let dispatch = DispatchGroup()

    dispatch.enter()
    f1(){
        // do sth
        dispatch.leave()
    }

    dispatch.enter()
    f2(){
        // do sth

        dispatch.leave()
    }

    dispatch.notify(queue:queue){
        // f1() and f2() complete 
        // it is time to do sth awesome!
    }
    

    // Apple Grand Center Dispatch(GCD) in Swift 4
    // You can use them to submit multiple work item and track when they all complete.
    // if we want to run the nodes in following order: node1.run(..) => node2.run(..)
    // then we need to use Apple GCD dispatchGroup, enter, leave and wait

    
    let dispatch = DispatchGroup()

    // dispatch.enter()
    //
    // dispatch.leave()
    //
    // dispatch.wait()
    // We can do sth useful now.

    let act1 = SKAction.move(..)
    let sequence = SKAction.sequence([act1], ...)
    for node in [node1, node2]{
        dispatch.enter()
        node.run(sequence, completion:{
            dispatch.leave()
            })
        dispatch.wait()
    }
    // running order: node1.run(..) done => node2.run(..) done