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iosswiftgrand-central-dispatchdispatchgroup
Lance Samaria
Lance Samaria

Reputation: 19572

Swift -is it necessary to call continue when leaving a dispatchGroup

I have a group of objects that I have to iterate through using a for-loop and DispatchGroup. When leaving the group inside the for-loop, is calling continue necessary?

let group = DispatchGroup()

for object in objects {

    group.enter()

    if object.property == nil {
         group.leave()
         continue // does calling this have any effect even though the group is handling it?
    }

    // do something with object and call group.leave() when finished
}
group.notify(...

Upvotes: 1

Views: 2051

Answers (2)

Rob
Rob

Reputation: 437442

Yes, the way this is written, the continue is critical, as you want to make sure that the enter call has a single leave call. Since you're calling enter before the if test, then you must leave and continue. If you don’t have the continue statement, it will proceed with the subsequent code which is calling leave already.

But this leave/continue pattern is not needed if you just call enter after the if statement:

let group = DispatchGroup()

for object in objects {    
    if object.property == nil {
         continue
    }

    group.enter()

    // do something with object and call group.leave() when finished
}
group.notify(queue: .main) { ... }

I'd then take that a step further and remove that if with the continue statement. Just add a where clause to the for loop, eliminating the need for the continue at all:

let group = DispatchGroup()

for object in objects where object.property != nil {
    group.enter()

    // do something with object and call group.leave() when finished
}

group.notify(queue: .main) { ... }

This accomplishes what your original code snippet did, but is more concise.

Upvotes: 1

David Pasztor
David Pasztor

Reputation: 54706

Yes, it is absolutely necessary to call continue, since you want to avoid continuing the execution of the body of your loop.

Calling DispatchGroup.leave does not exit the current scope, you need to call continue to achieve that. leave only affects whatever you are doing with the DispatchGroup - so consequent notify or wait calls.

Upvotes: 2

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