问题
I'm relatively new to Objective-C and really don't know much about it yet, so I apologise for what is probably a really amateurish question.
I'm trying to get the magnetic heading from CLHeading and CLLocationDirection. However I'm getting compile errors for this line of code:
locationLabel.text = [[[location course] magneticHeading] stringValue];
The errors are:
warning: invalid receiver type 'CLLocationDirection'
error: cannot convert to a pointer type
I don't really understand what I'm doing wrong here. Please help!
回答1:
Here are the steps needed to use the compass.
1) check the availability: if the headingAvailable property of the location manager is YES, then you can use the compass.
2) use the location manager method -(void) startUpdatingHeading to begin receiving the information you are searching for
3) actually retrieve this information using the delegate method (do not forget to set yourself as the delegate)
- (void)locationManager:(CLLocationManager *)manager didUpdateHeading:(CLHeading *)newHeading
Hope this helps.
回答2:
magneticHeading is of CLLocationDirection type, which is simply a typedef for the primitive data type "double". In your example you are trying to send a message to something that is not an object ! You should simply format the double like so:
locationLabel.text = [NSString stringWithFormat:@"Heading %.3f", [[location course] magneticHeading]];
回答3:
How are you allocating and initializing location? Make sure location is defined as a (CLLocationDirection *) and not just a (CLLocationDirection).
来源：https://stackoverflow.com/questions/1043740/iphone-3-0-compass-how-to-get-a-heading