Note: I am posting this as a reference for other developers that might run into the same issue.
Why do I have a memory leak with this code:
@interface SPWKThing : NSObject
@property (strong, nonatomic) NSArray *things;
@end
@implementation SPWKThing {
BOOL _isKVORegistered;
}
- (id)init
{
self = [super init];
if (self) {
NSLog(@"initing SPWKThing");
[self registerKVO];
}
return self;
}
- (void)didChangeValueForKey:(NSString *)key {
if ([key isEqualToString:@"things"]) {
NSLog(@"didChangeValueForKey: things have changed!");
}
}
#pragma mark - KVO
- (void)registerKVO
{
if (!_isKVORegistered) {
NSLog(@"Registering KVO, and things is %@", _things);
[self addObserver:self forKeyPath:@"things" options:0 context:NULL];
_isKVORegistered = YES;
}
}
- (void)unregisterKVO
{
if (_isKVORegistered) {
NSLog(@"Unregistering KVO");
[self removeObserver:self forKeyPath:@"things"];
_isKVORegistered = NO;
}
}
- (void)dealloc
{
NSLog(@"SPWKThing dealloc");
[self unregisterKVO];
}
@end
@implementation SPWKViewController
- (void)viewDidLoad
{
[super viewDidLoad];
[self runDemo];
}
- (void)runDemo
{
SPWKThing *thing = [[SPWKThing alloc] init];
thing.things = @[@"one", @"two", @"three"];
thing = nil;
}
@end
My output is:
initing SPWKThing
Registering KVO, and things is (null)
didChangeValueForKey: things have changed!
dealloc
is never called? Why? I am setting thing = nil
in the last line of runDemo
!
See a demo project here: https://github.com/jfahrenkrug/KVOMemoryLeak
The answer is:
Never override didChangeValueForKey:
(at least not without calling super
). The documentation does not warn you about this.
Use the correct method observeValueForKeyPath:ofObject:change:context:
instead.
This project clearly demonstrates this: https://github.com/jfahrenkrug/KVOMemoryLeak