Use of delegates in NSOperation

Question

I am trying to make use of CLLocationManager in an NSOperation. As part of this I require the ability to startUpdatingLocation then wait u

Answer 1

I think you have two options.

1. Create a separate thread, with its own run loop, for location services:

   #import "LocationOperation.h"
   #import <CoreLocation/CoreLocation.h>
   
   @interface LocationOperation () <CLLocationManagerDelegate>
   
   @property (nonatomic, readwrite, getter = isFinished)  BOOL finished;
   @property (nonatomic, readwrite, getter = isExecuting) BOOL executing;
   
   @property (nonatomic, strong) CLLocationManager *locationManager;
   
   @end
   
   @implementation LocationOperation
   
   @synthesize finished  = _finished;
   @synthesize executing = _executing;
   
   - (id)init
   {
       self = [super init];
       if (self) {
           _finished = NO;
           _executing = NO;
       }
       return self;
   }
   
   - (void)start
   {
       if ([self isCancelled]) {
           self.finished = YES;
           return;
       }
   
       self.executing = YES;
   
       [self performSelector:@selector(main) onThread:[[self class] locationManagerThread] withObject:nil waitUntilDone:NO modes:[[NSSet setWithObject:NSRunLoopCommonModes] allObjects]];
   }
   
   - (void)main
   {
       [self startStandardUpdates];
   }
   
   - (void)dealloc
   {
       NSLog(@"%s", __FUNCTION__);
   }
   
   #pragma mark - NSOperation methods
   
   - (BOOL)isConcurrent
   {
       return YES;
   }
   
   - (void)setExecuting:(BOOL)executing
   {
       if (executing != _executing) {
           [self willChangeValueForKey:@"isExecuting"];
           _executing = executing;
           [self didChangeValueForKey:@"isExecuting"];
       }
   }
   
   - (void)setFinished:(BOOL)finished
   {
       if (finished != _finished) {
           [self willChangeValueForKey:@"isFinished"];
           _finished = finished;
           [self didChangeValueForKey:@"isFinished"];
       }
   }
   
   - (void)completeOperation
   {
       self.executing = NO;
       self.finished = YES;
   }
   
   - (void)cancel
   {
       [self stopStandardUpdates];
       [super cancel];
       [self completeOperation];
   }
   
   #pragma mark - Location Manager Thread
   
   + (void)locationManagerThreadEntryPoint:(id __unused)object
   {
       @autoreleasepool {
           [[NSThread currentThread] setName:@"location manager"];
   
           NSRunLoop *runLoop = [NSRunLoop currentRunLoop];
           [runLoop addPort:[NSMachPort port] forMode:NSDefaultRunLoopMode];
           [runLoop run];
       }
   }
   
   + (NSThread *)locationManagerThread
   {
       static NSThread *_locationManagerThread = nil;
       static dispatch_once_t oncePredicate;
       dispatch_once(&oncePredicate, ^{
           _locationManagerThread = [[NSThread alloc] initWithTarget:self selector:@selector(locationManagerThreadEntryPoint:) object:nil];
           [_locationManagerThread start];
       });
   
       return _locationManagerThread;
   }
   
   #pragma mark - Location Services
   
   - (void)startStandardUpdates
   {
       if (nil == self.locationManager)
           self.locationManager = [[CLLocationManager alloc] init];
   
       self.locationManager.delegate = self;
       self.locationManager.desiredAccuracy = kCLLocationAccuracyBest;
       self.locationManager.distanceFilter = 500;
   
       [self.locationManager startUpdatingLocation];
   }
   
   - (void)stopStandardUpdates
   {
       [self.locationManager stopUpdatingLocation];
       self.locationManager = nil;
   }
   
   #pragma mark - CLLocationManagerDelegate
   
   - (void)locationManager:(CLLocationManager *)manager didUpdateLocations:(NSArray *)locations
   {
       CLLocation* location = [locations lastObject];
   
       // do whatever you want with the location
   
       // now, turn off location services
   
       if (location.horizontalAccuracy < 50) {
           [self stopStandardUpdates];
           [self completeOperation];
       }
   }
   
   @end
   

2. Alternatively, even though you're using an operation, you could just run location services on the main thread:

   #import "LocationOperation.h"
   #import <CoreLocation/CoreLocation.h>
   
   @interface LocationOperation () <CLLocationManagerDelegate>
   
   @property (nonatomic, readwrite, getter = isFinished)  BOOL finished;
   @property (nonatomic, readwrite, getter = isExecuting) BOOL executing;
   
   @property (nonatomic, strong) CLLocationManager *locationManager;
   
   @end
   
   @implementation LocationOperation
   
   @synthesize finished  = _finished;
   @synthesize executing = _executing;
   
   - (id)init
   {
       self = [super init];
       if (self) {
           _finished = NO;
           _executing = NO;
       }
       return self;
   }
   
   - (void)start
   {
       if ([self isCancelled]) {
           self.finished = YES;
           return;
       }
   
       self.executing = YES;
   
       [self startStandardUpdates];
   }
   
   #pragma mark - NSOperation methods
   
   - (BOOL)isConcurrent
   {
       return YES;
   }
   
   - (void)setExecuting:(BOOL)executing
   {
       if (executing != _executing) {
           [self willChangeValueForKey:@"isExecuting"];
           _executing = executing;
           [self didChangeValueForKey:@"isExecuting"];
       }
   }
   
   - (void)setFinished:(BOOL)finished
   {
       if (finished != _finished) {
           [self willChangeValueForKey:@"isFinished"];
           _finished = finished;
           [self didChangeValueForKey:@"isFinished"];
       }
   }
   
   - (void)completeOperation
   {
       self.executing = NO;
       self.finished = YES;
   }
   
   - (void)cancel
   {
       [self stopStandardUpdates];
       [super cancel];
       [self completeOperation];
   }
   
   #pragma mark - Location Services
   
   - (void)startStandardUpdates
   {
       [[NSOperationQueue mainQueue] addOperationWithBlock:^{
           if (nil == self.locationManager)
               self.locationManager = [[CLLocationManager alloc] init];
   
           self.locationManager.delegate = self;
           self.locationManager.desiredAccuracy = kCLLocationAccuracyBest;
           self.locationManager.distanceFilter = 500;
   
           [self.locationManager startUpdatingLocation];
       }];
   }
   
   - (void)stopStandardUpdates
   {
       [self.locationManager stopUpdatingLocation];
       self.locationManager = nil;
   }
   
   #pragma mark - CLLocationManagerDelegate
   
   - (void)locationManager:(CLLocationManager *)manager didUpdateLocations:(NSArray *)locations
   {
       CLLocation* location = [locations lastObject];
   
       // do whatever you want with the location
   
       // now, turn off location services
   
       if (location.horizontalAccuracy < 50) {
           [self stopStandardUpdates];
           [self completeOperation];
       }
   }
   
   @end
   

I think I'd be inclined to do the second approach (just making sure that I don't do anything too intensive in didUpdateLocations, or if I did, make sure to do it asynchronously), but both of these approaches appear to work.

Another approach is to keep the run loop alive until the operation is finished:

while (![self isFinished]) {
    [[NSRunLoop currentRunLoop] runUntilDate:[NSDate distantFuture]];
}

But this doesn't appear to work in conjunction with CLLocationManager, as runUntilDate doesn't immediately return (it's almost as if CLLocationManager is attaching its own source to the runloop, which prevents it from exiting). I guess you could change the runUntilDate to something a little closer than distantFuture (e.g. [NSDate dateWithTimeIntervalSinceNow:1.0]). Still, I think it's just as easy to run this operation start location services on the main queue, like the second solution above.

Having said that, I'm not sure why you would want to use location manager in an operation at all. It's already asynchronous, so I would just start the location manager from the main queue and call it a day.

Answer 2

UIWebView with UIWebViewDelegate method callbacks in an NSOperation

A server I wanted to grab a URL from a server that changes values based upon JavaScript execution from various browsers. So I slapped a dummy UIWebView into an NSOperation and use that to grab out the value I wanted in the UIWebViewDelegate method.

@interface WBSWebViewOperation () <UIWebViewDelegate>

@property (assign, nonatomic) BOOL stopRunLoop;
@property (assign, nonatomic, getter = isExecuting) BOOL executing;
@property (assign, nonatomic, getter = isFinished) BOOL finished;

@property (copy, nonatomic, readwrite) NSURL *videoURL;
@property (strong, nonatomic) UIWebView *webView;

@end



@implementation WBSWebViewOperation

- (id)init
{
    self = [super init];
    if (self) {
        _finished = NO;
        _executing = NO;
    }

    return self;
}

- (id)initWithURL:(NSURL *)episodeURL
{
    self = [self init];
    if (self != nil) {
        _episodeURL = episodeURL;
    }

    return self;
}

- (void)start
{
    if (![self isCancelled]) {
        self.executing = YES;

        [self performSelector:@selector(main) onThread:[NSThread mainThread] withObject:nil waitUntilDone:NO modes:[[NSSet setWithObject:NSRunLoopCommonModes] allObjects]];
    } else {
        self.finished = YES;
    }
}

- (void)main
{
    if (self.episodeURL != nil) {
        NSURLRequest *request = [NSURLRequest requestWithURL:self.episodeURL];
        UIWebView *webView = [[UIWebView alloc] init];
        webView.delegate = self;
        [webView loadRequest:request];

        self.webView = webView;
    }
}



#pragma mark - NSOperation methods

- (BOOL)isConcurrent
{
    return YES;
}

- (void)setExecuting:(BOOL)executing
{
    [self willChangeValueForKey:@"isExecuting"];
    _executing = executing;
    [self didChangeValueForKey:@"isExecuting"];
}

- (void)setFinished:(BOOL)finished
{
    [self willChangeValueForKey:@"isFinished"];
    _finished = finished;
    [self didChangeValueForKey:@"isFinished"];
}

- (void)completeOperation
{
    self.executing = NO;
    self.finished = YES;
}

- (void)cancel
{
    [self.webView stopLoading];
    [super cancel];
    [self completeOperation];
}



#pragma mark - UIWebViewDelegate methods

- (void)webViewDidFinishLoad:(UIWebView *)webView
{    
    NSString *episodeVideoURLString = [webView stringByEvaluatingJavaScriptFromString:@"document.getElementById('playerelement').getAttribute('data-media')"];
    NSURL *episodeVideoURL = [NSURL URLWithString:episodeVideoURLString];
    self.videoURL = episodeVideoURL;

    if ([self.delegate respondsToSelector:@selector(webViewOperationDidFinish:)]) {
        [self.delegate webViewOperationDidFinish:self];
    }

    [self completeOperation];
}

@end

Answer 3

Its going to call the delegate method in the same operation queue as main is running in. And NSOperation queues are serial by default. Your while loop is just spinning forever (because the operation is never cancelled) and the call to your delegate method is sitting in the queue behind it never able to run.

Get rid of the while loop entirely and let the operation finish. Then when the delegate method is called, if it's cancelled discard the result by returning.

Answer 4

Going back to the runloop discussion, this is how I generally solve that in my base NSOperation implementation:

// create connection and keep the current runloop running until
// the operation has finished. this allows this instance of the operation
// to act as the connections delegate
_connection = [[NSURLConnection alloc] initWithRequest:[self request]
                                              delegate:self];
while(!self.isFinished) {
    [[NSRunLoop currentRunLoop] runUntilDate:[NSDate distantFuture]];
}

I key off of isFinished, which I keep updated through setters for isCancelled and isFinished. Here's the isCancelled setter as an example:

- (void)setIsCancelled:(BOOL)isCancelled {
    _isCancelled = isCancelled;
    if (_isCancelled == YES) {
        self.isFinished = YES;
    }
}

That said, I second some of the questions about why this is necessary. If you don't need to kick something off until a location is found, why not just fire up your location manager on the main thread, wait for the appropriate delegate callback and then kick off the background operation?

Update: updated solution

While the original answer generally stands, I've fully implement a solution and it does require a slight change to how you manage the run loop. That said, all code is available on GitHub - https://github.com/nathanhjones/CLBackgroundOperation. Here is a detailed explanation of the approach.

Tl;dr

Change

[[NSRunLoop currentRunLoop] runUntilDate:[NSDate distantFuture]];

to

[[NSRunLoop currentRunLoop] runMode:NSRunLoopCommonModes
                         beforeDate:[NSDate distantFuture]];

Details

Within your operations interface define the following three properties. We'll be indicating that these operations are concurrent thus we'll manage their state manually. In the solution on GitHub these are part of NJBaseOperation.

@property(nonatomic,assign,readonly) BOOL isExecuting;
@property(nonatomic,assign,readonly) BOOL isFinished;
@property(nonatomic,assign,readonly) BOOL isCancelled;

Within your operations implementation you'll want to make those readwrite like so:

@interface NJBaseOperation ()

@property(nonatomic,assign,readwrite) BOOL isExecuting;
@property(nonatomic,assign,readwrite) BOOL isFinished;
@property(nonatomic,assign,readwrite) BOOL isCancelled;

@end

Next, you'll want to synthesize the three properties you defined above so that you can override the setters and use them to manage your operations state. Here's what I generally use, but sometimes there are some additional statements added to the setIsFinished: method depending on my needs.

- (void)setIsExecuting:(BOOL)isExecuting {
    _isExecuting = isExecuting;
    if (_isExecuting == YES) {
        self.isFinished = NO;
    }
}

- (void)setIsFinished:(BOOL)isFinished {
    _isFinished = isFinished;
    if (_isFinished == YES) {
        self.isExecuting = NO;
    }
}

- (void)setIsCancelled:(BOOL)isCancelled {
    _isCancelled = isCancelled;
    if (_isCancelled == YES) {
        self.isFinished = YES;
    }
}

Lastly, just so that we don't have to manually send the KVO notifications we'll implement the following method. This works because our properties are named isExecuting, isFinished and isCancelled.

+ (BOOL) automaticallyNotifiesObserversForKey:(NSString *)key {
    return YES;
}

Now that the the operations foundation is taken care of it's time to knockout the location stuff. You'll want to override main and within it fire up your location manager and instruct the current run loop to keep running until you tell it otherwise. This ensures that your thread is around to receive the location delegate callbacks. Here's my implementation:

- (void)main {

    if (_locationManager == nil) {
        _locationManager = [[CLLocationManager alloc] init];
        _locationManager.delegate = self;
        _locationManager.desiredAccuracy = kCLLocationAccuracyBest;
        [_locationManager startUpdatingLocation];
    }

    while(!self.isFinished) {
        [[NSRunLoop currentRunLoop] runMode:NSRunLoopCommonModes
                                 beforeDate:[NSDate distantFuture]];
    }
}

You should receive a delegate callback at which point you can do some work based on location and then finish the operation. Here's my implementation that counts to 10,000 and then cleans up.

- (void)locationManager:(CLLocationManager *)manager didUpdateLocations:(NSArray *)locations {
    NSLog(@"** Did Update Location: %@", [locations lastObject]);
    [_locationManager stopUpdatingLocation];

    // do something here that takes some length of time to complete
    for (int i=0; i<10000; i++) {
        if ((i % 10) == 0) {
            NSLog(@"Loop %i", i);
        }
    }

    self.isFinished = YES;
}

The source on GitHub includes a dealloc implementation, which simply logs that it's being called and also observes changes to the operationCount of my NSOperationQueue and logs the count - to indicating when it drops back to 0. Hope that helps. Let me know if you've got questions.