Multiple Images for iOS app's Background - iphone

Question

I have seen different apps that the image will change each time the app is opened on the view background. How is this accomplished?

Answer 1

It sounds like what you are seeing is the cached screenshot the iOS system is making of your app just before it puts it into the background.
This is handled automatically, and you do have the opportunity to intercept this.
Check out this answer, you can put an image over your app just as it's entering the background, this will be cached and used to relaunch the app.

Answer 2

add background_0.png, background_1.png, background_2.png, background_3.png etc to your project.
#define max_image_number 3
Add the following to your viewDidLoad method:
-(void)viewDidLoad {
[super viewDidLoad];
[self updatedBackgroundImage];
[[NSNotificationCenter defaultCenter] addObserver:self selector:#selector(updateBackgroundImage) name:UIApplicationWillEnterForegroundNotification object:nil];
}
-(void) updateBackgroundImage{
NSUserDefaults *defaults = [NSUserDefaults standardUserDefaults];
if (![defaults integerForKey:#"imageNumber"]) {
[defaults setInteger:0 forKey:#"imageNumber"];
}
int i = [defaults integerForKey:#"imageNumber"];
myUIimageView.image = [UIImage imageNamed:[NSString stringWithFormat:#"background_%d.png",i]];
i++;
if (i > max_image_number) {
[defaults setInteger:0 forKey:#"imageNumber"];
}else{
[defaults setInteger:i forKey:#"imageNumber"];
}
...
}
The above will update the image when the viewController is first loaded and also when it returns from running in the background. I assume this is what was missing for you :).
Remember to stop listening for the notification in the viewDidUnload method and you should be set.

Answer 3

Looking at the screenshots in the App Store of the application you mention, it looks like there's just a UIImageView at the back of the view hierarchy for the main menu. You can change this by assigning a UIImage object to its image property.

Answer 4

Suppose you want to change one or more view's background images, depending on the application launch.
Save an int x = 0 into NSUserDefaults. On each application launch increment it by 1. When you are to present a view, check that int and set a background image like this:
switch(x%3){// %3 just to make it a bit random
case 0://set this image;
break;
case 1://set that image
break;
//and so on
}