Constraint Attributes
Constraints use a limited geometric vocabulary consisting of attributes and relations. Attributes are the “nouns” of the constraint system, describing positions within a view’s alignment rectangle. Alignment rectangles will be described in detail shortly, but for the moment, you can think of them as being closely related to the view’s frame. Relations are “verbs,” specifying how the attributes compare to each other.
The attribute nouns speak to physical geometry. Constraints offer the following view attribute vocabulary:
left, right, top, and bottom—The edges of a view’s alignment rectangle on the left, right, top, and bottom ...
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