Ensure that the consumer of your api calls your callback

AssertComplete.swift

final class AssertComplete {
    var defused: Bool = false
    let function: String
    
    init(function: String = #function) {
        self.function = function
    }
    
    func defuse() { defused = true }
    
    deinit {
        assert(defused, "You must call complete when calling \(function)")
    }
}

func delegateActionAndComplete(action: (() -> Void) -> Void) {
    let assertComplete = AssertComplete()
    action {
        assertComplete.defuse()
        // when complete
    }
}

delegateActionAndComplete { complete in
    complete() // do not forget to call complete or else, assert will trigger !
}

print("done")