chapter 20

f_mtz · #1 (**permalink**) October 17th, 2013, 07:58 PM

'm newbie c Objective programmer, i'm trying to built location based service application, but i have no idea what happen here, could anyone help?? i'm working on xcode 4.6 and iphone 6 SDK

Code:

#import "ViewController.h"

@interface ViewController ()


@end

@implementation ViewController

- (void)viewDidLoad

{
    self.lm = [[CLLocationManager alloc]init];
    lm.delegate = self;
    lm.desiredAccuracy = kCLLocationAccuracyBest;
    lm.distanceFilter = kCLDistanceFilterNone;
    [lm startUpdatingLocation];
    [super viewDidLoad];
   // Do any additional setup after loading the view, typically from a nib.
}

    -(void) locationManagerDidResumeLocationUpdates:(CLLocationManager *)manager
                               didUpdateToLocation : (CLLocation *) newLocation
                                      fromLocation : (CLLocation *) oldLocation {
    
    //display latitude
        NSString *lat = [[NSString alloc]initWithFormat : @"%f", newLocation.coordinate.latitude];
        latitudeTextField.text = lat;
        
    //display longitude
        NSString *lng = [[NSString alloc]initWithFormat : @"%f", newLocation.coordinate.longitude];
        longitudeTextField.text = lng;
        
    //display accuracy
        NSString *acc = [[NSString alloc]initWithFormat : @"%f", newLocation.horizontalAccuracy];
        accuracyTextField.text = acc;
        
        
        [acc release];
        [lat release];
        [lng release];
        
    }

-(void) locationManagerDidResumeLocationUpdates:(CLLocationManager *)manager
                              didFailWithError : (NSError *) error {
    NSString *msg = [[NSString alloc]initWithTitle : @"Error"
                                           message : msg
                                          delegate : nil
                                 cancelButtonTitle : @"Done"
                                 otherButtonTitles : nil];
    
    [alert show]; //use of undeclared identifier 'alert'
    [message show];//use of undeclared identifier 'show'
    [alert release]; //use of undeclared identifier 'alert'

}

-(void)dealloc{
    [lm release];
    [latitudeTextField release];
    [longitudeTextField release];
    [accuracyTextField release];
    [super dealloc];
}


- (void)didReceiveMemoryWarning
{
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

@end

thepianoguy · #2 (**permalink**) October 17th, 2013, 10:13 PM

I assume that in the header file you have a property for a CLLocationManager named "lm" and since there are no @synthesize statements in the implementation file you are using Xcode's auto-generation of accessors. When Xcode generates the accessors the variable is redefined with a leading underscore, so lm becomes _lm. What this means is that you can only access lm by using self.lm, which calls the accessor or set the variable directly, bypassing the accessor by using _lm.
So, in the viewDidLoad method (changes in bold)

Code:

- (void)viewDidLoad

{
    self.lm = [[CLLocationManager alloc]init];
    self.lm.delegate = self;
    self.lm.desiredAccuracy = kCLLocationAccuracyBest;
    self.lm.distanceFilter = kCLDistanceFilterNone;
    [self.lm startUpdatingLocation];
    [super viewDidLoad];
   // Do any additional setup after loading the view, typically from a nib.
}

or bypassing the accessor and setting the variable directly

Code:

- (void)viewDidLoad

{
    _lm = [[CLLocationManager alloc]init];
    _lm.delegate = self;
    _lm.desiredAccuracy = kCLLocationAccuracyBest;
    _lm.distanceFilter = kCLDistanceFilterNone;
    [_lm startUpdatingLocation];
    [super viewDidLoad];
   // Do any additional setup after loading the view, typically from a nib.
}

Direct access of properties should be avoided except on initialization, so self.propertyName should usually be used. In this particular case, I don't think that the location manager should even be a property, and should actually be declared in the class extension in the implementation file, as it is a private variable, not a property. This would eliminate the need to go through the accessor. The author's inconsistent use of the accessor in the initialization is sloppy. The auto generation tries to enforce the proper use of accessors which is important for proper memory management, as it does not allow you to inadvertently set a variable directly.

This should address your problem. If not send your complete, nonworking project to
ipadhelper@me.com
and I can check it out more thoroughly..

Bob

thepianoguy · #3 (**permalink**) October 19th, 2013, 11:59 AM

Going through the project you sent me, I see that you have @synthesize declarations, that were not in the copy and paste of your original post. This will eliminate the issue that I addressed in my original reply.
I missed your annotations in the
-(void) locationManagerDidResumeLocationUpdates:(CLLocatio nManager *)manager
method.

The first issue with your implementation of this method is that you are initializing an NSString with an alertView initialization. NSString does not have an initWithTitle:message:delegate… message. You then attempted to pass the string as a the message argument, and failed to set the delegate.

This is a correct way to implement this method, with the incorrect code commented out

Code:

-(void) locationManagerDidResumeLocationUpdates:(CLLocationManager *)manager
                              didFailWithError : (NSError *) error {
//    NSString *msg = [[NSString alloc]initWithTitle : @"Error"
//                                           message : msg
//                                          delegate : nil
//                                 cancelButtonTitle : @"Done"
//                                 otherButtonTitles : nil];
    
    NSString *msg = @"Location Update failed";
    
    
    UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"Error"
                                                    message:msg
                                                   delegate:self
                                          cancelButtonTitle:@"Done"
                                          otherButtonTitles:nil];
   
    [alert show]; //use of undeclared identifier 'alert'
//    [message show];//use of undeclared identifier 'show'
    [alert release]; //use of undeclared identifier 'alert'
}

The NSString is initialized with the message to be displayed in the alertView.
The alertView is allocated and initialized with a title and the NSString is passed as an argument. The delegate needs to be set so that responses from the alertView can be monitored, and additional action taken if needed. As you had implemented there was no alert variable and no message variable since they were never declared, allocated or initialized. The initialization that was done which I commented out was not valid, since it is not an NSString method.
After making these corrections the program will compile without errors but will crash if you run it. Checking your storyboard, if you control-click on the view controller there are three outlets with yellow warning triangles. They need to be deleted. Also in the storyboard the three outlets that remain need to be connected to the corresponding textfields. Without these connections data generated will not be displayed.

Bob

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