When writing code in Swift it is often needed to observe changes in other objects. We can use Apple’s key-value observation but it has some implications: requires to use NSObject and dynamic dispatch through Objective-C runtime. This time, let’s build a simple key-value observation in Swift what does not require to use NSObject at all. Although it is far from being as feature complete as Apple’s implementation, it delivers the basic use-case which is often all what we need.
Custom KeyValueObservable protocol
The approach we take here is defining a protocol, providing default implementations for all the functions. Then we can make any class to conform to this protocol, but as we need to store observation related information, then the class needs to define a property holding an instance of ObservationStore. Secondly, it is required to send key-value change notification manually using didChangeValue(for:).
Add observer function returns an instance of Observation what can be used for removing the added observation. If the observer does not need to be removed during the lifetime of the observer, it can be ignored. Observation is always cleaned up automatically next time any key value changes happen after observer is deallocated. This is due to the fact that observation handler captures observer weakly and during key-value changes, it is checked if the object is still alive or not.
Adding default implementations
When adding observer, we create an observation handler what captures self and observer weakly. Handler returns boolean, what tells if the handler is still valid or not. Handler is not valid when observer has been deallocated since the last change. Otherwise handler is valid and should not be removed automatically.
Supporting objects to key-value observing
As mentioned before, ObservationStore is needed to added to every class conforming to KeyValueObservable protocol. It stores all the observations and restricts the access to modifying the observations directly from the observable class.
Observation is a simple struct containing an identifier and subtype defining the observation options. In this basic case, it just has initial option what assures handler is called immediately when adding an observer.
Conforming to KeyValueObservable
In this small example a class Event conforms to KeyValueObservable and ViewController observers the title change and updates a label.
This time we added basic support for observing key paths without using key-value observing APIs known already from Objective-C times. The added KeyValueObservable protocol is easy to add to existing classes but requires manually calling didChangeValue(for:) for every property change.
Inspiration came from Observers in Swift part 2 (Swift by Sundell).