How to check current thread in Swift 3?

Question

Looks like it's simply Thread.isMainThread in Swift 3.

Thread.isMainThread will return a boolean indicating if you're currently on the main UI thread. But this will not give you the current thread. It will only tell you if you are on the main or not.

Thread.current will return the current thread you are on.

I've made an extension to print the thread and queue:

extension Thread {
    class func printCurrent() {
        print("\r⚡️: \(Thread.current)\r" + "🏭: \(OperationQueue.current?.underlyingQueue?.label ?? "None")\r")
    }
}

Thread.printCurrent()

The result would be:

⚡️: <NSThread: 0x604000074380>{number = 1, name = main}
🏭: com.apple.main-thread

Swift 4 and above:

Thread.isMainThread returns Bool stating that if the user is on Main Thread or Not, in case if someone wants to print the name of the queue/thread this extension will be helpful

extension Thread {

    var threadName: String {
        if let currentOperationQueue = OperationQueue.current?.name {
            return "OperationQueue: \(currentOperationQueue)"
        } else if let underlyingDispatchQueue = OperationQueue.current?.underlyingQueue?.label {
            return "DispatchQueue: \(underlyingDispatchQueue)"
        } else {
            let name = __dispatch_queue_get_label(nil)
            return String(cString: name, encoding: .utf8) ?? Thread.current.description
        }
    }
}

How to use:

print(Thread.current.threadName)

When using GCD you can use dispatchPrecondition to check a dispatch condition necessary for further execution. This can be useful if you want to guarantee your code execution on correct thread. For example:

DispatchQueue.main.async {
    dispatchPrecondition(condition: .onQueue(DispatchQueue.global())) // will assert because we're executing code on main thread
}

In latest Swift 4.0 ~ 4.2, we can use Thread.current

See Returns the thread object representing the current thread of execution

Swift 5+

Normally, we only need to know which queue the code is dispatched. So I separate the threadName and queueName into different properties to make it more clear.

extension Thread {
    var threadName: String {
        if isMainThread {
            return "main"
        } else if let threadName = Thread.current.name, !threadName.isEmpty {
            return threadName
        } else {
            return description
        }
    }
    
    var queueName: String {
        if let queueName = String(validatingUTF8: __dispatch_queue_get_label(nil)) {
            return queueName
        } else if let operationQueueName = OperationQueue.current?.name, !operationQueueName.isEmpty {
            return operationQueueName
        } else if let dispatchQueueName = OperationQueue.current?.underlyingQueue?.label, !dispatchQueueName.isEmpty {
            return dispatchQueueName
        } else {
            return "n/a"
        }
    }
}

Use cases:

DispatchQueue.main.async {
   print(Thread.current.threadName)
   print(Thread.current.queueName)
}
// main
// com.apple.main-thread

DispatchQueue.global().async {
   print(Thread.current.threadName)
   print(Thread.current.queueName)
}
// <NSThread: 0x600001cd9d80>{number = 3, name = (null)}
// com.apple.root.default-qos