Swift - Store properties in an extension

gistfile1.txt

import Foundation

internal class StoredExtensionPropertyData {
    internal static var storedExtensionProperties = [ObjectIdentifier: StoredExtensionPropertyData]()
    var properties = [String: Any]()
}

internal protocol ExtensionProperties: AnyObject {
    func get<T>(key: String, owner: ExtensionProperties) -> T?
    func set<T>(key: String, value: T?, owner: ExtensionProperties)
    //func extensionPropertyCleanup()
}

internal extension ExtensionProperties {
    @inlinable
    func get<T>(key: String, owner: ExtensionProperties) -> T? {
        let store = StoredExtensionPropertyData.storedExtensionProperties[ObjectIdentifier(owner)]
        return store?.properties[key] as? T
    }
    
    @inlinable
    func set<T>(key: String, value: T?, owner: ExtensionProperties) {
        let store = StoredExtensionPropertyData()
        store.properties[key] = value
        StoredExtensionPropertyData.storedExtensionProperties[ObjectIdentifier(owner)] = store
    }
    
    func storedPropertyCleanup() {
        StoredExtensionPropertyData.storedExtensionProperties.removeValue(forKey: ObjectIdentifier(self))
    }
}

class MyClass {
    let doWop: String
    
    init() {
        self.doWop = "doo doo do dooo wop"
    }
    
    deinit {
        storedPropertyCleanup()
    }
}

extension MyClass: ExtensionProperties {
    var hansen: Int? {
        get { return get(key: "hansen", owner: self) }
        set(value) { set(key: "hansen", value: value, owner: self)}
    }
}

let mc = MyClass()

print(mc.doWop)

mc.hansen = 42
print(mc.hansen)

// manually call property cleanup so we can see it disappear.
mc.storedPropertyCleanup()
print(mc.hansen)