View frame changes between viewWillAppear: and vie

问题:

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?

回答1:

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.

回答2:

From the documentation:

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.

As a result the frames of the subviews aren't yet set in the viewWillAppear:

The appropriate method to modify your UI before the view is presented to the screen is:

viewDidLayoutSubviews

Notifies the view controller that its view just laid out its subviews.

回答3:

call

self.scrollView.layoutIfNeeded()

in your viewWillAppear method. Afterwards you can access it's frame and it will have the same value as you print in viewDidAppear

回答4:

In my case, moving all frame related methods to

override func viewWillLayoutSubviews()

worked perfectly(I was trying to modify constraints from storyboard).