Grand Central Dispatch, Say Whaa

Question

I need help with using the GCD. I am trying to fetch body weight and body fat from the Health Kit and multiply them together then list that out in a table view to the user.
Here's how I think it's supposed to be set up.
In my controller Instantiate the model object 
-> Model object makes an async call to the Health Kit for body weight in a dispatch group 
-> Model object makes another async call in the same dispatch group for body fat 
-> the group finishes and notifies the controller 
-> on the main dispatch queue I access the now fully instantiated model object to get the data 
-> insert rows into the table with the data
How do I notify the controller that the DispatchGroup in the model has completed?
Please let me know what code I need to provide to assist in answering. I have a lot of code and didn't want to paste it all.
Any help is much appreciated.

Martin Wildfeuer · Accepted Answer

Hey @Kyle Melton (https://teamtreehouse.com/kylemelton),
I have added a playground example that hopefully somehow matches your use case. It is partly taken from AllAboutSwift (https://www.allaboutswift.com/dev/2016/7/12/gcd-with-swfit3). I tend to not post cross links here, but
it is a huge topic and I think it would be too much to explain it here.
Also, when it comes to async programming and queues, you might want to have look at Operation Queue (https://developer.apple.com/reference/foundation/operationqueue).
Frameworks worth having a look at:
- PromiseKit (https://github.com/mxcl/PromiseKit)
- RxSwift (https://github.com/ReactiveX/RxSwift) and plenty other reactive libraries
Please note that these might be overkill for what you are trying to achieve. I just want to highlight that, as async tasks are literally everywhere in an app, there are great libraries to help you here. 
Finally, though, the code example
```swift
import UIKit
import PlaygroundSupport
PlaygroundPage.current.needsIndefiniteExecution = true
class Client {
let queue = DispatchQueue(label: "com.allaboutswift.dispatchgroup", attributes: .concurrent, target: .main)
let group = DispatchGroup()

func taskOne() {
    queue.async(group: group) {
        print("Waiting for task 1 to finish")
        Thread.sleep(forTimeInterval: 1)
        print("Task 1 finished")
    }
}

func taskTwo() {
    queue.async(group: group) {
        print("Waiting for task 2 to finish")
        Thread.sleep(forTimeInterval: 1)
        print("Task 2 finished")
    }
}

func taskOneAndTwo() {

taskOne()
    taskTwo()

group.notify(queue: .main) {
        print("Task 1 & 2 finished")
    }
}
}
let client = Client()
client.taskOneAndTwo()
```
Hope that helps :)

Kyle Melton · Answer

Thank you! Yes it does help. I would love a course that goes through incorporating asynchronous programming with Swift 3. Most of the resources available are outdated.

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Kyle Melton

Kyle Melton

Grand Central Dispatch, Say Whaa

2 Answers

Martin Wildfeuer

Martin Wildfeuer

Kyle Melton

Kyle Melton