Making some code only run once

Question

I have some code that I would like to run only once in my MainViewController. It should run every time the user starts the app, but only after the MainViewController has loa

Answer 1

I don't see any problem with that code. I like using a BOOL (as you did) and then assigning either YES/NO or TRUE/FALSE just so that the code reads more nicely. I would assign TRUE to firstRun in didFinishLaunching, and set it FALSE after the code executes. In my code these type of conditionals usually look like this:

@synthesize firstRun;

-(void)viewDidLoad {
   [super viewDidLoad];
    if (firstRun) {
        // code to run only once goes here
        firstRun = FALSE;
    }
}

Answer 2

for Swift2.2,Xcode 7.3:

static var token: dispatch_once_t = 0

dispatch_once(&YourClassName.token) {
  NSLog("Do it once")
}

Watch out "YourClassName.token"

Answer 3

Swift 1,2:

static var token: dispatch_once_t = 0

dispatch_once(&token) {
  NSLog("Do it once")
}

Objective-C

static dispatch_once_t once;
dispatch_once(&once, ^ {
  NSLog(@"Do it once");
});

Swift 3,4:

dispatch_once is no longer available in Swift. In Swift, you can use lazily initialized globals or static properties and get the same thread-safety and called-once guarantees as dispatch_once provided Apple doc

let myGlobal = { … global contains initialization in a call to a closure … }()
_ = myGlobal  // using myGlobal will invoke 
              // the initialization code only the first time it is used.

Answer 4

With Swift2.0, Xcode 7.0

var token: dispatch_once_t = 0

override func viewDidLoad() {
    super. viewDidLoad()
    dispatch_once(&token) {
        println("This is printed only on the first call to test()")
    }
    println("This is printed for each call to test()")
}