Executing asynchronous tasks sequentially in Swift

DispatchGroup.swift

let q = DispatchQueue(label: "q")
let group = DispatchGroup()
group.enter()

q.async {
    loadA(completion: {
        // Do something...
        group.leave()
    })
}

q.async {
    group.wait()
    group.enter()

    loadB(completion: {
        // Do something...
        group.leave()
    })
}

q.async {
    group.wait()
    group.enter()

    loadC(completion: {
        // Do something...
        group.leave()
    })
}