The NSLayoutAnchor class is a factory class for creating NSLayoutConstraint objects using a fluent API. Use these constraints to programatically define your layout using Auto Layout.
Language
- Swift
- Objective-C
SDKs
- iOS 9.0+
- macOS 10.11+
- tvOS 9.0+
Overview
Instead of creating NSLayoutConstraint objects directly, start with a UIView, NSView, or UILayoutGuide object you wish to constrain, and select one of that object’s anchor properties. These properties correspond to the main NSLayoutAttribute values used in Auto Layout, and provide an appropriate NSLayoutAnchor subclass for creating constraints to that attribute. Use the anchor’s methods to construct your constraint.
// Creating constraints using NSLayoutConstraint
NSLayoutConstraint(item: subview,
attribute: .Leading,
relatedBy: .Equal,
toItem: view,
attribute: .LeadingMargin,
multiplier: 1.0,
constant: 0.0).active = true
NSLayoutConstraint(item: subview,
attribute: .Trailing,
relatedBy: .Equal,
toItem: view,
attribute: .TrailingMargin,
multiplier: 1.0,
constant: 0.0).active = true
// Creating the same constraints using Layout Anchors
let margins = view.layoutMarginsGuide
subview.leadingAnchor.constraintEqualToAnchor(margins.leadingAnchor).active = true
subview.trailingAnchor.constraintEqualToAnchor(margins.trailingAnchor).active = true
As you can see from these examples, the NSLayoutAnchor class provides several advantages over using the NSLayoutConstraint API directly.
The code is cleaner, more concise, and easier to read.
The
NSLayoutAttributesubclasses provide additional type checking, preventing you from creating invalid constraints.
For more information on the anchor properties, see bottomAnchor in the UIView, NSView, or UILayoutGuide.