Check whether or not the current thread is the main thread

Question

Have a look at the NSThread API documentation.

There are methods like

- (BOOL)isMainThread

+ (BOOL)isMainThread

and + (NSThread *)mainThread

In Swift3

if Thread.isMainThread {
    print("Main Thread")
}

If you want a method to be executed on the main thread, you can:

- (void)someMethod
{
    dispatch_block_t block = ^{
        // Code for the method goes here
    };

    if ([NSThread isMainThread])
    {
        block();
    }
    else
    {
        dispatch_async(dispatch_get_main_queue(), block);
    }
}

If you want to know whether or not you're on the main thread, you can simply use the debugger. Set a breakpoint at the line you're interested in, and when your program reaches it, call this:

(lldb) thread info

This will display information about the thread you're on:

(lldb) thread info thread #1: tid = 0xe8ad0, 0x00000001083515a0 MyApp`MyApp.ViewController.sliderMoved (sender=0x00007fd221486340, self=0x00007fd22161c1a0)(ObjectiveC.UISlider) -> () + 112 at ViewController.swift:20, queue = 'com.apple.main-thread', stop reason = breakpoint 2.1

If the value for queue is com.apple.main-thread, then you're on the main thread.

The following pattern will assure a method is executed on the main thread:

- (void)yourMethod {
    // make sure this runs on the main thread 
    if (![NSThread isMainThread]) {
        [self performSelectorOnMainThread:_cmd/*@selector(yourMethod)*/
                               withObject:nil
                            waitUntilDone:YES];
        return;
    }
    // put your code for yourMethod here
}

Two ways. From @rano's answer,

[[NSThread currentThread] isMainThread] ? NSLog(@"MAIN THREAD") : NSLog(@"NOT MAIN THREAD");

Also,

[[NSThread mainThread] isEqual:[NSThread currentThread]] ? NSLog(@"MAIN THREAD") : NSLog(@"NOT MAIN THREAD");