Swift: How to let background thread wait for user input? - swift

Question

I have a background thread which needs some user input. Since it is not recommended to call an NSAlert window from a background thread, I like to do this in the main thread. But how can I let the background thread wait till the NSAlert window is closed?
let priority = DISPATCH_QUEUE_PRIORITY_DEFAULT
dispatch_async(dispatch_get_global_queue(priority, 0)) {
// some background task
dispatch_async(dispatch_get_main_queue()) {
// UI task asking for user input:
let alert = NSAlert()
alert.messageText = "Some text"
alert.informativeText = "Some information"
alert.addButtonWithTitle("Yes")
alert.addButtonWithTitle("No")
result = alert.runModal()
}
// some background task, treating user input (Yes/No)
}

Answer 1

The runModal method is synchronous. If you want to wait for it to finish, rather than dispatching asynchronously to the main queue with dispatch_async, you can most easily achieve what you asked for by dispatching it synchronously with dispatch_sync, e.g.:
let queue = dispatch_get_global_queue(QOS_CLASS_DEFAULT, 0)
dispatch_async(queue) {
// some background task
var result: NSModalResponse!
dispatch_sync(dispatch_get_main_queue()) { // note, `dispatch_sync`
// UI task asking for user input:
let alert = NSAlert()
...
result = alert.runModal()
}
// some background task, treating user input (Yes/No)
}
A more subtle problem rests with the synchronous nature runModal. This will block the main thread. This is not generally a good idea. It can interfere with other tasks your app might be performing (timers, animations, network operations using the main queue, etc.) that might require the main thread.
So, instead of the synchronous runModal, you might consider presenting the NSAlert asynchronously with beginSheetModalForWindow, with a completion handler that dispatches additional background processing to the background queue, e.g.:
let queue = dispatch_get_global_queue(QOS_CLASS_DEFAULT, 0)
dispatch_async(queue) {
// some background task
dispatch_async(dispatch_get_main_queue()) { // note, `dispatch_async` is OK here
// UI task asking for user input:
let alert = NSAlert()
...
alert.beginSheetModalForWindow(NSApp.mainWindow!) { result in
dispatch_async(queue) {
// some background task, treating user input (Yes/No)
}
}
}
}
This achieves what you want (triggering additional background tasks when the modal is dismissed), but has the virtue of not blocking the main thread at all.

Answer 2

You can use semaphore.
dispatch_async(dispatch_get_global_queue(priority, 0)) {
// some background task
//Create a semaphore with count = 0
let semaphore = dispatch_semaphore_create(0)
dispatch_async(dispatch_get_main_queue()) {
// UI task asking for user input:
let alert = NSAlert()
alert.messageText = "Some text"
alert.informativeText = "Some information"
alert.addButtonWithTitle("Yes")
alert.addButtonWithTitle("No")
result = alert.runModal()
//Signal semaphore
dispatch_semaphore_signal(semaphore)
}
//Wait for semaphore
dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER)
// some background task, treating user input (Yes/No)
}
You should not "wait" in the main thread. But background threads are other things.