在習慣了原來的OC開發，可能將多個block做為函數參數，亦或是存入集合類型（NSDictionary、NSArray）的實例中，但在Swift中將閉包存入一個Dictionary或Array中是不行的。其解決方案是用 unsafeBitCast 函數將Swift閉包轉為Objective-C的兼容的對象，如下：

//使用`@convention(block)` 關鍵字，Swift2.0以后@objc_block屬性由@convention(block)取代。
typealias CompletionBlock = @convention(block) () -> Void

        let aBlock:CompletionBlock = {
            print("Hello world!")
        }
        let nsArray = NSMutableArray()
        let blockObject = unsafeBitCast(aBlock, AnyObject.self)
        nsArray.addObject(blockObject)
        
        let closureObject = nsArray[0]
        let closure = unsafeBitCast(closureObject, CompletionBlock.self)
        closure()