CODEWITHSUNDEEP
iosswiftgrand-central-dispatch
daspianist
daspianist

Reputation: 5525

Ensuring that loop in async operation is completed before returning

My goal is to perform a resource-heavy operation asynchronously, and ensure that it is completed before returning the objects.

This is the structure:

func modifyGadgets() -> [Gadget] {

    var gadgetsToModify = [Gadget]() //this is already populated elsewhere

    DispatchQueue.global(qos: .userInitiated).async {
        //Do something to the original gadgets
        for gadget in gadgetsToModify {
            modifyThisGadget(gadget)
        }

        DispatchQueue.main.async {
            return gadgetsToModify //I get a warning here saying gadgetsToModify is unused
        }

    }
}

In addition to the warning, I also don't get any of the Gadget items back.

How should I structure this so I am able to return the items once it is finished?

Thanks.

Upvotes: 0

Views: 49

Answers (2)

user3151675
user3151675

Reputation: 58149

You should use a completion handler for this.

func modifyGadgets(_ callback: @escaping ([Gadget]) -> ()) {
    var gadgetsToModify = [Gadget]()

    DispatchQueue.global(qos: .userInitiated).async {
        for gadget in gadgetsToModify {
            modifyThisGadget(gadget)
        }

        DispatchQueue.main.async {
            callback(gadgetsToModify)
        }
    }
}

You can use it this way:

modifyGadgets { gadgets in
    print(gadgets)
}

Upvotes: 3

Charles Srstka
Charles Srstka

Reputation: 17060

If the operation is asynchronous, you will not be able to return a value in this way. What you want to do instead is to take a closure as a completion handler:

func modifyGadgets(completionHandler: @escaping ([Gadget]) -> ()) {
    let gadgets = ...

    DispatchQueue.global(qos: .userInitiated).async {
        //Do something to the original gadgets
        for eachGadget in gadgets {
            modifyThisGadget(eachGadget)
        }

        completionHandler(gadgets)
    }
}

Then you can call it like this:

modifyGadgets { gadgets in
    // do something with gadgets
}

Upvotes: 2

Related Questions