectMake(0, 0, IPHONE_WIDTH, IPHONE_HEIGHT) andManager:self];

 // Now set as visible
 [window addSubview:viewController.view];
 [window makeKeyAndVisible];
}

- (void) dealloc
{
 [viewController release];
    [window release];
    [super dealloc];
}

@end

Any feedback would be appreciated :-)

PS. Bonus points if you can tell me why all the books use "viewController.view" but for everything else seem to use "[object name]" format. Why not [viewController view]?

Answer 1

You have Cocos2D listed as a tag in your question but you're not actually using any Cocos2D code. Have you considered doing a Cocos2D implementation for your games? It will save you some unneeded hassle.

As for your syntax question [myObject view] is used for calling methods on myObject while myObject.view is used for setting/getting instance variables exposed as properties. I don't recall if you can retrieve instance variables using [myObject view] as well but if that works then I guess the only difference between the two is the syntax and you could use both methods to retrieve instance variables.

Hope some of that rambling is useful to you.

Answer 2

From many GL example by Apple, I think you should use a timer.

animationTimer = [NSTimer scheduledTimerWithTimeInterval:(1.0/60.0)
                                                  target:self
                                                selector:@selector(updateAndRender)
                                                userInfo:nil
                                                 repeats:TRUE];