Type Erasure with Pokemon---swift的类型擦除

Posted 2020-11-24 feng9exe

我感觉这个是swift的设计缺陷。

 

类型擦除：解决泛型类型作为公用类型的问题

 

是抽象的公用机制的一种实现方式。

1）类型擦除并不能解决类型不一致的兼容问题，只能解决类似继承一致性的兼容问题。

2）擦除类型后的抽象类型，其类型信息仍然存在，只是需要保持逻辑一致性。

 

import Foundation
	protocol Pokemon {
	associatedtype Power
	func attack() -> Power
	}
	struct Pikachu: Pokemon {
	func attack() -> ?? {
	return ??()
	}
	}
	struct Charmander: Pokemon {
	func attack() -> ?? {
	return ??()
	}
	}
	// power types
	struct ?? { }
	struct ?? { }
	// MARK: - Abstract base class
	class _AnyPokemonBase<Power>: Pokemon {
	init() {
	guard type(of: self) != _AnyPokemonBase.self else {
	fatalError("_AnyPokemonBase<Power> instances can not be created; create a subclass instance instead")
	}
	}
	func attack() -> Power {
	fatalError("Must override")
	}
	}
	// MARK: - Box container class
	fileprivate final class _AnyPokemonBox<Base: Pokemon>: _AnyPokemonBase<Base.Power> {
	var base: Base
	init(_ base: Base) { self.base = base }
	fileprivate override func attack() -> Base.Power {
	return base.attack()
	}
	}
	// MARK: - AnyPokemon Wrapper
	final class AnyPokemon<Power>: Pokemon {
	private let box: _AnyPokemonBase<Power>
	init<Base: Pokemon>(_ base: Base) where Base.Power == Power {
	box = _AnyPokemonBox(base)
	}
	func attack() -> Power {
	return box.attack()
	}
	}
	// Use AnyPokemon type directly
	let pokemon: AnyPokemon = AnyPokemon(Pikachu())
	pokemon.attack()
	// Add a new electric Pokemon
	class Jolteon: Eevee, Pokemon {
	func attack() -> ?? {
	return ??()
	}
	}
	class Eevee {}
	// Iterate over a collection of Electric Pokemon
	let electricPokemon = [AnyPokemon(Pikachu()), AnyPokemon(Jolteon())]
	electricPokemon.map() { $0.attack() }

 

 

 

 

 

1. class Pet { }
3. class Dog: Pet { }
5. class Cat: Pet { }
7. // 1. 隐藏具体宠物类型信息
9. let pets: [Pet] = [Dog(), Cat()]

 

https://github.com/bignerdranch/type-erasure-playgrounds/blob/master/playgrounds/Pokemon_erasure.playground/Contents.swift