🏡 Cully

A Swift-based DSL to make Auto Layout friendlier.

Motivation

Auto Layout has gotten nice, but it can still be pretty verbose. Cully provides handy lil’ helpers to reduce the amount of boilerplate code needed to layout views and dramatically improves the readability of that code.

Let’s say we want to position a fixed-size UIView in the center of another UIView.

// With `NSLayoutConstraint`s

let box = UIView()
view.addSubview(box)
box.translatesAutoresizingMaskIntoConstraints = false
NSLayoutConstraint(item: box, attribute: .centerX, relatedBy: .equal, toItem: view, attribute: .centerX, multiplier: 1.0, constant: 0).isActive = true
NSLayoutConstraint(item: box, attribute: .centerY, relatedBy: .equal, toItem: view, attribute: .centerY, multiplier: 1.0, constant: 0).isActive = true
NSLayoutConstraint(item: box, attribute: .width, relatedBy: .equal, toItem: nil, attribute: .notAnAttribute, multiplier: 1.0, constant: 100).isActive = true
NSLayoutConstraint(item: box, attribute: .height, relatedBy: .equal, toItem: nil, attribute: .notAnAttribute, multiplier: 1.0, constant: 100).isActive = true

Woof – that’s a lot to parse. The same layout with Cully:

// With Cully

let box = UIView()
view.addSubview(box)
box.constrain(width: 100, height: 100)
box.constrain(centerIn: view)

That’s friendlier.

Works directly with native types (NSLayoutConstraint) and retains the full-power of the native API
Adds just enough syntactic sugar to be useful, but not so much as to be overwhelming
Reads more naturally than the native API
Saves some time

Yes, one could use NSLayoutAnchors, and yes, perhaps when the world is all SwiftUI this becomes less relevant, and yes, _____________________, but this is the house I built and I intend on living in it. 🏡

Usage

import Cully

By default, Cully sets the receiver view’s translatesAutoresizingMaskIntoConstraints to false and activates the requested constraint.

Adding Constraints

Methods for adding constraints are namespaced constrain(...) on UIView.

myView.constrain(bottom: .equal, .top, of: someView)

Single constraint method names mirror NSLayoutContraint.Attributes. They all return a single NSLayoutConstraint. Some special case convenience helpers that require creating more than one constraint return [NSLayoutConstraint].

// Singly-activated `NSLayoutConstraint`s
let leftConstraint = myView.constrain(left: .equal, .left, of: someView)
let baselineConstraint = myView.constrain(lastBaseline: .lessThanOrEqual, .lastBaseline, of: someView)

// An array of `NSLayoutConstraint`s
let dimensionConstraints = myView.constrain(width: 50, height: 80)

Methods for accessing and modifying NSLayoutConstraints are namespaced constraint(...) and constraints(...) on UIView.

myView.constraints(for [.width, .height])
myView.constraints(activate: trailingConstraint)

KeyedConstraints is a typealias representing a Dictionary of NSLayoutConstraints keyed by NSLayoutConstraint.Attributes on Collection.

let keyedConstraints = myView.constraints.keyed
keyedConstraints[.width]?.constant = 150

You can also easily access the native NSLayoutConstraint initialization API, as well as a helper on UIView to activate and deactivate constraints.

let topConstraint = myView.constrain(.top, .equal, .top, of: view, multiplier: 1.0, constant: 100, isActive: false)
...
myView.constraints(activate: topConstraint)

Installation

Cully supports the Swift Package Manager in Xcode 11. Or give Carthage a try, how ‘bout?

Add the following to your Cartfile:
```
github "staykids/Cully" "1.0.1"
```
Follow the Carthage instructions for adding a framework to your project.

Improvements

Cully is opinionatedly tailored to my own needs and uses, but feel free to open a pull request if you think of something cool, notice an issue, or feel something’s missing.