"Stored properties" for Swift

PropertyStoring.swift

// Created by Nedrow, David E on 2/21/18.
// This work is licensed under Attribution 4.0 International (CC 4.0)

import Foundation

/// Mechanism that provides something resembling stored properties for extensions.
///
/// Usage:
///
/// 1. Extend your class with the PropertyStoring protocol.
/// 2. Create a private struct containing the properties to be used.
/// 3. Implement a public var{get{} set{}} for each of the private properties.
///
/// Example:
/// ```
/// protocol ToggleProtocol {
///     func toggle()
/// }
///
/// enum ToggleState {
///     case on
///     case off
/// }
///
/// extension UIButton: ToggleProtocol, PropertyStoring {
///
///     private struct CustomProperties {
///         static var toggleState = false
///     }
///
///     var toggleState: ToggleState {
///         get {
///             return getAssociatedObject(&CustomProperties.hidden, defaultValue: CustomProperties.hidden)
///         }
///         set {
///             return objc_setAssociatedObject(self, &CustomProperties.hidden, newValue, .OBJC_ASSOCIATION_RETAIN)
///         }
///     }
///
///     func toggle() {
///         toggleState = toggleState == .on ? .off : .on
///
///         if toggleState == .on {
///             // Shows background for status on
///         } else {
///             // Shows background for status off
///         }
///     }
/// }
/// ```
protocol PropertyStoring {

    /// Returns the value associated with a given object for a given key.
    ///
    /// - Parameters:
    ///   - object: The source object for the association.
    ///   - key: The key for the association.
    ///   - defaultValue: Value to return if association not found.
    /// - Returns: The value associated with the key for the object.
    /// - SeeAlso:
    ///     [objc_getAssociatedObject](https://developer.apple.com/documentation/objectivec/1418865-objc_getassociatedobject)
    func getAssociated<T>(_ object: Any!, _ key: UnsafeRawPointer!, defaultValue: T) -> T

    /// Sets an associated value for a given object using a given key and association policy.
    ///
    /// - Parameters:
    ///   - object: The source object for the association.
    ///   - value: The value to associate with the key key for object. Pass nil to clear an existing association.
    ///   - associativeKey: The key for the association.
    ///   - policy: The policy for the association. For possible values, see [Associated Objects](http://nshipster.com/associated-objects/)
    /// SeeAlso:
    ///   [objc_setAssociatedObject](https://developer.apple.com/documentation/objectivec/1418509-objc_setassociatedobject)
    func setAssociated<T>(object: AnyObject, value: T, associativeKey: UnsafeRawPointer, policy: objc_AssociationPolicy)
}

extension PropertyStoring {
    func getAssociated<T>(_ object: Any!, _ key: UnsafeRawPointer!, defaultValue: T) -> T {
        guard let value: T = objc_getAssociatedObject(object, key) as? T else {
            return defaultValue
        }
        return value
    }

    func setAssociated<T>(object: AnyObject, value: T, associativeKey: UnsafeRawPointer, policy: objc_AssociationPolicy) {
        objc_setAssociatedObject(object, associativeKey, value, policy)
    }
}

// This work is licensed under Attribution 4.0 International (CC 4.0)
// You are free to:
// Share — copy and redistribute the material in any medium or format
// Adapt — remix, transform, and build upon the material
// for any purpose, even commercially.
//
// This license is acceptable for Free Cultural Works.
// The licensor cannot revoke these freedoms as long as you follow the license terms.
// See https://creativecommons.org/licenses/by/4.0/