How to do some stuff in viewDidAppear only once?

问题:

I want to check the pasteboard and show an alert if it contains specific values when the view appears. I can place the code into viewDidLoad to ensure it's only invoked once, but the problem is that the alert view shows too quickly. I know I can set a timer to defer the alert's appearance, but it's not a good work-around I think.

I checked the question iOS 7 - Difference between viewDidLoad and viewDidAppear and found that there is one step for checking whether the view exists. So I wonder if there's any api for doing this?

Update: The "only once" means the lifetime of the view controller instance.

回答1:

There is a standard, built-in method you can use for this.

Objective-C:

- (void)viewDidAppear:(BOOL)animated {
    [super viewDidAppear:animated];

    if ([self isBeingPresented] || [self isMovingToParentViewController]) {
        // Perform an action that will only be done once
    }
}

Swift 3:

override func viewDidAppear(_ animated: Bool) {
    super.viewDidAppear(animated)

    if self.isBeingPresented || self.isMovingToParentViewController {
        // Perform an action that will only be done once
    }
}

The call to isBeingPresented is true when a view controller is first being shown as a result of being shown modally. isMovingToParentViewController is true when a view controller is first being pushed onto the navigation stack. One of the two will be true the first time the view controller appears.

No need to deal with BOOL ivars or any other trick to track the first call.

回答2:

rmaddy's answers is really good but it does not solve the problem when the view controller is the root view controller of a navigation controller and all other containers that do not pass these flags to its child view controller.

So such situations i find best to use a flag and consume it later on.

@interface SomeViewController()
{
    BOOL isfirstAppeareanceExecutionDone;
}
@end

@implementation SomeViewController

-(void)viewDidAppear:(BOOL)animated {
    [super viewDidAppear:animated];
    if(isfirstAppeareanceExecutionDone == NO) {
        // Do your stuff
        isfirstAppeareanceExecutionDone = YES;
    }
}

@end

回答3:

If I understand your question correctly, you can simply set a BOOL variable to recognize that viewDidAppear has already been called, ex:

- (void)viewDidAppear {
    if (!self.viewHasBeenSet) { // <-- BOOL default value equals NO

        // Perform whatever code you'd like to perform
        // the first time viewDidAppear is called

        self.viewHasBeenSet = YES; 
    }
}

回答4:

By reading other comments (and based on @rmaddy 's answer), I know this is not what OP asked for, but for those who come here because of title of the question:

extension UIViewController {
    var isPresentingForFirstTime: Bool {
        return isBeingPresented() || isMovingToParentViewController()
    }
}

UPDATE

You should use this method in viewDidAppear and viewWillAppear. (thanks to @rmaddy)

UPDATE 2

This method only works with modally presented view controllers and pushed view controllers. it's not working with a childViewController. using didMoveToParentViewController would be better with childViewControllers.

回答5:

This solution will call viewDidAppear only once throughout the life cycle of the app even if you create the multiple object of the view controller this won't be called after one time. Please refer to the rmaddy's answer above

You can either perform selector in viewDidLoad or you can use dispatch_once_t in you viewDidAppear. If you find a better solution then please do share with me. This is how I do the stuff.

- (void)viewDidLoad {
  [super viewDidLoad];
  [self performSelector:@selector(myMethod) withObject:nil afterDelay:2.0];
}

- (void)viewDidAppear:(BOOL)animated {
  [super viewDidAppear:animated];
  static dispatch_once_t once;
  dispatch_once(&once, ^{
    //your stuff
    [self myMethod];
  });
}

回答6:

You can use this function in ViewDidLoad method

performSelector:withObject:afterDelay:

it will call that function after delay. so you don't have to use any custom timer object. and For once you can use

dispatch_once DCD block.Just performSelector in the dispatch_once block it will call performSelector only once when ViewDidLoad is called

Hope it helps

回答7:

Try to set a BOOL value, when the situation happens call it.

@interface AViewController : UIViewController
   @property(nonatomic) BOOL doSomeStuff;
@end

@implementation AViewController
- (void) viewWillAppear:(BOOL)animated
{
    if(doSomeStuff)
    {
       [self doSomeStuff];
       doSomeStuff = NO;
    }
}

in somewhere you init AViewController instance:

AddEventViewController *ad = [AddEventViewController new];
ad.doSomeStuff = YES;

Not sure why you do this in ViewDidAppear? But if you want doSomeStuff is private and soSomeStuff was called only once, here is another solution by notification:

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(doSomeStuff) name:@"do_some_stuff" object:nil];

- (void) doSomeStuff
{}

Then post when somewhere:

[[NSNotificationCenter defaultCenter] postNotificationName:@"do_some_stuff" object:nil];