自动布局-添加多个约束的效果

Question

I look over this article, and i have a question.

Consider following constraints on text field object:

First Name Text Field.Top >= Top Layout Guide.Bottom + 20.0
First Name Text Field.Top = Top Layout Guide.Bottom + 20.0 (Priority 249)

I'm confused. If i add greater then or equal constraint, isnt that mean that i already say - set this distance greater then or equal 20? What the point to add second constraint - First Name Text Field.Top = Top Layout Guide.Bottom + 20.0 (Priority 249) . It also have lowered priority, so i suppose, it will never evaluated, because first constraint - First Name Text Field.Top >= Top Layout Guide.Bottom + 20.0 have greater priority.

Could someone explain that logic to me? Is there any point to add 2 constraint with different priorities? Any point to add greater-then-or-equal constraint and constraint, that equal to same value (20 in case)? Thanks.

Answer 1

A greater than or equal constraint is ambiguous by its very nature. You have told Auto Layout to make the distance between your first name text field and the top layout guide's bottom at least 20 . Note that 20 , 30 , and 50 would all satisfy that constraint.

By adding the second constraint, you tell Auto Layout that you'd prefer it to be 20 . By making the priority 249 you tell Auto Layout that this request is low (priorities go to 1000 ) and should be broken first if it needs to break a constraint to make things work.

So, using these 2 constraints, Auto Layout will make the distance 20 if it can. If not, it will use a value greater than 20 .