How can I tell when a screen is shown in iOS?

Question

I have a simple Tab bar navigation app

-------------------
|                 |
|                 |
|                 |
|                 |
|                 |
|                 |
|                 |
|       (o)       |
|                 |
[ Tab 1 ]_[ Tab 2 ]

When Tab 1 is touched, Screen 1 is shown. When Tab 2 is pressed, Screen 2 is shown.

How can I call an action each time a screen is shown (not just on the viewDidLoad event).

Do I have to listen to the touch event on the Tab bar or is there an event in the ViewController I can listen for?

"When Tab 1 is touched, Screen 1 is shown. When Tab 2 is pressed, Screen 2 is shown?" Yes it is. — Manlio, Feb 27 '12 at 15:06
*is there an event in the ViewController I can listen for?* What did you find when you looked at the [documentation for UIViewController](https://developer.apple.com/library/ios/#DOCUMENTATION/UIKit/Reference/UIViewController_Class/Reference/Reference.html)? — Caleb, Feb 27 '12 at 15:17

score 5 · Accepted Answer · edited May 23 '17 at 12:03

5

viewDidLoad gets called when the nib is being loaded into memory. When a view is actually appearing on the screen, viewDidAppear gets called.

[Edit]

This is the (natural) order of callbacks: initWithNibName:Bundle: -> awakeFromNib -> loadView -> viewDidLoad -> viewWillAppear -> viewDidAppear

[Edit2]

see here for an excellent explanation: https://stackoverflow.com/a/5109277/347353

edited May 23 '17 at 12:03

Community

1
1

answered Feb 27 '12 at 15:01

Pooria Azimi

8,585
5
30
41

sch · Answer 2 · 2012-02-27T15:10:25.693

2

You can use viewWillAppear and viewDidAppear.

viewWillAppear

Notifies the view controller that its view is about to be added to a view hierarchy.
viewDidAppear

Notifies the view controller that its view was added to a view hierarchy.

edited Feb 27 '12 at 15:10

answered Feb 27 '12 at 15:04

sch

27,436
3
68
83

I know — you had the correct quotes but swapped over. I just swapped them back :) – Amy Worrall Feb 27 '12 at 15:09
@Amorya - I just swapped them too at the same moment as you :) Thanks. – sch Feb 27 '12 at 15:11
Ah, all's well that ends well. Good answer btw. – Amy Worrall Feb 27 '12 at 15:12

How can I tell when a screen is shown in iOS?

2 Answers2