Cassowary

An incremental linear constraint-solving algorithm (Auto Layout) in Swift, originally from the paper:
Solving Linear Arithmetic Constraints for User Interface Applications (1997)

This repository consists of 3 frameworks:

• Simplex: Simplex tableau and its common operations
• Cassowary: Core constraint-solving algorithm using Simplex
• CassowaryUI: UIKit/AppKit wrapper on top of Cassowary

How to use

CassowaryUI

import Cassowary
import CassowaryUI
let rootSize = CGSize(width: 320, height: 480)
var solver = CassowaryUI.Solver()
try! solver.addConstraints(view1, view2) { v1, v2 in
    return [
        // `v1` has fixed size (4:3 aspect ratio)
        v1.width == rootSize.width - 40,
        v1.height == v1.width * 3 / 4,
        // `v2` has fixed origin.x & width (flexible in vertical)
        v2.width == v1.width,
        v1.centerX == rootFrame.width / 2,
        v2.centerX == v1.centerX,
        // equal spacing (vertical)
        v1.top == 40 ~ .high,
        v2.top - v1.bottom == v1.top ~ .high,
        rootSize.height - v2.bottom == v1.top ~ .high,
    ]
}
solver.applyLayout()

This will result:

<UIView: 0x7f8f1ee018c0; frame = (0 0; 320 480); layer = <CALayer: 0x608000220800>>
   | <UIView: 0x7f8f21001010; frame = (20 40; 280 210); layer = <CALayer: 0x60c000220da0>>
   | <UIView: 0x7f8f1ec015c0; frame = (20 290; 280 150); layer = <CALayer: 0x60c000220840>>

Acknowledgments

• Solving Linear Arithmetic Constraints for User Interface Applications (1997) by Alan Borning, Kim Marriott, Peter Stuckey, and Yi Xiao
• ユーザインタフェースのための線形等式・不等式制約解消系 (2002) by 細部 博史
• pybee/cassowary (Python version)
• robb/Cartography (Auto Layout DSL in Swift)

References

• AutoLayout Algorithm // Speaker Deck (in Japanese)

License

MIT