An API for storing arbitrary key/value pairs. Supporting this API enables extensions to add their properties to our type.

KeyValueStore.swift

//
//  KeyValueStore.swift
//  FoundationNG
//
//  Created by Hooman Mehr on 4/27/16.
//  Copyright © 2016 Hooman Mehr. See the MIT license at the bottom.
//





//==============================================================================================
//MARK: Common API for a Key Value Store
//==============================================================================================





/// An API for a value type that can store arbitrary key/value pairs.
///
/// As in any key/value system, the key needs to be `Hashable`.
/// 
/// A type that supports this API provides means for extensions to add their properties to the instance of a value type.

public protocol KeyValueStore {
    
    
    /// Adds/updates a (key,value) pair.
    mutating func set<K: Hashable,V>(key: K, to value: V)
    
    /// Removes a (key,value) pair; if the key exists.
    mutating func unset<K: Hashable>(key: K)
    
    /// Returns the value corresponding to key; if the key exists.
    func get<K: Hashable,V>(key: K) -> V?
}





/// An API for an object that can store arbitrary key/value pairs.
///
/// As in any key/value system, the key needs to be `Hashable`.
///
/// A class that supports this API provides means for extensions to add their properties to the instances of this class.

public protocol KeyValueStoreObject: class, KeyValueStore {}





//==============================================================================================
//MARK: A Key Value Store using Dictionary for Storage
//==============================================================================================





/// A protocol to facilitate support of Key Value Store API.
///
/// Any value type that implements this protocol, gets Key Value Store API for free.

public protocol AnyDictionaryStore {

    
    /// Provides storage for heterogenous (key,value) pairs.
    var anyDictionaryStore: [AnyKey:Any] {get set}
}





/// A Protocol to Facilitate Support of Key Value Store API.
///
/// Any value type that implements this protocols, gets Key Value Store API for free.

public protocol AnyDictionaryStoreObject: class, AnyDictionaryStore {}





/// A wrapper type that erases the type of any hashable key.

public struct AnyKey: Hashable {
    
    
    /// Function/arguments selector used as the input parameter of `AnyKey` core function.
    private enum Selector {
        
        case key
        case hash
        case equals(AnyKey)
    }
    
    
    /// The original key behind this `AnyKey`.
    public var key: Any { return function(.key) }
    
    /// The hash value for `Hashable` support.
    public var hashValue: Int { return function(.hash) as! Int }
    
    /// The core function that captures/encapsulates all data/functionality of this type.
    private let function: (Selector)->Any
    
    
    /// Initializer of `AnyKey`.
    public init<T: Hashable>(_ key: T) {
        
        function = { arg in switch arg {
            
        case .key: return key
        
        case .hash: return key.hashValue
        
        case let .equals(other):
            
            guard let otherKey = other.key as? T else { return false }
            
            return key == otherKey
        }}
    }
}





/// Equality test operator for `Hashable` support of `AnyKey`.

public func ==(lhs: AnyKey, rhs:AnyKey) -> Bool {
    
    return lhs.function(.equals(rhs)) as! Bool
}





//==============================================================================================
//MARK: Implementation of Key Value Store using Heterogenous Dictionary for storage
//==============================================================================================





/// Default Implementation of `KeyValueStore` for any type that has an `anyDictionaryStore`.

public extension KeyValueStore where Self: AnyDictionaryStore {
    
    
    /// Adds/updates a (key,value) pair.
    mutating func set<K: Hashable,V>(key: K, to value: V) { anyDictionaryStore[AnyKey(key)] = value }
    
    /// Removes a (key,value) pair; if the key exists.
    mutating func unset<K: Hashable>(key: K) { anyDictionaryStore[AnyKey(key)] = nil }
    
    /// Returns the value corresponding to key; if the key exists.
    func get<K: Hashable,V>(key: K) -> V? { return anyDictionaryStore[AnyKey(key)] as? V }
}







/// Default Implementation of `KeyValueStoreObject` for any type that has an `anyDictionaryStore`.

public extension KeyValueStoreObject where Self: AnyDictionaryStoreObject {
    
    
    /// Adds/updates a (key,value) pair.
    func set<K: Hashable,V>(key: K, to value: V) { anyDictionaryStore[AnyKey(key)] = value } //FIXME: Cannot assign through subscript: ‘self’ is immutable
    
    /// Removes a (key,value) pair; if the key exists.
    func unset<K: Hashable>(key: K) { anyDictionaryStore[AnyKey(key)] = nil } //FIXME: Cannot assign through subscript: ‘self’ is immutable
    
    /// Returns the value corresponding to key; if the key exists.
    func get<K: Hashable,V>(key: K) -> V? { return anyDictionaryStore[AnyKey(key)] as? V }
}






// Copyright (c) 2016 Hooman Mehr (hooman@mac.com)
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in
// all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
// THE SOFTWARE.