Reloading json data with nstimer goes crazy after a minute - ios

Question

My log window goes crazy, constantly reloading after a minute. Did I use NSTimer at the end correctly? Is performSelectorOnMainThread correct use? Thanks
-(void)URL
{
dispatch_async(myQueue, ^{
NSString* myURL= #"https://url.json";
NSData* data = [NSData dataWithContentsOfURL: [NSURL URLWithString:myURL]];
NSLog(#"url: %#", myURL);
if ((!data) || (data==nil))//v1.2
{
NSLog(#"url: loading ERROR");
[NSTimer scheduledTimerWithTimeInterval:5 target:self selector:#selector(URL) userInfo:nil repeats:YES];
}else
{
[self performSelectorOnMainThread:#selector(fetchedData:) withObject:data waitUntilDone:YES];
}
});
}
- (void)fetchedData:(NSData *)responseData {
NSLog=#"fetch";
NSError* error;
NSDictionary* json = [NSJSONSerialization JSONObjectWithData:responseData //1
options:kNilOptions
error:&error];
NSNumber* spot= [json objectForKey:#"amount"];
float spotfloat = [spot floatValue];
timer = [NSTimer scheduledTimerWithTimeInterval:15 target:self selector:#selector(URL) userInfo:nil repeats:YES];
}

Answer 1

set the repeats to NO or set the actual maximum timer. it's an infinite loop if you don't set the maximum time.
[NSTimer scheduledTimerWithTimeInterval:15 target:self selector:#selector(URL) userInfo:nil repeats:NO];
- (void)fetchedData:(NSData *)responseData {
NSLog=#"fetch";
NSError* error;
NSDictionary* json = [NSJSONSerialization JSONObjectWithData:responseData //1
options:kNilOptions
error:&error];
NSNumber* spot= [json objectForKey:#"amount"];
float spotfloat = [spot floatValue];
if (timer < 60)
{
timer = [NSTimer scheduledTimerWithTimeInterval:15 target:self selector:#selector(URL) userInfo:nil repeats:YES];
}
else
{
[timer invalidate];
}
}

Answer 2

A couple of observations:
You are repeatedly creating new repeating timers, but never calling invalidate on the old ones and as a result, you will undoubtedly end up with a cascade of timer events as the old ones will keep firing. (A repeating timer will continue firing until you explicitly call invalidate. Simply nil-ing or replacing the object in your class property/ivar is insufficient.) If you want to replace a repeating timer, make sure to invalidate the old one first (otherwise the old will keep firing).
Generally, though, you'd either create a repeating timer once and let it keep firing, or you'd create non-repeating timer, and at the end of the method, schedule another non-repeating timer. Given that you're dealing with network requests that may take an indeterminate amount of time (i.e. it could still be trying the previous request by the time the next repeating timer fires), I'd suggest using non-repeating timers that schedule the next one at the end of the method. Or, given the hassles in creating timers in background queues, just use dispatch_after.
BTW, timers need a run loop to function properly, so your attempt to create a timer in myQueue if the request failed is unlikely to succeed. If you wanted to schedule a timer from the background queue, the easiest way to do this would be to create the timer with timerWithTimeInterval and then manually add it to the main run loop. (The alternative, of creating a new background NSThread and scheduling your timer on that, is overkill for such a simple problem.)
As an aside, I'd be inclined to run fetchedData on the background queue, too. Just dispatch the UI and/or model update back to the main queue. That way you minimize how much you're doing on the main queue.
I'm unclear as to how you're determining when to stop this process. Maybe you've concluded you don't need that at this point, but I'd suggest you include some cancellation logic, even if you don't avail yourself of it at this point.
You're using a shorter delay if you encounter network problem. That might be a dangerous solution because if your server failed because it was overwhelmed with requests coming in every 15 seconds from too many users, having clients start sending requests every 5 seconds might only make the situation worse.
Frankly, in an ideal scenario, you'd look at the exact cause of the error, and decide the correct behavior at that point. For example, if a request failed because the device doesn't have Internet connectivity, use Reachability to determine when the network is restored, rather than blindly trying every five seconds.
You're sending requests ever 5-15 seconds. If you really need that sort of interactivity, you might consider a different architecture (e.g. sockets). That's beyond the scope of this question, but something for you to research/consider.
Anyway, you might consider something like:
- (void)viewDidLoad
{
[super viewDidLoad];
self.myQueue = ...
[self schedulePollWithDelay:0];
}
- (void)viewDidDisappear:(BOOL)animated
{
[super viewDidDisappear:animated];
self.stopPolling = YES;
}
- (void)schedulePollWithDelay:(CGFloat)delay
{
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(delay * NSEC_PER_SEC)), self.myQueue, ^{
[self retrieveData];
});
}
-(void)retrieveData
{
if (self.stopPolling)
return;
NSString* myURL= #"https://url.json";
NSData* data = [NSData dataWithContentsOfURL: [NSURL URLWithString:myURL]];
if (!data) {
NSLog(#"url: loading ERROR");
} else {
[self processFetchedData:data];
}
[self schedulePollWithDelay:15];
}
- (void)processFetchedData:(NSData *)responseData {
NSError* error;
NSDictionary* json = [NSJSONSerialization JSONObjectWithData:responseData //1
options:kNilOptions
error:&error];
NSNumber* spot = [json objectForKey:#"amount"];
float spotfloat = [spot floatValue];
dispatch_sync(dispatch_get_main_queue(), ^{
// update your UI or model here
});
}