Using GCD to call function after another function in Swift

Question

I have a function A that contains two other functions B & C. Inside function A I need to call function C once function B has completed. I'm thinking I need to use Grand Central Dispatch's dispatch_group_notify for it, but am not too sure how to use it. I'm calling asynchronous methods in both functions B & C. Here are my functions:

func A() {

    func B() {

    // Three asynchronous functions

    }

    func C() {

    // More asynchronous functions that handle results from func B()

    }

    funcB()
    funcC()
}

EDIT: In func B(), I have three async functions that take awhile to finish. When I use the following method, func C() is still being called before the methods within func B() are completely finished. How do I make sure they're completely finished before the second dispatch group is called?

func A() {

var group = dispatch_group_create()

        dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0)) { () -> Void in

            // async function 1
            // async function 2
            // async function 3
        }

        dispatch_group_notify(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), { () -> Void in

           // async function

        })

}

Can you provide more on how you are implementing B() and C()? Also do you expect A() to return immediately before B() and C() complete the work? Your code is already calling C() after B(), unless B() is starting another queue inside the method and returns immediately. — hebinda
– hebinda, Commented Jul 16, 2015 at 4:08
See updated code. I have some async functions inside func C() that must only be called once the three async functions in func B() have finished. — slider
– slider, Commented Jul 16, 2015 at 4:11
possible duplicate of Waiting until two async blocks are executed before starting another block — zrzka
– zrzka, Commented Jul 16, 2015 at 6:04
Check answer in linked question or read documentation of NSOperationQueue, NSOperation, addDependency(_:), ... — zrzka
– zrzka, Commented Jul 16, 2015 at 6:05

zrzka · Accepted Answer · 2015-07-20 09:31:50Z

1

Here's an example how to wait till your tasks are finished:

func asyncTaskSimulation(delay: NSTimeInterval, completion: (NSTimeInterval) -> ()) {
  dispatch_after(dispatch_time(DISPATCH_TIME_NOW,
    Int64(delay * Double(NSEC_PER_SEC))), dispatch_get_global_queue(QOS_CLASS_UTILITY, 0)) {
      completion(delay)
  }
}

func A() {

  func B(completion: () -> ()) {

    print("Function B start")

    let group = dispatch_group_create()

    dispatch_group_enter(group)
    asyncTaskSimulation(1.0) { (delay) in
      print("First task after \(delay)s")
      dispatch_group_leave(group)
    }

    dispatch_group_enter(group)
    asyncTaskSimulation(2.0) { (delay) in
      print("Second task after \(delay)s")
      dispatch_group_leave(group)
    }

    dispatch_group_enter(group)
    asyncTaskSimulation(0.5) { (delay) in
      print("Third task after \(delay)s")
      dispatch_group_leave(group)
    }

    dispatch_group_wait(group, DISPATCH_TIME_FOREVER)
    completion()

    print("Function B end")
  }

  func C() {
    print("Function C start")
    print("Whatever")
    print("Function C end")
  }

  B() {
    C()
  }

}

A()

Here's the output:

Function B start
Second task after 0.5s
First task after 1.0s
Second task after 2.0s
Function C start
Whatever
Function C end
Function B end

answered Jul 20, 2015 at 9:31

zrzka

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4 Comments

slider Over a year ago

When calling function B(), Cannot reference a local function with captures from another local function

zrzka Over a year ago

I just tested my code in Swift 1.2 & Swift 2.0 and it does work. If you have an issue, it's probably because you did extend it somehow and in a wrong way. No one has a crystal ball, you have to be specific if you need help. Edit your question and show what you did or I can't help.

slider Over a year ago

It may have worked for you. But I had an async function saving objects to a backend in the asyncTaskSimulation, and another retrieving those very objects in func C(). So the function itself may have worked, in fact it returned all outputs in correct order for me, but it wasn't enough to save and retrieve objects in one call. So I just put a 6 second delay on func C() and it worked accordingly. Thanks for your help.

Lapinou Over a year ago

I prefere use the block way: dispatch_group_notify(group, dispatch_get_main_queue(),^{ print("Function B end") });

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