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

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 = {
        let key = DispatchSpecificKey()
        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