Objective-C Language Key Value Coding / Key Value Observing Key Value Observing
Example
Setting up key value observing.
In this case, we want to observe the contentOffset on an object that our observer owns
//
// Class to observe
//
@interface XYZScrollView: NSObject
@property (nonatomic, assign) CGPoint contentOffset;
@end
@implementation XYZScrollView
@end
//
// Class that will observe changes
//
@interface XYZObserver: NSObject
@property (nonatomic, strong) XYZScrollView *scrollView;
@end
@implementation XYZObserver
// simple way to create a KVO context
static void *XYZObserverContext = &XYZObserverContext;
// Helper method to add self as an observer to
// the scrollView's contentOffset property
- (void)addObserver {
// NSKeyValueObservingOptions
//
// - NSKeyValueObservingOptionNew
// - NSKeyValueObservingOptionOld
// - NSKeyValueObservingOptionInitial
// - NSKeyValueObservingOptionPrior
//
// can be combined:
// (NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld)
NSString *keyPath = NSStringFromSelector(@selector(contentOffset));
NSKeyValueObservingOptions options = NSKeyValueObservingOptionNew;
[self.scrollView addObserver: self
forKeyPath: keyPath
options: options
context: XYZObserverContext];
}
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSString *,id> *)change context:(void *)context {
if (context == XYZObserverContext) { // check the context
// check the keyPath to see if it's any of the desired keyPath's.
// You can observe multiple keyPath's
if ([keyPath isEqualToString: NSStringFromSelector(@selector(contentOffset))]) {
// change dictionary keys:
// - NSKeyValueChangeKindKey
// - NSKeyValueChangeNewKey
// - NSKeyValueChangeOldKey
// - NSKeyValueChangeIndexesKey
// - NSKeyValueChangeNotificationIsPriorKey
// the change dictionary here for a CGPoint observation will
// return an NSPoint, so we can take the CGPointValue of it.
CGPoint point = [change[NSKeyValueChangeNewKey] CGPointValue];
// handle point
}
} else {
// if the context doesn't match our current object's context
// we want to pass the observation parameters to super
[super observeValueForKeyPath: keyPath
ofObject: object
change: change
context: context];
}
}
// The program can crash if an object is not removed as observer
// before it is dealloc'd
//
// Helper method to remove self as an observer of the scrollView's
// contentOffset property
- (void)removeObserver {
NSString *keyPath = NSStringFromSelector(@selector(contentOffset));
[self.scrollView removeObserver: self forKeyPath: keyPath];
}
@end
