Pass method as an SEL argument to another class method

Question

I have an appDelegate that init an instance of a class called LocationService. I just want to pass to this instance by init a method that this class will run.

I got this exception:

2011-08-16 20:38:15.233 WalklogAnywhere[8258:307] -[LocationService setBackgroundActionMethod:]: unrecognized selector sent to instance 0x17a740
2011-08-16 20:38:15.249 WalklogAnywhere[8258:307] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[LocationService setBackgroundActionMethod:]: unrecognized selector sent to instance 0x17a740'
*** Call stack at first throw:
(
        0   CoreFoundation        0x314d0987 __exceptionPreprocess + 114
        1   libobjc.A.dylib       0x319a149d objc_exception_throw + 24
        2   CoreFoundation        0x314d2133 -[NSObject(NSObject) doesNotRecognizeSelector:] + 102
        3   CoreFoundation        0x31479aa9 ___forwarding___ + 508
        4   CoreFoundation        0x31479860 _CF_forwarding_prep_0 + 48
        5   WalklogAnywhere       0x00004013 -[LocationService initWithBackgroundMethod:] + 206
        6   WalklogAnywhere       0x0000246b -[WalklogAnywhereAppDelegate application:didFinishLaunchingWithOptions:] + 110
        7   UIKit                 0x338dabc5 -[UIApplication _callInitializationDelegatesForURL:payload:suspended:] + 772
        8   UIKit                 0x338d6259 -[UIApplication _runWithURL:payload:launchOrientation:statusBarStyle:statusBarHidden:] + 272
        9   UIKit                 0x338a248b -[UIApplication handleEvent:withNewEvent:] + 1114
        10  UIKit                 0x338a1ec9 -[UIApplication sendEvent:] + 44
        11  UIKit                 0x338a1907 _UIApplicationHandleEvent + 5090
        12  GraphicsServices      0x35d66f03 PurpleEventCallback + 666
        13  CoreFoundation        0x314656ff __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE1_PERFORM_FUNCTION__ + 26
        14  CoreFoundation        0x314656c3 __CFRunLoopDoSource1 + 166
        15  CoreFoundation        0x31457f7d __CFRunLoopRun + 520
        16  CoreFoundation        0x31457c87 CFRunLoopRunSpecific + 230
        17  CoreFoundation        0x31457b8f CFRunLoopRunInMode + 58
        18  UIKit                 0x338d5309 -[UIApplication _run] + 380
        19  UIKit                 0x338d2e93 UIApplicationMain + 670
        20  WalklogAnywhere       0x000021bb main + 70
        21  WalklogAnywhere       0x00002170 start + 40
)
terminate called after throwing an instance of 'NSException'

this is my app delegate class method that init the LocationService instance:

-(void) backgroundLocationUpdate {
    NSLog(@"location is updated in background");
}


#pragma mark -
#pragma mark Application lifecycle

- (BOOL)application:(UIApplication *)application
     didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {    

    // Override point for customization after application launch.

    [self.window makeKeyAndVisible];
    locationService = [[LocationService alloc]
            initWithBackgroundMethod:@selector(backgroundLocationUpdate:)];
    [locationService startForegroundService];
    storageService = [[StorageService alloc] init];
    [self loadApplicationData];

    return YES;
}

And this is the LocationService class method:

-(id) initWithBackgroundMethod:(SEL)selector {
    self = [super init];
    if (self != nil) {
        self.locationManager = [[CLLocationManager alloc] init];
        self.locationManager.delegate = self;
        locationManager.desiredAccuracy = kCLLocationAccuracyBest;
        self.backgroundActionMethod = selector;
    }
    return self;
}

Please help me guys.
Thanks a lot!

score 2 · Accepted Answer · answered Aug 16 '11 at 17:50

A selector is just a method's name. It's not related to a class, nor an instance of a class.

If you want to have a kind of callback, using selectors, you'll have to provide a target object as well as the selector.

Then, invoke the selector to the target object, using the performSelector method, inherited from NSObject.

For error checking, see the respondToSelector method.

score 1 · Answer 2 · answered Aug 16 '11 at 17:48

Remove the colon next to backgroundLocationUpdate as in below:

[self.window makeKeyAndVisible]; locationService = [[LocationService alloc] initWithBackgroundMethod:@selector(backgroundLocationUpdate)];

As the colon makes it to look for a method with (id)sender parameter.

score 1 · Answer 3 · answered Aug 16 '11 at 17:49

1

The error is coming from this line:

self.backgroundActionMethod = selector;

I'm guessing you don't actually have a backgroundActionMethod property, because there is no -setBackgroundActionMethod: method.

answered Aug 16 '11 at 17:49

jtbandes

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Pass method as an SEL argument to another class method

3 Answers3