Background mode with NSTimers and server calls - ios

Question

I'm working on an sdk that uses NSTimer to make a server call when it expires at x mins and the server call resets the timer back to x mins after doing some work. The problem I'm having is that my timer stops running when my app is in the background and resumes immediately when my app is back in the foreground. How do I get this to work?
//initialize self.myTimer somewhere in my code
//called once somewhere in my code-->[self resetTimer:self.myTimer expiry:30];
- (void)resetTimer:(NSTimer *)timer expiry:(float)seconds {
[timer invalidate];
NSTimer *newTimer = [NSTimer timerWithTimeInterval:seconds
target:self
selector:#selector(updateTimer:)
userInfo:nil
repeats:NO];
[[NSRunLoop mainRunLoop] addTimer:newTimer forMode:NSRunLoopCommonModes];
self.myTimer = newTimer;
}
- (void) updateTimer:(NSTimer *)timer {
[timer invalidate];
dispatch_queue_t queue = dispatch_queue_create("update", NULL);
dispatch_async(queue, ^{
[self serverCall];
});
}
-(void)serverCall{
//make server call and do some work
[self resetTimer:self.myTimer expiry:30];
}

Answer 1

The timer won't continue to fire when the app is in the background. If you're willing to let the OS set the pace of the server calls, you can accomplish this by consulting the section "Fetching Small Amounts of Content Opportunistically" in this background execution doc.
The gist is to set your app's UIBackgroundModes key == fetch in info.plist.
When iOS decides to grant your app some cycles, you can run a short task in:
- (void)application:(UIApplication *)application performFetchWithCompletionHandler:(void (^)(UIBackgroundFetchResult result))completionHandler {
[self serverCall]; // BUT! read on
}
But you'll need to refactor your serverCall to tell the caller when its done. Otherwise, you won't know when to invoke the completionHandler:.
- (void)application:(UIApplication *)application performFetchWithCompletionHandler:(void (^)(UIBackgroundFetchResult result))completionHandler {
[self newServerCallWithCompletion:^(BOOL gotNewData) {
UIBackgroundFetchResult result = (gotNewData)? UIBackgroundFetchResultNewData : UIBackgroundFetchResultNoData;
completionHandler(result);
}];
}
See the docs for more options on UIBackgroundFetchResult.
With this, iOS will set the pace of requests, so you won't be able to make them more often than when you're app is given the chance. By persisting the time of the last request, you can make requests less often. Just check if the interval since the last request is <= to some desired max frequency. If it is, just call the completionHandler right away with "no data".