Extra constraints are logged when I NSLog myBtn.constraints

Question

I have a button inside a view. I have set image for my button in interface builder. The constraints that I have added to my button in interface builder is:

Right space to superview = 10

top space to superview = 10

bottom space to superview >= 10

left space to a label >= 10

All thing work fine and I have no problem with my layout. But when I log the button constraints with this code:

NSLog(@"constraints for btnBack is: %@", self.btnBack.constraints);

the console logs:

constraints for btnBack is:(

"<NSContentSizeLayoutConstraint:0x7c35c400 H:[UIButton:0x7c3588a0(102)] Hug:250 CompressionResistance:750>"

"<NSContentSizeLayoutConstraint:0x7c35c450 V:[UIButton:0x7c3588a0(92)] Hug:250 CompressionResistance:750>")

I know that the top and left and right and bottom constraints shouldn't log here because they are the superview constraints not the button constrains. But here I didn't add any width and height constrains to my button(even in code).

Why constraints height and width are logged in the console?

Answer 1

If a view has a natural width based on its content, it returns that width as intrinsicContentSize.width. Otherwise, it sets intrinsicContentSize.width to UIViewNoIntrinsicMetric.

Similarly, if it has a natural height, it returns that width as intrinsicContentSize.height. Otherwise, it sets intrinsicContentSize.height to UIViewNoIntrinsicMetric.

A button has a natural size (both width and height), determined by its title, font, image, and maybe other details. So its intrinsicContentSize.width and intrinsicContentSize.height are both valid sizes, not UIViewNoIntrinsicMetric.

When intrinsicContentSize.width != UIViewNoIntrinsicMetric, auto layout installs a special constraint of type NSContentSizeLayoutConstraint on the view. Similarly, when intrinsicContentSize.height != UIViewNoIntrinsicMetric, auto layout installs an NSContentSizeLayoutConstraint on the view.

For the details of what these special constraints do, see this answer.