锁定在左右边缘的视图可能会随着高度的变化而变化……我该如何加以约束？

Question

I am new to interface builder.

I want to create a view that is centered horizontally and vertically. The view has to be the width of the screen but the height can vary slightly. How do I constrain that view?

I have added trailing and leading space to superview, to lock the right and left edges to the screen border. I have added center Y to superview.

Xcode whines "need constrain for Y position or height". As far as I know a constrain I have added to center Y on superview is exactly that. How do I solve that?

Answer 1

You always need two constraints for X, and two constraints for Y. The constraints determine position and size . You have both constraints for X, but only a position constraint for Y. You need an additional constraint for Y that determines the vertical size of the view.

When using auto layout, you don't programmatically change the height of a view. Instead, you programmatically change the constraint that determines the height of the view. It's a level of indirection that's confusing at first (and annoying forever), but that's the way auto layout works.

You can control-drag from a constraint in storyboard into the code to create an IBOutlet for the constraint, just like you would create an IBOutlet for the view itself. Then you can change the properties of the constraint (eg the constant property) at runtime to change the height of the view.