Example of using Swift property change observation to bind models to arguments passed to DSL closures

dsltest.swift

import UIKit


enum GraphicsCommand {
  case setColour(colour: UInt32)
  case moveTo(x: Float, y: Float)
  case lineTo(x: Float, y: Float)
}

class GraphicsDSL {
  var commands = [GraphicsCommand]()
  var colour: UInt32 = 0xffffff {
    didSet {
      if oldValue != colour {
        commands.append(.setColour(colour: colour))
      }
    }
  }
  func moveTo(x: Float, y: Float){
    commands.append(.moveTo(x: x, y: y))
  }
  func lineTo(x: Float, y: Float){
    commands.append(.lineTo(x: x, y: y))
  }
}

class GraphicsBuilder {
  /// Called with a closure that can access a special list of functions and properties
  func build(code: (GraphicsDSL) -> ()) -> [GraphicsCommand] {
    let dsl = GraphicsDSL()
    code(dsl)
    return dsl.commands
  }
}

let dsl = GraphicsBuilder()
/// This is what it looks like to use it. You can see what you can do in the arg list.
/// It is likely that moving these things to a struct is better. This would in itself be a nice pattern,
/// using structs to define the functionality of a DSL. However we're demonstrating the possibilities
/// of mutable, observable local vars here...
let commands = dsl.build { b in
  b.colour = 0xff0000
  b.moveTo(x: 50, y: 50)
  b.colour = 0x00ff00
  b.lineTo(x: 100, y: 100)
  b.colour = 0x0000ff
  b.lineTo(x: 0, y: 100)
  b.colour = 0xff00ff
  b.lineTo(x: 50, y: 50)
}

print(commands.count)
for c in commands {
  print(c)
}