Question

In short, I would like to, in Objective-C cocoa, program something that functions the same way as the following Java pseudocode:

public class MainClass
{
    public void mainmethod() //Gets called at start of program
    {
        UILabel label = CreateAButton();
        new DaemonClass(label).start();
        //Do things without being interrupted by the Daemon class sleeping or lagging
    }
}

public class DaemonClass extends Thread
{
    public UILabel label;
    public DaemonClass(UILabel lbl)
    {
        setDaemon(true);
        label = lbl;
    }

    public void run()
    {
        int i = 0;
        while(true)
        {
            i++;
            i = i%2;
            UILabel.setText("" + i);
            Thread.sleep(1000);
        }
    }
}

In other words… I’d like to spawn a daemon thread that can be as slow as it likes, without interrupting the progress or speed of any other threads, INCLUDING, the main one.

I have tried using things like the Dispatch Queue, as well as NSThread.

When using either of these, I tried to create a simple label-changer thread that toggled the label’s text from 1 to 0, indefinitely. It appeared to me, the user, to constantly be locked either at 1, or 0, randomly chosen at startup.

When using either of these, and attempting to use [NSThread sleepForTimeInterval:1];, the thread would stop executing all together after the sleepForTimeInterval call.

Furthermore, having skimmed the docs, I picked up on the fact that the run loop is not called while [NSThread sleep... is sleeping!

If it is any help, I was invoking my threads from the - (void)viewDidLoad; method.

My question for you is:

How do I stop [NSThread sleepForTimeInterval:1]; from crashing my thread, OR:
How do I start a daemon thread that invokes a method or code block (preferably a code block!)

P.S. if it makes any difference, this is for iOS

Answer 1

The reason for the problems you’ve seen is most likely that UIKit isn’t thread-safe, i.e. you can only use a UILabel from the main thread. The easiest way to do that is to enqueue a block on the main queue (which is associated with the main thread) using GCD:

dispatch_async(dispatch_get_main_queue(), ^{
    myLabel.text = @"whatever";
});