Monday, August 11, 2014

Swift Tip - Singleton Pattern

When you are developing iOS applications, you often need to share data between View Controllers. One way to do that would be to create singletons for sharing data. Here is the singleton pattern in Swift that you can use:

class MySingleton {
    //---typed property to return a shared instance of the current class---
    class var sharedInstance: MySingleton {
    struct Static {
        //---to contain the one and only instance of this class---
        static var instance: MySingleton!
        //---a token to ensure that the class is only instantiated once---
        static var token: dispatch_once_t = 0
        //---executes a block object once and only once for the lifetime of an application---
        dispatch_once(&Static.token) {
            //---creates an instance of this class---
            Static.instance = MySingleton()
        //---returns the instance of this class---
        return Static.instance
    //---the properties in your class---
    var num:Int = 0
    var str:String = ""


In the above code snippet, MySingleton is a singleton, i.e. you can only create one and only one instance of this class. To make use of this singleton, use the following statements:

        var s1 = MySingleton.sharedInstance
        s1.num = 5
        s1.str = "Hello Singleton!"
        println(s1.num)   //---5---
        println(s1.str)   //---Hello Singleton!---
        var s2 = MySingleton.sharedInstance
        println(s2.num)   //---5---
        println(s2.str)   //---Hello Singleton!---

Have fun with Swift!

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