最近常问的一道面试题，自己也整理一下。
题目：4个任务ABCD，先做A, 再同时做B,C，最后做D

    print("start")

    //Execute 4 tasks A, B, C, D on order: A -> B,C -> D
    let workA = DispatchWorkItem {
        print(Date(), "+++ A started")
        Thread.sleep(forTimeInterval: 2)
        print(Date(), "--- A finished")
    }
    
    let workB = DispatchWorkItem {
        print(Date(), "+++ B started")
        Thread.sleep(forTimeInterval: 4)
        print(Date(), "--- B finished")
    }
    
    let workC = DispatchWorkItem {
        print(Date(), "+++ C started")
        Thread.sleep(forTimeInterval: 2)
        print(Date(), "--- C finished")
    }
    
    let workD = DispatchWorkItem {
        print(Date(), "+++ D started")
        Thread.sleep(forTimeInterval: 4)
        print(Date(), "--- D finished")
    }

    let concurrent = DispatchQueue(label: "concurrent", qos: .utility, attributes: .concurrent)
    let group = DispatchGroup()

    //#1.1 Use DispatchQueue barrier
    print("Use DispatchQueue barrier")
    concurrent.sync(execute: workA)
    concurrent.async(execute: workB)
    concurrent.async(execute: workC)
    concurrent.sync(flags:.barrier) {
        workD.perform()
    }
    
    //#1.2 Use DispatchWorkItem barrier
    print("Use DispatchWorkItem barrier")
    let barrierWorkD = DispatchWorkItem(flags:.barrier) {
        workD.perform()
    }
    concurrent.sync(execute: workA)
    concurrent.async(execute: workB)
    concurrent.async(execute: workC)
    concurrent.sync(execute: barrierWorkD)

    //#2.1 Use DispatchGroup
    print("Use DispatchGroup")
    workA.perform()
    concurrent.async(group: group, execute: workB)
    concurrent.async(group: group, execute: workC)
    group.wait()
    workD.perform()
    
    //#2.2 Use DispatchGroup enter leave
    print("Use DispatchGroup enter leave")
    workA.perform()
    group.enter()
    concurrent.async {
        workB.perform()
        group.leave()
    }
    group.enter()
    concurrent.async {
        workC.perform()
        group.leave()
    }
    group.wait()
    workD.perform()

    //#3. Use DispatchSemaphore
    print("Use DispatchSemaphore")
    let semaphore = DispatchSemaphore(value: 0)
    workA.perform()
    concurrent.async {
        workB.perform()
        semaphore.signal()
    }
    concurrent.async {
        workC.perform()
        semaphore.signal()
    }
    semaphore.wait()
    semaphore.wait()
    workD.perform()
    
    //#4. Use OperationQueue
    print("Use OperationQueue")
    let queue = OperationQueue()
    let boA = BlockOperation {
        workA.perform()
    }
    let boB = BlockOperation {
        workB.perform()
    }
    boB.addDependency(boA)
    let boC = BlockOperation {
        workC.perform()
    }
    boC.addDependency(boA)
    let boD = BlockOperation {
        workD.perform()
    }
    boD.addDependency(boB)
    boD.addDependency(boC)
    queue.addOperations([boA, boB, boC, boD], waitUntilFinished: true)

    print("end")