Object.assign in Swift

Assign.swift

import Foundation

// MARK: - Function

/// This function is used to copy the values of all enumerable own properties from one or more
/// source struct to a target struct. It will return the target struct.
public func assign<T>(_ value: T, changes: (inout T) -> Void) -> T {
  guard Mirror(reflecting: value).displayStyle == .struct else {
    fatalError("'value' must be a struct.")
  }
  var copy = value
  changes(&copy)
  return copy
}

// MARK: - Example

struct Aa: DescConvertible {
  var text = "foo"
}

struct A: DescConvertible {
  var nested = Aa()
  var number = 42
}

let a = A()
var b = assign(a) { (argument: inout A) in
  argument.nested.text = "bar"
  argument.number = 3
}

// MARK: - Debug

public protocol DescConvertible : CustomStringConvertible { }
extension DescConvertible {
  /// Returns a representation of the state in the form:
  /// Type(prop1: 'value', prop2: 'value'..)
  func reflectionDescription(delimiters: String = "") -> String {
    let mirror = Mirror(reflecting: self)
    var str = "{"
    var first = true
    for (label, value) in mirror.children {
      if let label = label {
        if first { first = false } else {  str += ", "  }
        str += "\(label):\(value)"
      }
    }
    str += "}"
    return str
  }

  public var description: String {
    return reflectionDescription()
  }
}