AutoLayout using Operators

NiceConstraints.swift

/// - note: The equivalence operators don't work because `NSObject` implements `Equatable`.

import UIKit

// MARK: Constraint From Anchor

public func == <AnchorType>(constrained: NSLayoutAnchor<AnchorType>, reference: NSLayoutAnchor<AnchorType>) -> NSLayoutConstraint {
    return constrained.constraint(equalTo: reference)
}

public func >= <AnchorType>(constrained: NSLayoutAnchor<AnchorType>, reference: NSLayoutAnchor<AnchorType>) -> NSLayoutConstraint {
    return constrained.constraint(greaterThanOrEqualTo: reference)
}

public func <= <AnchorType>(constrained: NSLayoutAnchor<AnchorType>, reference: NSLayoutAnchor<AnchorType>) -> NSLayoutConstraint {
    return constrained.constraint(lessThanOrEqualTo: reference)
}

// MARK: - Offset Layout Anchors

public struct Partial<AnchorType: AnyObject> {
    var anchor: NSLayoutAnchor<AnchorType>
    var constant: CGFloat

    public static func + (partial: Partial, constant: CGFloat) -> Partial {
        var partial = partial
        partial.constant += constant
        return partial
    }

    public static func - (partial: Partial, constant: CGFloat) -> Partial {
        var partial = partial
        partial.constant -= constant
        return partial
    }
}

public func + <AnchorType>(anchor: NSLayoutAnchor<AnchorType>, constant: CGFloat) -> Partial<AnchorType> {
    return Partial(anchor: anchor, constant: constant)
}

public func - <AnchorType>(anchor: NSLayoutAnchor<AnchorType>, constant: CGFloat) -> Partial<AnchorType> {
    return Partial(anchor: anchor, constant: -constant)
}

// MARK: - Constraint from Partial

public func == <AnchorType>(constrained: NSLayoutAnchor<AnchorType>, partial: Partial<AnchorType>) -> NSLayoutConstraint {
    return constrained.constraint(equalTo: partial.anchor, constant: partial.constant)
}

public func >= <AnchorType>(constrained: NSLayoutAnchor<AnchorType>, partial: Partial<AnchorType>) -> NSLayoutConstraint {
    return constrained.constraint(greaterThanOrEqualTo: partial.anchor, constant: partial.constant)
}

public func <= <AnchorType>(constrained: NSLayoutAnchor<AnchorType>, partial: Partial<AnchorType>) -> NSLayoutConstraint {
    return constrained.constraint(lessThanOrEqualTo: partial.anchor, constant: partial.constant)
}

// MARK: - Multiplied Layout Dimensions

public struct MultipliedPartial {
    var anchor: NSLayoutDimension
    var constant: CGFloat
    var multiplier: CGFloat

    public static func + (partial: MultipliedPartial, constant: CGFloat) -> MultipliedPartial {
        var partial = partial
        partial.constant += constant
        return partial
    }

    public static func - (partial: MultipliedPartial, constant: CGFloat) -> MultipliedPartial {
        var partial = partial
        partial.constant -= constant
        return partial
    }
}

public func * (anchor: NSLayoutDimension, multiplier: CGFloat) -> MultipliedPartial {
    return MultipliedPartial(anchor: anchor, constant: 0, multiplier: multiplier)
}

// MARK: - Constraint from MultipliedPartial

extension NSLayoutDimension {
    public static func == (constrained: NSLayoutDimension, partial: MultipliedPartial) -> NSLayoutConstraint {
        return constrained.constraint(equalTo: partial.anchor, multiplier: partial.multiplier, constant: partial.constant)
    }

    public static func >= (constrained: NSLayoutDimension, partial: MultipliedPartial) -> NSLayoutConstraint {
        return constrained.constraint(greaterThanOrEqualTo: partial.anchor, multiplier: partial.multiplier, constant: partial.constant)
    }

    public static func <= (constrained: NSLayoutDimension, partial: MultipliedPartial) -> NSLayoutConstraint {
        return constrained.constraint(lessThanOrEqualTo: partial.anchor, multiplier: partial.multiplier, constant: partial.constant)
    }
}