CODEWITHSUNDEEP
objective-cios
Jezen Thomas
Jezen Thomas

Reputation: 13800

Making some code only run once

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 loaded.

I don't want to run it in -(void)applicationDidFinishLaunching:(UIApplication *)application.

Here's the idea I had:

MainViewController.h

@interface IpadMainViewController : UIViewController <UISplitViewControllerDelegate> {
    BOOL hasRun;
}

@property (nonatomic, assign) BOOL hasRun;

MainViewController.m

@synthesize hasRun;

-(void)viewDidLoad {
    [super viewDidLoad];
    if (hasRun == 0) {
        // Do some stuff
        hasRun = 1;
    }
}

Any ideas?

Upvotes: 31

Views: 18283

Answers (6)

Dylan
Dylan

Reputation: 967

Possible variation (Swift 5):

// Whether this is the first code run since app (re)started
var firstRun: Bool = true

public final class Foo {
    public init() {
        if firstRun {
            // Code that has to be run only once goes
            // HERE
            firstRun = false
        }
    }
}

Upvotes: 1

JPetric
JPetric

Reputation: 3918

Since there is no dispatch_once_t ins swift 3 and above, we can use lazy initialized global variables.

Make a lazy global variable

lazy var doOnlyOnce: () -> Void = {
    .. add code to run only once here ..
    return {}
}()

To run the code:

_ = doOnlyOnce

Upvotes: 0

Andrey
Andrey

Reputation: 1206

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.

Upvotes: 86

PeiweiChen
PeiweiChen

Reputation: 413

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"

Upvotes: -2

Tai Le
Tai Le

Reputation: 9296

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()")
}

Upvotes: -2

Dylan
Dylan

Reputation: 126

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;
    }
}

Upvotes: 7

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