tfrank377 tfrank377 - 11 months ago 220
Swift Question

Modifying struct instance variables within a Dispatch closure in Swift

I'm using the

version of Swift. I cannot seem to get around this issue, I've tried using
in various places, but I still have the following error:

Closure cannot implicitly capture a mutating self parameter

To better explain, here is a simple example:

public struct ExampleStruct {
let connectQueue = dispatch_queue_create("connectQueue", nil)
var test = 10

mutating func example() {
if let connectQueue = self.connectQueue {
dispatch_sync(connectQueue) {
self.test = 20 // error happens here

Something must have changed in these Swift binaries that is now causing my previously working code to break. A workaround I want to avoid is making my struct a class, which does help in fixing the issue. Let me know if there is another way.


I cannot test it, because I'm not using a build with that error, but I'm pretty sure by capturing self explicitly you can fix it:

dispatch_sync(connectQueue) { [self] in
    self.test = 20

EDIT: Apparently it doesn't work, maybe you can try this (not very nice tbh):

var copy = self
dispatch_sync(connectQueue) {
    copy.test = 20
self = copy

If you want to read more on why, here is the responsible Swift proposal.

The new dispatch API makes the sync method @noreturn so you wouldn't need the explicit capture:

connectQueue.sync {
    test = 20