running several NSTimer instances in various queues - iphone

Question

In the iPhone App am coding, I need to do several tasks in parallel :
PART 1 : All the time (even if the App is not active currently):
Fetch some data from a remote DB and persist it in local Sqlite
To do this, am firing a NSTimer in a separate queue in AppDelegate like this :
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
...
...
self.queueForDbFetch = dispatch_queue_create("queueForDbFetch", NULL);
self.queueForDbFetchTimer = dispatch_queue_create("queueForDbFetchTimer", NULL);
[[NSNotificationCenter defaultCenter] addObserver:self
selector:#selector(getDbData:) name:kNotif_GetDbData object:nil];
dispatch_async(self.queueForDbFetchTimer, ^(void) {
self.timerDbNotifier = [NSTimer scheduledTimerWithTimeInterval:60.0
target:self selector:#selector(scheduleNotificationToFetchDbData)
userInfo:nil repeats:YES];
[[NSRunLoop currentRunLoop] addTimer:self.timerDbNotifier forMode:NSDefaultRunLoopMode];
});
...
...
}
PART 2 :
And then, I need to asynchronously update the UI with the fetched data (which is from the local sqlite DB), which an doing with queues & timers (similar to the above) like this in a UIViewController class :
-(void) initializeThisView {
// Make sure the queues are created only once
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
self.queueForUiRefresh = dispatch_queue_create("queueForUiRefresh", NULL);
self.queueForUiRefreshTimer = dispatch_queue_create("queueForUiRefreshTimer", NULL);
});
[self scheduleUiDataRefresher];
}
-(void) scheduleUiDataRefresher {
dispatch_async(self.queueForUiRefreshTimer, ^(void) {
self.timerUiDataRefresh = [NSTimer scheduledTimerWithTimeInterval:60.0
target:self selector:#selector(loadUiData)
userInfo:nil repeats:YES];
[[NSRunLoop currentRunLoop] addTimer:self.timerUiDataRefresh forMode:NSDefaultRunLoopMode];
});
}
-(void) loadUiData {
dispatch_async(self.queueForUiRefresh, ^(void) {
[self refreshWithUiData:dict];
});
}
THE ISSUE :
The NSTimer instances (in both Part 1 and part 2) get fired once, and that's it. They don't repeat.
1. will creating NSTimer to repeat in the main queue block other user interaction with the App?
2. Is there any issue (or better way) in my structuring of the activities?

Answer 1

Don't create the timers in the dispatch_async() call. As soon as the block you pass to dispatch_async() has finished running (and it finishes directly after creating the timer), all data belonging to it is freed. I'm surprised it doesn't crash. Also, when you use scheduledTimerWithTimeInterval:target:selector:userInfo:repeats, the timer will already be scheduled to the main runloop. The call to NSRunLoop:addTimer: is not necessary, it will either have no effect or you will have a conflict in scheduling the timer.
To answer your questions:
Create the timers without dispatch_async(), just call NSTimer:scheduledTimerWithTimeInterval: directly. Do not use NSRunLoop:addTimer (unless you know exactly why you want it that way). Then in the selector that is being called by your timers, use dispatch_async() to fire off your asynchronous tasks.
However, if you are sure that these tasks do not take long, you might as well avoid using dispatch_async() altogether.