Replaces a call to *addSubview* with an implementation that ensures it is always invoked on the main thread.

UIView+MainThread.swift

import UIKit

/**
 Swizzles `addSubview` of `UIView`, replacing it with an implementatio that ensures it is always 
 invoked on the main thread.
 
 Call this method in your application entry point, e.g. the `UIApplicationDelegate`.
 */
func swizzleAddSubview() {
    let clazz = UIView.self
    let method: Method = class_getInstanceMethod(clazz, #selector(UIView.addSubview(_:)))
    let swizzled: Method = class_getInstanceMethod(clazz, #selector(UIView.addSubviewOnMainThread(_:)))
    
    method_exchangeImplementations(method, swizzled)
}

extension UIView {
    
    /**
     Forwards a call to `addSubview` on the main thread. If this method is called on the main 
     thread, it will call `addSubview` immediately. If it is invoked on another thread, it will 
     dispatch a call to `addSubview` on the main thread using GCD.
     
     - Parameter view: Child view to add
     */
    @objc dynamic func addSubviewOnMainThread(view: UIView) {
        // Since we've replaced the method signature of `addSubview` with this one, it means that 
        // to access the original `addSubview`, we must call `addSubviewOnMainThread`, because 
        // the two implementations swapped names.
        if (NSThread.isMainThread()) {
            addSubviewOnMainThread(view)
        } else {
            dispatch_sync(dispatch_get_main_queue()) {
                self.addSubviewOnMainThread(view)
            }
        }
    }
    
}