Test a GCD block as the async process is taking some time - objective-c

Question

I am developing an app that contacts a RESTful server to get some data and then with the returned JSON response to display that data.
Using UniRest calls and all is working well. The main call is 'runUnirestRequest'
The uni rest call is an async GCD dispatch call. My problem is that because I am testing locally the call is so quick I can't see the activity indicator rolling. It simply disappears before I can see it.
The GCD block occur within the viewController viewDidLoad call.
What I need to achieve: Have the async unirest call take several seconds to simulate a server response that is slow (Dont want to actually stop the iOS app in its tracks).
Please excuse any coding errors/bad habits, only been doing objective c for a week but am happy for any additional constructive crit. :)
I have tried
sleep(5); // But bad idea as far as I can see.
Also tried
[NSThread sleepForTimeInterval:5.0]; // but this doesn't seem to do anything.
viewDidLoad
- (void)viewDidLoad
{
[super viewDidLoad];
[self createActivityIndicator];
NSLog(#"viewDidLoad");
NSLog(#"viewDidLoad->thread: %#", [NSThread currentThread]);
[messageLabel setText:#""];
unirestQueue = dispatch_queue_create("com.simpleweb.pbs.dayDataUnirestRequest", NULL);
// Do any additional setup after loading the view from its nib.
daySalesFigures = [[PBSDaySales alloc] init];
responseVal = [[HttpJsonResponse alloc] init];
// Use Grand Central Dispatch to run async task to server
dispatch_async(unirestQueue, ^{
[self runUnirestRequest:self.requestUrl];
});
dispatch_after(unirestQueue, dispatch_get_main_queue(), ^(void){
[activityIndicator stopAnimating];
});
}
runUniRestRequest function
- (void) runUnirestRequest:(NSString*)urlToGet
{
[NSThread sleepForTimeInterval:5.0];
NSLog(#"runUnirestRequest called");
HttpJsonResponse* response = [[Unirest get:^(SimpleRequest* request) {
[request setUrl:#"http://x.x.x.x:9000/Sales/Day/2013-02-14"];
}] asString];
NSString *jsonStr = [response body];
SBJsonParser *jsonParser = [SBJsonParser new];
id response2 = [jsonParser objectWithString:jsonStr];
[self deserializeJsonPacket:(NSDictionary*)response2];
}

Answer 1

dispatch_after's first parameter is time. You are passing in unirestQueue, which is dispatch_queue_t queue according to
unirestQueue = dispatch_queue_create("com.simpleweb.pbs.dayDataUnirestRequest", NULL);
proper code for dispatch_after, i.e. performing block after some delay, is like this:
double delayInSeconds = 2.0;
dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, (int64_t)(delayInSeconds * NSEC_PER_SEC));
dispatch_after(popTime, dispatch_get_main_queue(), ^(void){
// Do whatever you want
});
Edit: Oh, I probably see what you are trying to accomplish :-) You thought the dispatch_after means "do something after this queue" right? Nope, it's "do something after some time"
Edit 2: You can use code like below to do something time consuming in background and update UI when its done
// Start block on background queue so the main thread is not frozen
// which prevents apps UI freeze
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
// Do something taking a long time in background
// Here we just freeze current (background) thread for 5s
[NSThread sleepForTimeInterval:5.0];
// Everything in background thread is done
// Call another block on main thread to do UI stuff
dispatch_async(dispatch_get_main_queue(), ^{
// Here you are in the main thread again
// You can do whatever you want
// This example just stops UIActivityIndicatorView
[activityIndicator stopAnimating];
});
});
Edit 3: I recommend this great article about GCD at raywenderlich.com for more detailed info