Check whether or not the current thread is the main thread

Have a look at the NSThread API documentation.

There are methods like

- (BOOL)isMainThread

+ (BOOL)isMainThread

and + (NSThread *)mainThread

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In Swift3

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

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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);
    }
}

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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.

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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
}

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void ensureOnMainQueue(void (^block)(void)) {

    if ([[NSOperationQueue currentQueue] isEqual:[NSOperationQueue mainQueue]]) {

        block();

    } else {

        [[NSOperationQueue mainQueue] addOperationWithBlock:^{

            block();

        }];

    }

}

note that i check the operation queue, not the thread, as this is a more safer approach

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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");

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For Monotouch / Xamarin iOS you can perform the check in this way:

if (NSThread.Current.IsMainThread)
{
    DoSomething();
}
else
{
    BeginInvokeOnMainThread(() => DoSomething());
}

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Details

Swift 5.1, Xcode 11.3.1

Solution 1. Detect any queue

Get current DispatchQueue?

Solution 2. Detect only main queue

import Foundation

extension DispatchQueue {

    private struct QueueReference { weak var queue: DispatchQueue? }

    private static let key: DispatchSpecificKey<QueueReference> = {
        let key = DispatchSpecificKey<QueueReference>()
        let queue = DispatchQueue.main
        queue.setSpecific(key: key, value: QueueReference(queue: queue))
        return key
    }()

    static var isRunningOnMainQueue: Bool { getSpecific(key: key)?.queue == .main }
}

Usage

if DispatchQueue.isRunningOnMainQueue { ... }

Sample

func test(queue: DispatchQueue) {
    queue.async {
        print("--------------------------------------------------------")
        print("queue label: \(queue.label)")
        print("is running on main queue: \(DispatchQueue.isRunningOnMainQueue)")
    }
}

test(queue: DispatchQueue.main)
sleep(1)
test(queue: DispatchQueue.global(qos: .background))
sleep(1)
test(queue: DispatchQueue.global(qos: .unspecified))

Result (log)

--------------------------------------------------------
queue label: com.apple.root.background-qos
is running on main queue: false
--------------------------------------------------------
queue label: com.apple.root.default-qos
is running on main queue: false
--------------------------------------------------------
queue label: com.apple.main-thread
is running on main queue: true

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Swift Version

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

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let isOnMainQueue = (dispatch_queue_get_label(dispatch_get_main_queue()) == dispatch_queue_get_label(DISPATCH_CURRENT_QUEUE_LABEL))

check this answer from https://stackoverflow.com/a/34685535/1530581

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Here is a way to detect what the current queue is
extension DispatchQueue {
    //Label of the current dispatch queue.
    static var currentQueueLabel: String { String(cString: __dispatch_queue_get_label(nil)) }

    /// Whether the current queue is a `NSBackgroundActivityScheduler` task.
    static var isCurrentQueueNSBackgroundActivitySchedulerQueue: Bool { currentQueueLabel.hasPrefix("com.apple.xpc.activity.") }

    /// Whether the current queue is a `Main` task.
    static var isCurrentQueueMainQueue: Bool { currentQueueLabel.hasPrefix("com.apple.main-thread") }
}

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UPDATE: seems that is not correct solution, according to queue.h header as mentioned @demosten

The first thought was brought to me, when I was needed this functionality was the line:

dispatch_get_main_queue() == dispatch_get_current_queue();

And had looked to the accepted solution:

[NSThread isMainThread];

mine solution 2.5 times faster.

PS And yes, I'd checked, it works for all threads

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