Obj-C: Usage of callbacks - objective-c

Question

There is a method in the Google Maps SDK that uses callbacks like these, I guess it´s blocks? anyways I have never used these before. I just want to see if the method requestPanoramaNearCoordinate returns a valid Panorama near the coordinate i give it. How do i use the callback to see what the callback returns? If it returns a valid Panorama at the coordinate I want to return it - if not I´ll recursively call the method until a valid location is found
- (void) requestPanoramaNearCoordinate: (CLLocationCoordinate2D)coordinate
callback: (GMSPanoramaCallback)callback
Retrieves information about a panorama near the given coordinate. This
is an asynchronous request, callback will be called with the result.
CLLocationCoordinate2D ranLatLng = CLLocationCoordinate2DMake(ranLatitude, ranLongitude);
if ([panoService requestPanoramaNearCoordinate:ranLatLng callback:<#^(GMSPanorama *panorama, NSError *error)callback#>)

Answer 1

The panorama parameter contains the GMSPanorama object that the request found for you. It doesn't seem to be documented very clearly, but the usual pattern in Cocoa is that if nothing appropriate was found, the result will be nil, and the NSError object will give you a little information about why. As for re-making the request, that would look something like this I guess:
// Necessary if GMSPanoService is ill-behaved and keeps the Block
// around after calling it.
__weak GMSPanoramaService weak_panoService = panoService;
__block GMSPanoramaCallback cb;
cb = [^(GMSPanorama *panorama, NSError *error){
if( !panorama ){
// Do something with error if desired
CLLocationCoordinate2D newLatLng = /* Calc new coordinates somehow */;
[/*weak_*/panoService requestPanoramaNearCoordinate:newLatLng
callback:cb];
}
else {
// Got a panorama, do something with it
cb = nil; // Release Block
}
} copy];
[panoService requestPanoramaNearCoordinate:ranLatLng
callback:cb];

Answer 2

It's a block. You make the api call like this:
[aPanoramaService requestPanoramaNearCoordinate:aCoordiante callback:^(GMSPanorama *panorama, NSError *error) {
NSLog(#"the service returned a panorama=%# and an error=%#", panorama, error);
}];
One way to do the recursion is to build a method with a similar signature to the google method. An important point is to find a way for the recursion to stop, possibly because you get tired of searching.
- (void)findPanoramaAtLocation:(CLLocationCoordinate2D)location withCompletion:(void (^)(GMSPanorama *, NSError *))completion {
[self.panoramaService requestPanoramaNearCoordinate:location callback:^(GMSPanorama *panorama, NSError *error) {
if (error) {
completion(nil, error);
} else if (panorama) {
completion(panorama, nil);
} else {
if (/* should we continue searching? */) {
// compute a new location to try
double latitude = // compute new latitude
double longitude = // compute new longitude
CLLocationCoordinate2D newLocation = CLLocationCoordinate2DMake(latitude, longitude);
[self findPanoramaAtLocation:newLocation withCompletion:completion];
} else {
// no panorama and no error means we gave up
completion(nil, nil);
}
}
}];
}
This will run, calling the "callback" (it's a block) when either a panorama location is found, when an error occurs, or when your logic determines that it ought to stop searching.