Why does popViewController only work every other time

Question

I am totally stumped, here's the situation:

My app uses the Core Location framework to get the current location of the user and then pings my server at TrailBehind for interesting places nearby and displays them as a list. No problems.

To conserve batteries, I turn off the GPS service after I get my data from the server. If the user moves around while using the app and wants a new list he clicks "Refresh" on the navigation controller and the CLLocation service is again activated, a new batch of data is retrieved from the server and the table is redrawn.

While the app is grabbing data from my server I load a loading screen with a spinning globe that says "Loading, please wait" and I hide the navigation bar so they don't hit "back".

So, the initial data grab from the server goes flawlessly.

The FIRST time I hit refresh all the code executes to get a new location, ping the server again for a new list of data and updates the cells. However, instead of loading the table view as it should it restores the navigation controller bar for the table view but still shows my loading view in the main window. This is only true on the device, everything works totally fine in the simulator.

The SECOND time I hit refresh the function works normally.

The THIRD time I hit refresh it fails as above.

The FOURTH time I hit refresh it works normally.

The FIFTH time I hit refresh it fails as above.

etc etc, even refreshes succeed and odd refreshes fail. I stepped over all my code line by line and everything seems to be executing normally. I actually continued stepping over the core instructions and after a huge amount of clicking "step over" I found that the table view DOES actually display on the screen at some point in CFRunLoopRunSpecific, but I then clicked "continue" and my loading view took over the screen.

I am absolutely baffled. Please help!! Many thanks in advance for your insight.

Video of the strange behavior:

Relevant Code:

RootViewControllerMethods (This is the base view for this TableView project)

- (void)viewDidLoad {
    //Start the Current Location controller as soon as the program starts.  The Controller calls delegate methods
    //that will update the list and refresh
    [MyCLController sharedInstance].delegate = self;
    [[MyCLController sharedInstance].locationManager startUpdatingLocation];
    lv = [[LoadingViewController alloc] initWithNibName:@"Loading" bundle:nil];
    [self.navigationController pushViewController:lv animated:YES];
    [super viewDidLoad];
}



- (void)updateClicked {
    //When the location is successfully updated the UpdateCells method will stop the CL manager from updating, so when we want to update the location
    //all we have to do is start it up again.  I hope.
    [[MyCLController sharedInstance].locationManager startUpdatingLocation];
    [self.navigationController pushViewController:lv animated:YES];
    //LV is a class object which is of type UIViewController and contains my spinning globe/loading view.
}



-(void)updateCells {
    //When the Core Location controller has updated its location it calls this metod.  The method sends a request for a JSON dictionary
    //to trailbehind and stores the response in the class variable jsonArray.  reloadData is then called which causes the table to
    //re-initialize the table with the new data in jsonArray and display it on the screen.  

    [[MyCLController sharedInstance].locationManager stopUpdatingLocation];

    if(self.navigationController.visibleViewController != self) {
        self.urlString = [NSString stringWithFormat:@"http://www.trailbehind.com/iphone/nodes/%@/%@/2/10",self.lat,self.lon];
        NSURL *jsonURL = [NSURL URLWithString:self.urlString];
        NSString *jsonData = [[NSString alloc] initWithContentsOfURL:jsonURL];
        NSLog(@"JsonData = %@ \n", jsonURL);
        self.jsonArray = [jsonData JSONValue];
        [self.tableView reloadData];
        [self.navigationController popToRootViewControllerAnimated:YES];
        [jsonData release];
    }
}

CLController Methods: Basically just sends all the data straight back to the RootViewController

// Called when the location is updated
- (void)locationManager:(CLLocationManager *)manager
    didUpdateToLocation:(CLLocation *)newLocation
           fromLocation:(CLLocation *)oldLocation
{
    NSLog(@"New Location: %@ \n", newLocation);
    NSLog(@"Old Location: %@ \n", oldLocation);
    @synchronized(self) {
        NSNumber *lat = [[[NSNumber alloc] init] autorelease];
        NSNumber *lon = [[[NSNumber alloc] init] autorelease];
        lat = [NSNumber numberWithFloat:newLocation.coordinate.latitude];
        lon = [NSNumber numberWithFloat:newLocation.coordinate.longitude];
        [self.delegate noteLat:lat];
        [self.delegate noteLon:lon];
        [self.delegate noteNewLocation:newLocation];
        [self.delegate updateCells];
    }
}

Answer 1

The first thought is that you may not want to send startUpdatingLocation to the CLLocationManager until after you've pushed your loading view. Often the first -locationManager:didUpdateToLocation:fromLocation: message will appear instantly with cached GPS data. This only matters if you're acting on every message and not filtering the GPS data as shown in your sample code here. However, this would not cause the situation you've described - it would cause the loading screen to get stuck.

I've experienced similarly weird behavior like this in a different situation where I was trying to pop to the root view controller when switching to a different tab and the call wasn't being made in the correct place. I believe the popToRootViewController was being called twice for me. My suspicion is that your loading view is either being pushed twice or popped twice.

I recommend implementing -viewWillAppear:, -viewDidAppear:, -viewWillDisappear: and -viewDidDisappear: with minimal logging in your LoadingViewController.

- (void)viewWillAppear:(BOOL)animated {
    NSLog(@"[%@ viewWillAppear:%d]", [self class], animated);
    [super viewWillAppear:animated];
}

- (void)viewDidAppear:(BOOL)animated {
    NSLog(@"[%@ viewDidAppear:%d]", [self class], animated);
    [super viewDidAppear:animated];
}

- (void)viewWillDisappear:(BOOL)animated {
    NSLog(@"[%@ viewWillDisappear:%d]", [self class], animated);
    [super viewWillDisappear:animated];
}

- (void)viewDidDisappear:(BOOL)animated {
    NSLog(@"[%@ viewDidDisappear:%d]", [self class], animated);
    [super viewDidDisappear:animated];
}

Then, run a test on your device to see if they are always being sent to your view controller and how often. You might add some logging to -updateClicked to reveal double-taps.

Another thought, while your @synchronized block is a good idea, it will only hold off other threads from executing those statements until the first thread exits the block. I suggest moving the -stopUpdatingLocation message to be the first statement inside that @synchronized block. That way, once you decide to act on some new GPS data you immediately tell CLLocationManager to stop sending new data.