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Question

I have this code and I want to change content of a label with a text from web

var url = NSURL(string: "SOME_URL");
var task = NSURLSession.sharedSession().dataTaskWithURL(url!, completionHandler: {
(data, response, error) -> Void in


if error == nil {
self.label.text = "SOME_TEXT" // not working

dispatch_async(dispatch_get_main_queue()){
     self.label.text = "SOME_TEXT" // working
     }

}
else{
    println("Error")
}

})

task.resume()

Why do I have to use dispatch_async(dispatch_get_main_queue()){ ... } to change the content of the label ?

Answer 1

The principe of iOS: you have only 1 thread that can modify your UI. It's called UI Thread. Whenever you want to change the UI content, all functions that change your UI content must be called in the UI Thread. In your case, the handler is executed in a background thread, hence you have to put the self.label.text = "SOME_TEXT" in the UI Thread.

Answer 2

It's a very common pattern in GUI programming.

Basically the main message loop runs single threaded without the cost of synchronizing everything. That means that if you want to interact with your window you need to do it on the main GUI thread -- that's what dispatch does, it posts a message on the message queue and the loop interprets it, bringing your code on its thread.

It's both for simplicity and for performance.