NSLayoutConstraint

NSLayoutConstraint is a fundamental class in Apple's UIKit and AppKit frameworks used for defining the layout of user interface elements. It represents a single constraint, which is a rule that dictates a relationship between two or more attributes of views. These constraints are the building blocks of Auto Layout, a dynamic view layout system.

A constraint is typically defined by specifying a first item, a first attribute, a relation, a second item (which can be nil), a second attribute, and a multiplier and constant. The relation can be less than or equal to, greater than or equal to, or equal to. For example, a constraint might state that the leading edge of view A should be equal to the leading edge of view B plus 20 points. This ensures that view A is always positioned 20 points to the right of view B.

When constraints are applied to a view hierarchy, the Auto Layout engine resolves these relationships to determine the final size and position of each view. This system allows for flexible and adaptive layouts that can adjust automatically to different screen sizes and orientations. NSLayoutConstraint objects are usually created programmatically or by using Interface Builder. The system automatically generates many constraints for you when using Auto Layout in Interface Builder.