Jumhyn Jumhyn - 1 month ago 29
Objective-C Question

View frame changes between viewWillAppear: and viewDidAppear:

I have discovered a strange behavior in my application, where a connected

IBOutlet
has its connected view's frame between the calls in my view controller to
viewWillAppear:
and
viewDidAppear:
. Here is the relevant code in my
UIViewController
subclass:

-(void)viewWillAppear:(BOOL)animated {
NSLog(@"%@", self.scrollView);
}

-(void)viewDidAppear:(BOOL)animated {
NSLog(@"%@", self.scrollView);
}


and the resulting log output:

MyApp[61880:c07] <UIScrollView: 0x1057eff0; frame = (0 0; 0 0); clipsToBounds = YES; autoresize = TM+BM; gestureRecognizers = <NSArray: 0x10580100>; layer = <CALayer: 0x1057f210>; contentOffset: {0, 0}>
MyApp[61880:c07] <UIScrollView: 0x1057eff0; frame = (0 44; 320 416); clipsToBounds = YES; autoresize = TM+BM; gestureRecognizers = <NSArray: 0x10580100>; layer = <CALayer: 0x1057f210>; contentOffset: {0, 0}>


Which clearly shows that the frame is changing between the two calls. I wanted to do setup with the view in the
viewDidLoad
method, but if the content is not available for me to change until it is on the screen, that seems pretty useless. What could be happening?

Answer

Autolayout made a huge change in how we design and develop the GUI of our views. One of the main differences is that autolayout doesn't change our view sizes immediately, but only when is triggered, that means at a specific time, but we can force it to recalculate our constraints immediately or mark them as "in need" of layout. It works like -setNeedDisplay.
The big challenge for me was to understand and accept that, we do not need to use autoresizing masks anymore, and frame has become a useless property in placing our views. We do not need to think about view position anymore, but we should think how we want to see them in a space related to each other.
When we want to mix old autoresizing mask and autolayout is when problems arise. We should think about autolayout implementation really soon and try to avoid to mix the old approach in a view hierarchy based on autolayout.
Is fine to have a container view that uses only autoresizing masks, such as a main view of a view controller, but is better if we do not try to mix.
I never used storyboard, but most proably it is correct. Using Autolayout, frame of your views are set when the autolayout engine starts its calculation. Try to ask the same thing right after super of - (void)viewDidLayoutSubviews method of your view controller.
This method is called when the autolayout engine has finished to calculate your views' frames.