生成器模式（Builder Pattern）又称为建造模式，是 Gang of Four design patterns 中二十三种设计模式之一，属于 creational patterns。
Builder pattern 通过逐步提供输入来创建复杂对象，而不是通过初始化程序预先要求所有输入。生成器模式包含以下三部分：
- Director：接受输入并与 builder 协调。通常为视图控制器，或视图控制器所使用的帮助程序。
- Product：要创造的复杂对象。其可以是 struct 或 class，具体取决于所需引用类型。一般为 model，但也可以根据需要改变。
- Builder：接受输入并负责创建 product。为了方便引用，一般为 class 类型。
1. 何时使用 builder pattern
当需要使用一系列操作创建复杂对象时，使用 builder pattern。
当 product 需要多个输入时，生成器模式特别有效。Builder 按照 director 指令顺序，将输入进行抽象创建产品。
例如，我们有一个 Car 类。Car 有很多选项（如，座位数、颜色、GPS等），选项的组合将会产生大量构造函数。因此，我们将创建 builder 类CarBuilder，通过向CarBuilder一步一步发送选项来构建 Car。下面使用伪代码（pseudocode）实现上述功能：
// Product
class Car is
Can have GPS, trip computer and various numbers of seats.
Can be a city car, a sports car, or a cabriolet.
// Builder
class CarBuilder is
method getResult() is
output: a Car with the right options
Construct and return the car.
method setSeats(number) is
input: the number of seats the car may have.
Tell the builder the number of seats.
method setCityCar() is
Make the builder remember that the car is a city car.
method setCabriolet() is
Make the builder remember that the car is a cabriolet.
method setSportsCar() is
Make the builder remember that the car is a sports car.
method setTripComputer() is
Make the builder remember that the car has a trip computer.
method unsetTripComputer() is
Make the builder remember that the car does not have a trip computer.
method setGPS() is
Make the builder remember that the car has a global positioning system.
method unsetGPS() is
Make the builder remember that the car does not have a global positioning system.
// Director
Construct a CarBuilder called carBuilder
carBuilder.setSeats(2)
carBuilder.setSportsCar()
carBuilder.setTripComputer()
carBuilder.unsetGPS()
car := carBuilder.getResult()
在上述代码中，Builder 是 CarBuilder，Product 是 Car Model，Director 根据输入协调 builder 制造 car。
2. Demo
使用 builder design pattern 实现一个 hamburger builder。Product 是 hamburger 模型，其中包含肉类选择、浇头、调味汁等输入；Director 是掌握制作汉堡技能的员工，或接受用户输入的试图控制器；Builder 接受输入制作 hamburger。
2.1 Product
首先，定义 product，代码如下：
// MARK: - Product
// Hamburger 有 Meat、Sauces、Toppings三个属性。制作完成后不可修改成份。
public struct Hamburger {
public let meat: Meat
public let sauce: Sauces
public let toppings: Toppings
}
extension Hamburger: CustomStringConvertible {
public var description: String {
return meat.rawValue + " github.com/pro648"
}
}
// 每个汉堡有且只有一种肉
public enum Meat: String {
case beef
case chicken
case kitten
case tofu
}
// OptionSet 允许添加多种Sauces
public struct Sauces: OptionSet {
public static let mayonnaise = Sauces(rawValue: 1 << 0)
public static let mustard = Sauces(rawValue: 1 << 1)
public static let ketchup = Sauces(rawValue: 1 << 2)
public static let secret = Sauces(rawValue: 1 << 3)
public let rawValue: Int
public init(rawValue: Int) {
self.rawValue = rawValue
}
}
public struct Toppings: OptionSet {
public static let cheese = Toppings(rawValue: 1 << 0)
public static let lettuce = Toppings(rawValue: 1 << 1)
public static let pickles = Toppings(rawValue: 1 << 2)
public static let tomatoes = Toppings(rawValue: 1 << 3)
public let rawValue: Int
public init(rawValue: Int) {
self.rawValue = rawValue
}
}
2.2 Builder
// MARK: - Builder
public class HamburgerBuilder {
// meat、sauces、toppings 为变量，使用 private(set) 限制为只有 HamburgerBuilder 可以修改。
public private(set) var meat: Meat = .beef
public private(set) var sauces: Sauces = []
public private(set) var toppings: Toppings = []
private var soldOutMeats: [Meat] = [.kitten]
// 因为 property 使用了 private(set)，必须提供 public 方法修改属性。
public func addSauces(_ sauce: Sauces) {
sauces.insert(sauce)
}
public func removeSauces(_ sauce: Sauces) {
sauces.remove(sauce)
}
public func addToppings(_ topping: Toppings) {
toppings.insert(topping)
}
public func removeToppings(_ topping: Toppings) {
toppings.remove(topping)
}
public func setMeat(_ meat: Meat) throws {
self.meat = meat
}
public func build() -> Hamburger {
return Hamburger(meat: meat, sauce: sauces, toppings: toppings)
}
}
private(set)限制必须使用public设值方法，这样可以在设值时进行验证，避免设置为不符合规范的值。
例如，在设置meat前需要确保该meat有货。在原来的属性下添加以下属性：
private var soldOutMeats: [Meat] = [.kitten]
当添加已经销售完毕的meat时，将会抛出异常，更新后如下：
public func setMeat(_ meat: Meat) throws {
guard isAvailable(meat) else { throw Error.soldOut }
self.meat = meat
}
public func isAvailable(_ meat: Meat) -> Bool {
return !soldOutMeats.contains(meat)
}
public enum Error: Swift.Error {
case soldOut
}
2.3 Director
最后，声明 director。
// Director
public class Employee {
public func createCombol() throws -> Hamburger {
let builder = HamburgerBuilder()
try builder.setMeat(.beef)
builder.addSauces(.secret)
builder.addToppings([.lettuce, .tomatoes, .pickles])
return builder.build()
}
public func createKittenSpecial() throws -> Hamburger {
let builder = HamburgerBuilder()
try builder.setMeat(.kitten)
builder.addSauces(.mustard)
builder.addToppings([.lettuce, .tomatoes])
return builder.build()
}
}
现在，Employee 掌握了制作两种 hamburger 方法。
使用下面代码创建 hamburger :
let burgerFlipper = Employee()
guard let combol = try? burgerFlipper.createCombol() else {
print("Sorry, no beef burgers here...")
return
}
print("Beef burgers " + combol.description)
这里创建了Employee对象的实例burgerFlipper，并创建combol，控制台输出如下：
Beef burgersbeef github.com/pro648
继续添加以下代码：
guard let kittenBurger = try? burgerFlipper.createKittenSpecial() else {
print("Sorry, no kitten burgers here...")
return
}
print("Nom nom nom " + kittenBurger.description)
这里创建kittenBurger，但由于kitten已经售罄，控制台会输出以下信息：
Sorry, no kitten burgers here...
总结
以下是 builder pattern 的关键点：
- Builder pattern 非常适合逐步创建复杂对象，它涉及三个对象：director、product、builder。
- Director 接受输入并与 builder 协调，product 是创建的复杂对象。builder 根据输入步骤创建对象。
如果 product 没有多个输入，或无法一步一步创建，那么生成器模式就不适合使用。相反，考虑提供 convenience initializer 来创建对象。
Demo名称：BuilderPattern
源码地址：https://github.com/pro648/BasicDemos-iOS/tree/master/BuilderPattern
参考资料：
Builder pattern
Using the builder pattern in Swift
Swift builder design pattern
欢迎更多指正：https://github.com/pro648/tips/wiki