Swift extension to add some methods to the NSThread class to run a block on any thread you have a reference to.

NSThread+blocks.swift

//
//  NSThread+blocks.swift
//  Swift extensions
//

import Foundation

public typealias Block = @convention(block) () -> Void

extension NSThread {
    
    /**
     Perform block on current thread
     
     - parameter block: block to be executed
     - parameter wait:
     */
    func performBlock(block: Block, waitUntilDone wait: Bool) {
        
        NSThread.performSelector("runBlock:", onThread: self, withObject: block as? AnyObject, waitUntilDone: wait)
    }

    /**
     Perform block on main thread
     
     - parameter block: block to be executed
     */
    class func performBlockOnMainThread(block: Block) {
        
        NSThread.mainThread().performBlock(block)
    
    }
    
    /**
     Perform block in background thread
     
     - parameter block: block to be executed
     */
    class func performBlockInBackground(block: Block) {
        
        NSThread.performSelectorInBackground("runBlock:", withObject: block as? AnyObject)
    
    }
    
    /**
     Execute block
     
     - parameter block: block to be executed
     */
    class func runBlock(block: Block) {
                
        block()
    
    }
    
    /**
     Perform block on current thread
     
     - parameter block: block to be executed
     */
    func performBlock(block: Block) {
        
        if NSThread.currentThread().isEqual(self) {
            block()
        }
        else {
            self.performBlock(block, waitUntilDone: false)
        }
    }
    
    /**
     Perform block
     
     - parameter block: block to be executed
     - parameter delay: 
     */
    func performBlock(block: Block, afterDelay delay: NSTimeInterval) {
        
        self.performSelector("performBlock:", withObject: block as? AnyObject, afterDelay: delay)
    }
}

Thanks for this. Here's a Swift 3 version:

extension Thread {

    private typealias Block = @convention(block) () -> Void
    
    /**
     Execute block, used internally for async/sync functions.
     
     - parameter block: Process to be executed.
     */
    @objc private func run(block: Block) {
        block()
    }

    /**
     Perform block on current thread asynchronously.
     
     - parameter block: Process to be executed.
     */
    public func async(execute: Block) {
        guard Thread.current != self else { return execute() }
        perform(#selector(run(block:)), on: self, with: execute, waitUntilDone: false)
    }

    /**
     Perform block on current thread synchronously.
     
     - parameter block: Process to be executed.
     */
    public func sync(execute: Block) {
        guard Thread.current != self else { return execute() }
        perform(#selector(run(block:)), on: self, with: execute, waitUntilDone: true)
    }
}

Then you use it like this:

let thread = Thread.current
...
thread.async {
    // Do something
}

Where would I put the extension?