Question

On iOS, if there is a single view app, and a new thread is created using:

[NSThread detachNewThreadSelector:@selector(consumeData:) 
     toTarget:self.consumer withObject:self.queue];

where the consumer is a Consumer object that will process data inside the method consumeData, and the queue is a Queue object, which is where the data comes from for the consumer to process.

But what if the thread needs to check whether a Switch on the main view is set to on or off? That is to toggle whether the Consumer object should do the work or pause at the moment. Should the withObject:self be used instead, so that the whole ViewController reference is passed to the thread, and then the thread will use viewController.view.______ to access the switch’s value, and use viewController.queue to access the queue, or is there a better or alternative method?

Answer 1

Absolutely not. Nothing UI-related can ever be touched from another thread. It’s simply not safe. If the other thread needs to know the switch’s current value, then it needs to call back to the main thread before asking for it.