Strutter

Strutter is a Swift µFramework for defining AutoLayout constraints with a few custom infix operators. It uses iOS 9 / OS X 10.11’s NSLayoutAnchor system for defining these constraints.

For backwards compatibility, there is a subspec called OALayoutAnchor and an alternate framework called Strutter iOS OALayout which uses OALayoutAnchor instead of NSLayoutAnchor. This version works on iOS 8.

Strutter uses the custom operators |=|, |>=|, and |<=| to define equal, greater than equal, and less than equal constraints between two NSLayoutAnchors. The Apple documentation explains the basics of how NSLayoutAnchor works.

Strutter automatically activates constraints and disables the translatesAutoresizingMaskIntoConstraints attribute of the left-hand view. It doesn’t automatically disable both views so positioning subviews of view controllers and navigation bars works as expected.

Since Strutter is just syntactic sugar on top of NSLayoutAnchor’s methods, it retains the type safety enforced by NSLayoutAnchor which prevents creating invalid constraints, such as between dimension and location metrics.

Installation

Strutter supports Cocoapods and Carthage.

How To

This verbose blob aligns the trailing edge of one view with the leading edge of another on iOS 8:

NSLayoutConstraint(item: view1,
    attribute: .Trailing,
    relatedBy: .Equal,
    toItem: view2,
    attribute: .Leading,
    multiplier: 1,
    constant: 0).active = true

Using layout anchors shortens this to the following:

view1.trailingAnchor.constraintEqualToAnchor(view2.leadingAnchor).active = true

Strutter shortens this futher to

view1.trailingAnchor |=| view2.leadingAnchor

The value of a Strutter expression is the created constraint, so it can be deactivated after creation if desired:

let constraint = view1.trailingAnchor |=| view2.leadingAnchor
constraint.active = false

Constants and Multipliers

Strutter allows specifying a constant and/or multiplier via tuples:

subview.topAnchor |=| (superview.topAnchor, 10)

is equivalent to

subview.topAnchor.constraintEqualToAnchor(
    superview.topAnchor, constant: 10).active = true
// or
NSLayoutConstraint(item: subview, 
    attribute: .Top, 
    relatedBy: .Equal, 
    toItem: superview, 
    attribute: .Top, 
    multiplier: 1, 
    constant: 10).active = true

and

subv.heightAnchor |=| (v.heightAnchor, multiplier: 2, constant: -20)

is equivalent to

subv.heightAnchor.constraintEqualToAnchor(
    v.heightAnchor, multiplier: 2, constant: -20)

Inequalities

To define less-than and greater-than attributes:

subv.heightAnchor |<=| v.heightAnchor

Is the same as

subv.heightAnchor.constraintLessThanOrEqualToAnchor(v.heightAnchor)

Fixed Values

Lastly, Strutter allows constraining to constant values for dimension axes:

subv.heightAnchor |=| 200

is equivalent to

subv.heightAnchor.constraintEqualToConstant(200)

Playground

Take a look at the playground, which demonstrates basic Strutter syntax. To use it, open the Strutter.xcworkspace project, build the Strutter iOS scheme with a target of an iPhone 5s simulator or newer, and select Example.playground in the project navigator.

License

Strutter is released under the MIT License. Contributions appreciated.