OperationGroup.swift

class OperationGroup<T1, T2> {
    private var item1: T1?
    private var item2: T2?
    private var observers: [(T1, T2) -> Void] = []
    
    func notify1(_ item: T1) {
        item1 = item
        _notifyIfNeeded()
    }
    
    func notify2(_ item: T2) {
        item2 = item
        _notifyIfNeeded()
    }
    
    func subscribe(_ observer: @escaping (T1, T2) -> Void) {
        observers.append(observer)
        _notifyIfNeeded()
    }
    
    private func _notifyIfNeeded() {
        guard let item1 = item1, let item2 = item2 else {
            return
        }
        while let observer = observers.popLast() {
            observer(item1, item2)
        }
    }
}