Duplicate UIViewControllers on UINavigationController stack after application wakes - ios

Question

I just learnt IOS programming and am writing my first iPhone app.
My app provides info on an MKMapView, the view controller of which is the root view controller of a UINavigationController. If the mobile signal is bad i use mapViewDidFailLoadingMap:withError: to make the app push one of two different view controllers onto the navigation controller stack depending on what the user is doing. Code follows:
- (void)mapViewDidFailLoadingMap:(MKMapView *)mapView withError:(NSError *)error
{
NSLog(#"mapViewDidFailLoadingMap: %#", [error localizedDescription]);
[aiView stopAnimating];
if (mapTypeInt == 0) {
NSString *message =
[NSString stringWithFormat:#"Your signal is not currently
strong enough to download a map. Switching to table view."];
UIAlertView *av = [[UIAlertView alloc] initWithTitle:#"Maps Unavailable"
message:message
delegate:nil
cancelButtonTitle:#"OK"
otherButtonTitles:nil];
[av show];
if (currentMode == #"retracingSteps")
{
RetraceViewController *rvc =
[[RetraceViewController alloc] initWithNibName:#"RetraceViewController" bundle:nil];
[[self navigationController] pushViewController:rvc animated:YES];
}
else{
TripTableViewController *ttvc = [[TripTableViewController alloc] init];
[[self navigationController] pushViewController:ttvc animated:YES];
}
}
else{
[self setMapType:0];
NSString *message = [NSString stringWithFormat:
#"Your signal is not currently strong enough to download a satellite map.
Switching to Standard Map view."];
UIAlertView *av = [[UIAlertView alloc] initWithTitle:#"Can't Use Satellite Maps"
message:message
delegate:nil
cancelButtonTitle:#"OK"
otherButtonTitles:nil];
[av show];
}
}
Yesterday I tested the poor mobile signal in a rural valley and the correct second view controller was pushed onto the stack. What I then noticed when I locked the phone and rechecked the app after a few minutes was that on waking, the root view controller was displayed quickly followed by the view controller I expected. In effect what this did was to push an identical copy of the second view controller onto the stack. I discovered this when I had to tap the back button half a dozen times to get back to the root view controller.
What I would like the app to do on wake up is to immediately display the view controller that was live when the phone is locked rather than the root view controller. I do not know how to do this.

Answer 1

This is probably happened because even you lock the screen your app still receiving locations update (location services can run code in a real background mode), also when you push a view controller the previous one in the stack still exist and still receiving location update and doing everything has been designed to do, so even you push another controller, if something happen that for the implemented logic has to push another controller, this view controller has access to the navigation controller so has just to add to the stack another controller and push it (the navigation controller still the same for all the VC in the stack)
So what you have to do is stop to handle this cases when you push another controller and restart to do that when you pop back the controller

Answer 2

This bug was caused by the handler mapViewDidFailLoadingMap:withError: being called multiple times before control passed to the pushed view controller. I fixed the bug by making three changes to the handler method:
1) I removed the UIAlertView show and moved it to the pushed view controller;
2) I used a flag (initialized in viewDidAppear:) to return from the handler without executing the push if the handler was being called a second time;
3) I checked for the pushed view controller's existence before allocing it.
When all three of these changes were made, transitions between view controllers happened correctly. Code follows:
- (void)mapViewDidFailLoadingMap:(MKMapView *)mapView withError:(NSError *)error
{
[aiView stopAnimating];
if (mapViewFailed > 0) {
mapViewFailed ++;
return;
}
if (mapTypeInt == 0) {
[map setUserTrackingMode:MKUserTrackingModeNone];
[self removeAsNoticationsObserver];
if ([currentMode isEqualToString:#"retracingSteps"])
{
if (!rvc) {
rvc = [[RetraceViewController alloc] initWithNibName:#"RetraceViewController" bundle:nil];
}
[[self navigationController] pushViewController:rvc animated:YES];
}
else{
if (!ttvc) {
ttvc = [[TripTableViewController alloc] init];
}
[[self navigationController] pushViewController:ttvc animated:YES];
}
mapViewFailed ++;
}
else{
[self setMapType:0];
NSString *message = [NSString stringWithFormat:#"Your signal is not currently strong enough to download a satellite map. Switching to Standard Map view."];
UIAlertView *av = [[UIAlertView alloc] initWithTitle:#"Can't Use Satellite Maps"
message:message
delegate:nil
cancelButtonTitle:#"OK"
otherButtonTitles:nil];
[av show];
}
}
No doubt a more elegant solution is available which I don't know about yet, being an iOS newb.