How to use multithread in Swift - swift

Question

I have two tasks : task1 and task2. I want to execute task2 after task1 finishes.
let globalQueueDefault = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0)
dispatch_sync(globalQueueDefault){
self.activityIndicatorView.hidden = false
self.activityIndicatorView.startAnimating()
task1()
sleep(6)
dispatch_sync(globalQueueDefault) { () -> Void in
task2()
}
}
I searched in internet, I find NSLock,NSConditionLock and objc_sync_enter...I have try them, but it doesn't work...
let lock = NSLock()
let globalQueueDefault = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0)
dispatch_sync(globalQueueDefault){
self.activityIndicatorView.hidden = false
self.activityIndicatorView.startAnimating()
self.lock.lock()
task1()
self.lock.unlock()
sleep(6)
dispatch_sync(globalQueueDefault) { () -> Void in
self.lock.lock()
task2()
self.lock.unlock()
}
}
I also tried NSConditionLock and objc_sync_enter...It doesn't work. How I can use lock in swift ? Could you give me a example base on my code? Thank you.
PS: I don't want to use callback here...because I have tried it, I think multithread is more closer to my answer, Thank you.

Answer 1

I'm going out on a limp and making some guesses about your program structures. The first problem with your code is that it's trying to access a view on a background thread. GUI elements should always be accessed on the main thread. The second problem is sleep: don't use it to write concurrent code. It makes assumptions about how the asynchronous task is going take. You should treat that time as unknown and use a sync pattern or a call back.
Since you mentioned that task1() download JSON, it's likely asynchronous. Here's how I'd do it:
func task1(finish: () -> Void) {
// Set up your connection to the website
let task = NSURLSession.sharedSession().dataTaskWithRequest(request) {
// Handle the response, parse the json, etc
...
// Now call the completion handler
finish()
}
}
func task2() {
// Do whatever here
}
// In the function that triggers the JSON download
func downloadJSON() {
self.activityIndicatorView.hidden = false
self.activityIndicatorView.startAnimating()
task1(task2)
}