KeyValueCoding

KeyValueCoding provides a mechanism by which you can access the properties of pure Swift struct or class instances indirectly by a string key or a key path.

• Getting Started
  • Basics
  • Relationship
  • Class Inheritance
  • Protocols
  • Advanced functions
• Methods
  • metadata
  • [key]
  • [keyPath]
• API
  • swift_metadata
  • swift_value
  • swift_setValue
• Installation
• License

Getting Started

Basics

The basic approach of KeyValueCoding protocol for accessing an instance’s properties values is subscripting by string key or a key path. In order to make your types key-value coding compliant just adopt them from this protocol, for instance:

import KeyValueCoding

struct Resolution: KeyValueCoding {
  let width: Int
  let height: Int
}

var resolution = Resolution(width: 640, height: 480)

resolution["width"] = 1920
resolution["height"] = 1080

print(resolution) // Prints: Resolution(width: 1920, height: 1080)

NOTE: An instance variable must be declared as var, otherwise you’ll have the following error: Cannot use mutating getter on immutable value.

The same works with the key paths as well:

resolution[\Resolution.width] = 2560
resolution[\Resolution.height] = 1440

print(resolution) // Prints: Resolution(width: 2560, height: 1440)

You can also read properties values in the same way:

if let width: Int = resolution[\Resolution.width], let height: Int = resolution[\Resolution.height] {
  print("\(width)x\(height)") // Prints: 2560x1440
}

The following properties can be accessible for classes and structs in a consistent manner including:

• Constant let and variable var properties.
• lazy, @objc and optional properties.
• Properties with any access level: public, internal, private etc.
• Properties of any type: enum, struct, class, tuple etc.
• Relationship properties by a string key or a key path.

But there are some limitations:

• Computed properties are not addressable.
• The willSet and didSet observers aren’t being called on changing values.
• weak, unowned and the property wrappers are not supported.

Relationship

KeyValueCoding can access to relationship properties by a string key (“relationship.property”) or a key path, for example:

import KeyValueCoding

struct Resolution {
  let width: Int
  let height: Int
}

class VideoMode: KeyValueCoding {
  let name: String
  let resolution: Resolution
  
  init(name: String, resolution: Resolution) {
    self.name = name
    self.resolution = resolution
  }
}

var videoMode = VideoMode(name: "HD", resolution: Resolution(width: 1920, height: 1080))
print("\(videoMode.name) - \(videoMode.resolution.width)x\(videoMode.resolution.height)")
// Prints: HD - 1920x1080

videoMode[\VideoMode.name] = "4K"
videoMode[\VideoMode.resolution.width] = 3840
videoMode[\VideoMode.resolution.height] = 2160
print("\(videoMode.name) - \(videoMode.resolution.width)x\(videoMode.resolution.height)")
// Prints: 4K - 3840x2160

NOTE: Your parent instance can access to its children’s properties without conforming the children to KeyValueCoding protocol.

Class Inheritance

Properties from inherited classes are also accessible by KeyValueCoding protocol:

import KeyValueCoding

class Mode {
  let name: String
  
  init(name: String) {
    self.name = name
  }
}

class VideoMode: Mode, KeyValueCoding {
  let frameRate: Int
  
  init(name: String, frameRate: Int) {
    self.frameRate = frameRate
    super.init(name: name)
  }
}

var videoMode = VideoMode(name: "HD", frameRate: 30)
print("\(videoMode.name) - \(videoMode.frameRate)fps")
// Prints: HD - 30fps

videoMode[\VideoMode.name] = "4K"
videoMode[\VideoMode.frameRate] = 25
print("\(videoMode.name) - \(videoMode.frameRate)fps")
// Prints: 4K - 25fps

Protocols

You can inherit any protocol from KeyValueCoding and then all instances of this protocol will be to accessible to read and write their properties:

import KeyValueCoding

protocol Size: KeyValueCoding {
  var width: Int { get }
  var height: Int { get }
}

struct Resolution: Size {
  let width: Int
  let height: Int
}

var resolution: Size = Resolution(width: 1920, height: 1080)
print(resolution)
// Prints: Resolution(width: 1920, height: 1080)

resolution[\Resolution.width] = 3840
resolution[\Resolution.height] = 2160

if let width: Int = resolution[\Resolution.width], let height: Int = resolution[\Resolution.height] {
  print("\(width)x\(height)")
  // Prints: 3840x2160
}

Advanced functions

In additional you can use pure API functions for getting and setting values of an instance’s properties without adopting KeyValueCoding protocol at all:

import KeyValueCoding

struct Resolution {
  let width: Int
  let height: Int
}

var resolution = Resolution(width: 1920, height: 1080)
print(resolution)
// Prints: Resolution(width: 1920, height: 1080)

swift_setValue(3840, to: &resolution, keyPath: \Resolution.width)
swift_setValue(2160, to: &resolution, keyPath: \Resolution.height)

if let width = swift_value(of: &resolution, keyPath: \Resolution.width) as? Int,
    let height = swift_value(of: &resolution, keyPath: \Resolution.height) as? Int
{
  print("\(width)x\(height)")
  // Prints: 3840x2160
}

Methods

Swift instances of struct or class that adopt KeyValueCoding protocol are key-value coding compliant for their properties and they are addressable via essential subscriptions [key] and [keyPath].

metadata

Returns the metadata of the instance which includes its type, kind, size and a list of accessible properties:

import KeyValueCoding 

struct Resolution: KeyValueCoding {
  let width: Int
  let height: Int
}

let resolution = Resolution(width: 1920, height: 1080)
print(resolution.metadata)

Prints:

Metadata(type: Resolution, kind: .struct, size: 16, properties: [
  Property(name: 'width', isStrong: true, isLazy: false, isVar: false, offset: 0), 
  Property(name: 'height', isStrong: true, isLazy: false, isVar: false, offset: 8)
])

[key]

Gets and sets a value for a property identified by a string key.

import KeyValueCoding

struct Resolution: KeyValueCoding {
  let width: Int
  let height: Int
}

var resolution = Resolution(width: 1920, height: 1080)

resolution["width"] = 2048

if let width: Int = resolution["width"] {
  print(width) // Prints: 2048
}

[keyPath]

Gets and sets a value for a property identified by a key path.

import KeyValueCoding

struct Resolution: KeyValueCoding {
  let width: Int
  let height: Int
}

var resolution = Resolution(width: 1920, height: 1080)

resolution[\Resolution.width] = 2048

if let width: Int = resolution[\Resolution.width] {
  print(width) // Prints: 2048
}

API

Global API functions to set, get and retrieve metadata information from any instance or type even without adopting KeyValueCoding protocol.

swift_metadata

Returns the metadata of an instance or a type which includes its type, kind, size and a list of accessible properties:

import KeyValueCoding

struct Resolution {
  let width: Int
  let height: Int
}

let resolution = Resolution(width: 1920, height: 1080)
    
var metadata = swift_metadata(of: resolution)
// OR
metadata = swift_metadata(of: type(of: resolution))
// OR
metadata = swift_metadata(of: Resolution.self)

print(metadata)

Prints:

Metadata(type: Resolution, kind: .struct, size: 16, properties: [
 Property(name: 'width', isStrong: true, isLazy: false, isVar: false, offset: 0), 
 Property(name: 'height', isStrong: true, isLazy: false, isVar: false, offset: 8)
])

swift_value

Returns the value for the instance’s property identified by a given string key or a key path.

import KeyValueCoding

struct Resolution {
  let width: Int
  let height: Int
}

var resolution = Resolution(width: 1920, height: 1080)
    
if let width = swift_value(of: &resolution, key: "width") as? Int {
  print(width) // Prints: 1920
}
// OR   
if let width = swift_value(of: &resolution, keyPath: \Resolution.width) as? Int {
  print(width) // Prints: 1920
}

swift_setValue

Sets a property of an instance specified by a given string key or a key path to a given value.

import KeyValueCoding

struct Resolution {
  let width: Int
  let height: Int
}

var resolution = Resolution(width: 1920, height: 1080)
    
swift_setValue(2048, to: &resolution, key: "width")
// OR
swift_setValue(2048, to: &resolution, keyPath: \Resolution.width)
    
print(resolution) // Prints: Resolution(width: 2048, height: 1080)

Installation

XCode

1. Select Xcode > File > Add Packages...
2. Add package repository: https://github.com/ikhvorost/KeyValueCoding.git
3. Import the package in your source files: import KeyValueCoding

Swift Package

Add KeyValueCoding package dependency to your Package.swift file:

let package = Package(
    ...
    dependencies: [
        .package(url: "https://github.com/ikhvorost/KeyValueCoding.git", from: "1.0.0")
    ],
    targets: [
        .target(name: "YourPackage",
            dependencies: [
                .product(name: "KeyValueCoding", package: "KeyValueCoding")
            ]
        ),
        ...
    ...
)

License

KeyValueCoding is available under the MIT license. See the LICENSE file for more info.