Why does NSOperation disable automatic key-value observing?

Question

When working with a custom NSOperation subclass I noticed that the automatic key-value observing is disabled by the [NSOperation automaticallyNotifiesObserversForKey] class method (which returns NO at least for some key paths). Because of that the code inside of NSOperation subclasses is littered by manual calls to willChangeValueForKey: and didChange…, as visible in many code samples on the web.

Why does NSOperation do that? With automatic KVO support people could simply declare properties for the operation lifecycle flags (isExecuting etc.) and trigger the KVO events through the accessors, ie. the following code:

[self willChangeValueForKey:@"isExecuting"];
executing = NO;
[self didChangeValueForKey:@"isExecuting"];
[self willChangeValueForKey:@"isFinished"];
finished = YES;
[self didChangeValueForKey:@"isFinished"];

…could be replaced by this:

[self setIsExecuting:NO];
[self setIsFinished:YES];

Is there a catch somewhere? I just overrode the automaticallyNotifiesObserversForKey to return YES and things seem to work fine.

Answer 1

The most likely explanation is that the kvo keys don't match the standard conventions. Normally one has methods like -isExecuting and -setExecuting:, where the key path is @"executing". In the case of NSOperation, the key path is @"isExecuting" instead.

The other possibility is that most NSOperations don't actually have a method named -setIsExecuting: to change that value. Instead, they base the executing/finished flags on other internal state. In this case, one absolutely needs to use the explicit willChange/didChange notifications. For example, if I have an NSOperation that wraps an NSURLConnection, I may have 2 ivars, one named data that holds the downloaded data, and one named connection which holds the NSURLConnection, and I may implement the getters like so:

- (BOOL)isExecuting {
    return (connection != nil);
}

- (BOOL)isFinished {
    return (data != nil && connection == nil);
}

Now my -start method can use

[self willChangeValueForKey:@"isExecuting"];
data = [[NSMutableData alloc] init]; // doesn't affect executing, but is used later
connection = [[NSURLConnection connectionWithRequest:request delegate:self] retain];
[self didChangeValueForKey:@"isExecuting"];

to start executing, and

[self willChangeValueForKey:@"isExecuting"];
[self willChangeValueForKey:@"isFinished"];
[connection cancel];
[connection release];
connection = nil;
[self didChangeValueForKey:@"isFinished"];
[self didChangeValueForKey:@"isExecuting"];

to finish.

Answer 2

While I agree that overriding automaticallyNotifiesObserversForKey appears to work, but I personally forgo the isExecuting and isFinished properties altogether and instead define executing and finished properties, which, as Kevin suggests, is more consistent with modern conventions:

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

I then write custom setters for these two properties, which do the necessary isExecuting and isFinished notifications:

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

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

This yields:

• a more customary BOOL property declaration;
• custom setters satisfy the strange notifications that NSOperation requires; and
• I can now just use the executing and finished setters throughout my operation implementation, without littering my code with notifications.

I must confess that I like the elegance of overriding automaticallyNotifiesObserversForKey, but I just worry about unintended consequences.

---

Note, if doing this in iOS 8 or Yosemite, you will also have to explicitly synthesize these properties in your @implementation:

@synthesize finished  = _finished;
@synthesize executing = _executing;

Answer 3

I don't know why you are talking about NSOperation can not use automatic KVO. But I just try to verify that, therefore it can use KVO.

[self addObserver:self
       forKeyPath:@"isReady"
          options:NSKeyValueObservingOptionNew | NSKeyValueObservingOptionInitial
          context:&ctxKVO_CSDownloadOperation];

[self addObserver:self
       forKeyPath:@"isExecuting"
          options:NSKeyValueObservingOptionNew | NSKeyValueObservingOptionInitial
          context:&ctxKVO_CSDownloadOperation];

[self addObserver:self
       forKeyPath:@"isFinished"
          options:NSKeyValueObservingOptionNew | NSKeyValueObservingOptionInitial
          context:&ctxKVO_CSDownloadOperation];

[self addObserver:self
       forKeyPath:@"isCancelled"
          options:NSKeyValueObservingOptionNew | NSKeyValueObservingOptionInitial
          context:&ctxKVO_CSDownloadOperation];

...

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
    if (context == &ctxKVO_CSDownloadOperation) {
        NSLog(@"KVO: %@", keyPath);
    } else {
        [super observeValueForKeyPath:keyPath ofObject:object change:change context:context];
    }
}

The result:

2017-08-02 14:29:58.831 CSDownloader[77366:5089399] isReady : 1
2017-08-02 14:29:58.831 CSDownloader[77366:5089399] KVO: isReady
2017-08-02 14:29:58.831 CSDownloader[77366:5089399] isExecuting : 0
2017-08-02 14:29:58.831 CSDownloader[77366:5089399] KVO: isExecuting
2017-08-02 14:29:58.831 CSDownloader[77366:5089399] isFinished : 0
2017-08-02 14:29:58.832 CSDownloader[77366:5089399] KVO: isFinished
2017-08-02 14:29:58.832 CSDownloader[77366:5089399] isCancelled : 0
2017-08-02 14:29:58.832 CSDownloader[77366:5089399] KVO: isCancelled
2017-08-02 14:29:58.832 CSDownloader[77366:5089399] isReady : 1
2017-08-02 14:29:58.832 CSDownloader[77366:5089399] KVO: isReady
2017-08-02 14:29:58.833 CSDownloader[77366:5089399] isExecuting : 0
2017-08-02 14:29:58.833 CSDownloader[77366:5089399] KVO: isExecuting
2017-08-02 14:29:58.833 CSDownloader[77366:5089399] isFinished : 0
2017-08-02 14:29:58.833 CSDownloader[77366:5089399] KVO: isFinished
2017-08-02 14:29:58.833 CSDownloader[77366:5089399] isCancelled : 0
2017-08-02 14:29:58.833 CSDownloader[77366:5089399] KVO: isCancelled
2017-08-02 14:29:58.834 CSDownloader[77366:5089399] isReady : 1
2017-08-02 14:29:58.834 CSDownloader[77366:5089399] KVO: isReady
2017-08-02 14:29:58.834 CSDownloader[77366:5089399] isExecuting : 0

So I am really confused about this question and answers...

Answer 4

The NSOperationQueue is not observing isFinished or isExecuting, it is observing finished and executing.

isFinished is simply the the synthesized get accessor for the property finished. Automatic key-value observing notifications will be sent for this property unless your subclass has specifically opted out of automatic KVO notifications by implementing +automaticallyNotifiesObserversForKey or +automaticallyNotifiesObserversOf<Key> to return NO. If you have not opted out of automatic KVO notifications, you do not need to perform manual notifications using will/DidChangeValueForKey:. In your case, you were sending manual notifications for isFinished and isExecuting, which are not the key paths that NSOperationQueue observes.

TL;DR: These aren't the key paths that NSOperationQueue is looking for.

executing and finished are the correct key paths, and they should be sending automatic KVO notifications.

If you are truly paranoid about KVO and want to send notifications for the get accessor key paths such as isFinished, register your property as a dependency of the key path:

+ (NSSet *) keyPathsForValuesAffectingIsFinished {
    NSSet   *result = [NSSet setWithObject:@"finished"];
    return result;
}