UIStackView - Distribute views evenly from the centre - ios

Question

I have a UIStackView,inside a UIScrollView to show dynamically added subviews horizontally arranged. Current solution will start displaying items from the left, I would like to start distributing items from the centre without changing the width and height of the subviews. How do I do that? Im also open to solutions which doesn't use UIStackView as well. That way I could support devices < iOS9.
(Current)
(Expected)

Answer 1

Short answer :
ScrollView constraints
Leading >= 0, Trailing >= 0, Top >= 0, Bottom >= 0
Center X and Center Y
StackView constraints
Leading = 0, Trailing = 0, Top = 0, Bottom = 0
StackView width = ScrollView width (priority low :250)
StackView height = ScrollView height
Long answer
Firstly, your structure is good, we have :
UIScrollView
UIStackView (horizontal)
Subviews
So, to reach the goal we should :
Center the UIScrollView
Set the contentSize of the UIScrollView equal to the intrinsic content
size of the UIStackView
Here is how to do it :
Step 1: Center the frame of the UIScrollView
CenterY and CenterX constraints used to center the frame of the UIScrollView
Leading Space >= 0, Trailling Space >= 0, Top Space >= 0, Bottom Space >= 0 are used to prevent the frame of the UIScrollView to exceed the frame of the parent view
I used placeholder intrinsic size to don't show errors related to the contentSize of the UIScrollView (because we don't have yet the subviews so the contentSize).
Now, the frame of our UIScrollView is Ok, go to the next step :)
Step 2: add the horizontal UIStackView
Top, Bottom, Leading, Trailing constraints are used to fix
UIStackView frame
Equal Height and equal width used to calculate the
contentSize of the UIScrollView
PS. Any change in the frame of the UIStackView, change the contentSize of the UIScrollView
Step 3: add subviews
Because we use Fill Distribution in the UIStackView all subviews must have a intrinsic content size (or height and width constraints (not preferred)).
For example, if we use Fill Equally, only one subview with intrinsic content size (or height and width constraints (not preferred)) sufficient, the other subviews size will be equal to this one.
For Example: I will add 3 labels (please remove the placeholder intrinsic size of the UIScrollView)
It works !! no, no, not yet try to add fourth and five labels :)
Why ?
To understand we will calculate the frame of each element of the view with two examples :
The parent view size : 200, 200
The first label intrinsic content size : 120, 50
the second label intrinsic content size : 150, 50
First example (only the first label in the UIStackView)
UIStackView intrinsic content size = 120, 50
UIScrollView contentSize = 120, 50
UIScrollView frame = 40, 75, 120, 50
All frames are OK
Second example (with the two labels)
UIScrollView frame = 0, 0, 200, 50
UIScrollView contentSize = 200, 50
UIStackView intrinsic content size = 200, 50
So, the UIStackView can't show correctly the two labels (because the width of UIStackView lower than the two labels width), and we don't have the scroll because, the UIStackView constraint width is equal to UIScrollView width's.
It works when the UIStackView intrinsic content size is lower than the max UIScrollView frame.
To Fix that, we change the priority of the width constraint to a value lower than the UIStackView intrinsic content size priority value, and all works fine :)
Hope that helps.
Code source