Glyphs

Type Method

observe(_:forKeyPath:withObserver:options:context:)observe:forKeyPath:withObserver:options:context:observe_forKeyPath_withObserver_options_context_

Creates a new observation proxy that observes the given object, forwarding observation messages to the given observer.

class func observe(_ object: NSObject?, forKeyPath keyPath: String, withObserver observer: NSObject, options: NSKeyValueObservingOptions = [], context: UnsafeMutableRawPointer?) -> Self?
+ (instancetype) observe:(NSObject *) object forKeyPath:(NSString *) keyPath withObserver:(NSObject *) observer options:(NSKeyValueObservingOptions) options context:(void *) context;
observe_forKeyPath_withObserver_options_context_(object: NSObject, keyPath: str, observer: NSObject, options: NSKeyValueObservingOptions, context) -> GSKeyValueObservation

Discussion

The returned observation proxy adds itself as an observer of object with the given key path, options, and context. Any observation message is then forwarded to the observer, to which the proxy holds a weak reference. Before forwarding the message, the proxy checks whether the observer is nil and if so, removes itself as an observer from object.

The proxy also removes itself as an observer when it is deallocated. This allows for a convenient pattern, where the observation is stored in a private property. Replacing the value if the property stops the current observation, if any, and keeps track of the current observation. When the containing object is deallocated, so is the observation property and the observation is stopped.

@interface Controller ()
@property (strong) GSKeyValueObservation *someObservation;
@end

@implementation Controller
- (void)thingDidUpdate:(Thing *)newThing {
    _someObservation = [GSKeyValueObservation observe:newThing ...];
}

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context {
    // `_someObservation` forwards messages to this method
}
@end

When object is nil or NSNull, this method returns nil.