KVO broken in iOS 9.3

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This might be an awful bug in iOS 9.3 (release).

When adding a single observer to [NSUserDefaults standardUserDefaults] I've noticed that the responding method -observeValueForKeyPath:ofObject:change:context: is called multiple times.

In the simple example below, every time a UIButton is pressed once, observeValueForKeyPath fires twice. In more complicated examples it fires even more times. It is only present on iOS 9.3 (both on sim and devices).

This can obviously wreak havoc on an app. Anyone else experiencing the same?

// ViewController.m (barebones, single view app)

- (void)viewDidLoad {
    [super viewDidLoad];
    NSLog(@"viewDidLoad");
    [[NSUserDefaults standardUserDefaults] addObserver:self forKeyPath:@"SomeKey" options:NSKeyValueObservingOptionNew context:NULL];
}

- (IBAction)buttonPressed:(id)sender {
    NSLog(@"buttonPressed");
    [[NSUserDefaults standardUserDefaults] setInteger:1 forKey:@"SomeKey"];
}

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSString *,id> *)change context:(void *)context {
    NSLog(@"observeValueForKeyPath: %@", keyPath);
}
ios9.3 key-value-observing objective-c ios
3 Answers

Adding this answer for MacOS (10.13) which definitely has the bug getting multiple notifications for KVO of NSUserDefault Keys, and which also addresses deprecations. It is better to use a calculation for elapsed nano seconds that gets it for the machine you are running on. Do it like so:

#include <mach/mach.h>
#include <mach/mach_time.h>
static mach_timebase_info_data_t _sTimebaseInfo;

uint64_t  _newTime, _previousTime, _elapsed, _elapsedNano, _threshold;
NSString  *_previousKeyPath;

-(BOOL)timeThresholdForKeyPathExceeded:(NSString *)key thresholdValue:(uint64_t)threshold
{
   _previousTime = _newTime;
   _newTime = mach_absolute_time();

    if(_previousTime > 0) {
        _elapsed = _newTime - _previousTime;
        _elapsedNano = _elapsed * _sTimebaseInfo.numer / _sTimebaseInfo.denom;
    }

    if(_elapsedNano > threshold || ![key isEqualToString:_previousKeyPath]) {
        if(![key isEqualToString:_previousKeyPath]) _previousKeyPath = key;
            return YES;
        }
        return NO;
    }
}

-(void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
    if(![self timeThresholdForKeyPathExceeded:keyPath thresholdValue:5000000]) return;  // Delete this line of MacOS bug ever fixed
    }
    // Else this is the KeyPath you are looking for Obi Wan, process it.
}

This is based on Listing 2 of this Apple Doc: https://developer.apple.com/library/content/qa/qa1398/_index.html

user_7544940
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