iOS: Background / Foreground Events - ios

Question

I´m working on an application in which, when it is pushed in background via the home button, a timer should start and when the application gets back to foreground and the timer has passed a certain amount of time, something should be executed.
My questions are
How do I handle the events when my app goes to
background/foreground?
Is there a special method or an other technique?
Thanks a lot.

Answer 1

A possible implementation could look like:
#define YOUR_TIME_INTERVAL 60*60*5 //i.e. 5 hours
- (void)applicationDidEnterBackground:(UIApplication *)application
{
//... your oder code goes here
NSNumber *timeAppClosed = [NSNumber numberWithDouble:[[NSDate date] timeIntervalSince1970]];
NSUserDefaults *defaults = [NSUserDefaults standardUserDefaults];
[defaults timeAppClosed forKey:#"time.app.closed"];
[defaults synchronize];
}
and
- (void)applicationWillEnterForeground:(UIApplication *)application
{
NSNumber *timeAppClosed = [[NSUserDefaults standardUserDefaults] valueForKey:#"time.app.closed"];
if(timeAppClosed == nil)
{
//No time was saved before so it is the first time the user
//opens the app
}
else if([[NSDate date] timeIntervalSinceDate:[NSDate dateWithTimeIntervalSince1970:[timeAppClosed doubleValue]]] > YOUR_TIME_INTERVAL)
{
//Place your code here
}
}

Answer 2

In the appDelegate of the app you have some delegate methods which you can implement.
You can check out the UIApplicationDelegate protocol to which you AppDelegate should conform.
When the app is pushed in the background the function applicationDidEnterBackground: will be called. When entering the foreground applicationWillEnterForeground: is called.
Better not use a timer, but store a NSDate reference in the applicationDidEnterBackground: method. When your app is entering the foreground you can calculate the timeDifference using the stored NSDate using the
- (NSTimeInterval)timeIntervalSinceDate:(NSDate *)anotherDate
function.