On which thread is a Swift property's didSet{} being executed?

Question

I have defined a property in Swift like:

var foo : String {
  didSet {
    doSomething()
  }
}

So everytime I set foo like foo = "bar" my didSet will be executed.

However I can not find out on which thread didSet is being executed.

For example what happens in this case:

dispatch_async(some_queue, { () -> Void in
     self.foo = "bar"
})

This will cause the didSet code to be executed. Will didSet be run on some_queue or always on the main_queue?

Answer 1

The property observers willSet and didSet are directly called from within the setter method, immediately before/after the new value is stored, so all this executes on the same thread.

You can verify that by setting a breakpoint in didSet and inspecting the stack backtrace, which would look like this:

* thread #2: tid = 0x4df3, 0x000000010024f9eb swtest2`swtest2.foo.didset : Swift.String(oldValue="foo") + 27 at main.swift:6, queue = 'myQueue', stop reason = breakpoint 1.1
  * frame #0: 0x000000010024f9eb swtest2`swtest2.foo.didset : Swift.String(oldValue="foo") + 27 at main.swift:6
    frame #1: 0x000000010024fb91 swtest2`swtest2.foo.setter : Swift.String(newValue="bar") + 177 at main.swift:4
    frame #2: 0x000000010024fe3a swtest2`swtest2.(closure #1) + 42 at main.swift:13
    ...