MVVM and data binding

While data binding and two-way data binding are not required to use MVVM, you saw in the previous example that binding actions and values is tedious, a source of boilerplate, and prone to errors.

While data binding is usually found in Reactive programming frameworks, it revolves around a simple object: the Observable object. The responsibility of an observable is to call the observers whenever a change in the internal value occurs.

Implementing the Observable class

We'll implement Observable as a generic class, as it should be able to wrap any kind of object:

class Observable<Type> {
    typealias Observer = (Type) -> ()
    typealias Token = NSObjectProtocol

    private var observers = [(Token, Observer)]()

    var value: Type {
        didSet {
            notify()
        }
    }

    init(_ value: Type) {
        self.value = value
    }

    @discardableResult
    func bind(_ observer: @escaping Observer) -> Token {
        defer { observer(value) }
...