iOS - Ensure execution on main thread [duplicate]

Question

I want to know how to call my function on the main thread.

How do I make sure my function is called on the main thread?

(this follows a previous question of mine).

Asthenia

I would imagine only classes that call your delegate methods using [NSObject performSelectorOnMainThread] can be considered safe. Otherwise if the object just calls [NSObject performSelector] your delegate methods are running in the context of the caller's thread. – Parthenos

Answer 1

there any rule I can follow to be sure that my app executes my own code just in the main thread?

Typically you wouldn't need to do anything to ensure this — your list of things is usually enough. Unless you're interacting with some API that happens to spawn a thread and run your code in the background, you'll be running on the main thread.

If you want to be really sure, you can do things like

[self performSelectorOnMainThread:@selector(myMethod:) withObject:anObj waitUntilDone:YES];

to execute a method on the main thread. (There's a GCD equivalent too.)

Sibeal

How would you go about the case where you spawn a thread to run code in the background? – Wyandotte

Answer 2

This will do it:

[[NSOperationQueue mainQueue] addOperationWithBlock:^ {

   //Your code goes in here
   NSLog(@"Main Thread Code");

}];

Corrosive

It does, thanks. Do you know if this is subject to the same kind of restrictions as meccans' answer (which is the GCD version I guess), say 'don't call this from the main thread'? – Asthenia

Well I just find this way easier because if you do it this way you don't have to make a new method every time you want to do something on the main thread. – Corrosive

This helped me with a socket programming mysterious hang when upgrading my XCode to the latest iOS 6.1 simulator. The code was writing to a socket immediately after reading some data from it. For some strange reason, that worked like a charm in iOS 5.1 simulator, but not in 6.0 or 6.1 - using this suggestion to execute the write operation on the socket in the main thread solved my problem, and it works in all simulator versions and devices! – Walkyrie

So I was using the code block in the above answer: (dispatch_sync(dispatch_get_main_queue(), ^{ //Your code goes in here NSLog(@"Main Thread Code"); });) since gcd is known to work well however, I ran into an issue where the main thread wouldn't take priority and the app would stall in one of the iPod5's I have. I switched to using the NSOperationQueue instead and it worked fine. Weird odd case but I thought, I'd document it here. – Soften

So this, unlike dispatch_get_main_queue, you can do directly on the main thread? – Glyn

This is better as I can "throw" whatever code inside without having to "design" a function for a performselector. Also, whatever that's executed from within there goes on the main thread as well. Thanks! – Servo

Although I prefer this option (looks nicer) it is slow with hard UI (e.g. progress updates lag). For that you should use dispatch_async. – Caras

Answer 3

When you're using iOS >= 4

dispatch_async(dispatch_get_main_queue(), ^{
  //Your main thread code goes in here
  NSLog(@"Im on the main thread");       
});

Ludhiana

Just so the OP is clear, this is not something you'd want to do if you're already on the main thread. – Fabe

What horrible thing will happen, exactly? (just for curiosity) – Epizoic

the main thread freezes! here is the source: https://mcmap.net/q/24016/-dispatch_sync-on-main-queue-hangs-in-unit-test – Ludhiana

Also see #5662860 (you should be able to use dispatch_async and not have a problem when on the main thread). – Additional

Would using dispatch_sync work without issue too? – Stavros

yes, it would work without issues if you call dispatch_sync on a background thread, but you will run into a deadlock if you call dispatch_sync on the main thread with dispatch_get_main_queue – Ludhiana

Answer 4

i think this is cool, even tho in general its good form to leave the caller of a method responsible for ensuring its called on the right thread.

if (![[NSThread currentThread] isMainThread]) {
    [self performSelector:_cmd onThread:[NSThread mainThread] withObject:someObject waitUntilDone:NO];
    return;
}

Tallulah