nsarray switching image texture randomly everyone one second - iphone

Question

I have an array that has 2 balloon textures,one green and one red.I have 12 green balloon object at the start of the app.Basically I want one random green balloon to turn red every one second.If anyone can help it would be appreciated.
init method
balloonTextures = [NSMutableArray array];
[balloonTextures addObject:[SPTexture textureWithContentsOfFile:#"greenballoon.png"]];
[balloonTextures addObject:[SPTexture textureWithContentsOfFile:#"redballoon.png"]];
[balloonTextures retain];
playFieldSprite = [SPSprite sprite];
[self addChild:playFieldSprite];
[self addBalloon];
add ballon method
-(void)addBalloon
{
for(int i = 0; i < 12; i++)
{
SPImage *image = [SPImage imageWithTexture:[balloonTextures objectAtIndex:0]];
image.x = 40*i;
image.y = 10 ;
[playFieldSprite addChild:image];
}
}

Answer 1

For getting random number:
#include <stdlib.h>
int r = arc4random() % 12;
Use a NSTimer to be called every second: How do I use NSTimer?

Answer 2

You could use CADisplayLink instead of NSTimer. The reason is that CADisplayLink is synchronized its drawing to the refresh rate of the display.
How to setup CADisplayLink:
id displayLink = [NSClassFromString(#"CADisplayLink") displayLinkWithTarget:self selector:#selector(changeBalloonTexture)];
[displayLink setFrameInterval:60]; // refresh rate is 60fps, 1=60fps, 60=1fps
[displayLink addToRunLoop:[NSRunLoop currentRunLoop] forMode:NSRunLoopCommonModes];
How to check whether CADisplayLink is supported or not on the particular device:
// code from cocos2d
displayLinkSupported = FALSE;
NSString *reqSysVer = #"3.1";
NSString *currSysVer = [[UIDevice currentDevice] systemVersion];
if ([currSysVer compare:reqSysVer options:NSNumericSearch] != NSOrderedAscending)
displayLinkSupported = TRUE;

Answer 3

For this you'll need to keep track of Green Balloons & change one of them to red randomly.
Put all your balloon sprites in one NSArray say balloonArray & schedule a timer to run a method every second. In that method, iterate the balloonArray, & collect all green balloons into another array say greenBalloonsArray.
Then use arch4random() method to generate a random number (index) from 0 to the length of greenBalloonsArray. Use random number as an index of newly created greenBalloonsArray to fetch the balloonSprite and change it's texture to red.
// Sudo Code
// Add all objects to one array
NSMutableArray *balloonsArray = [[NSMutableArray alloc] initWithObjects: ..... ];
- (void) timerMethod
{
__block NSMutableArray *greenBalloonsArray = [[NSMutableArray alloc] init];
[balloonsArray enumerateObjectsUsingBlock:^(BalloonSprite *object, NSUInteger idx, BOOL *stop) {
if (object.isGreen)
[greenBalloonsArray addObject:object];
}];
int index = arc4random() % greenBalloonsArray.count;
GreenBallonArray * greenBalloon = [greenBalloonsArray objectAtIndex:index];
[greenBallon turnToRed];
}