Check whether or not the current thread is the main thread

Question

Is there any way to check whether or not the current thread is the main thread in Objective-C?

I want to do something like this.

  - (void)someMethod         


        

Answer 1

Swift Version

---

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

Answer 2

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

Answer 3

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");

Answer 4

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") }
}

Answer 5

Have a look at the NSThread API documentation.

There are methods like

- (BOOL)isMainThread

+ (BOOL)isMainThread

and + (NSThread *)mainThread

Answer 6

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
}