枚举联合类型的参数？

Question

我有两个枚举

enum Fruit { Apples, Oranges };
enum Vegetable { Garlic, Cucumber };

和类型

type Food = Fruit | Vegetable;

现在我想制作另一种类型，它是一个函数，并以食物为参数

type OnViewFood = (food: Food) => void;

但是当我这样做时

viewFruit: OnViewFood = (fruit: Fruit) => `This is ${fruit}`;

我收到一个错误，即 Food 不能分配给 Fruit。 如何实现我的目标？

Answer 1

编译器不希望你这样做的原因是它本质上是不安全的。考虑这个例子是否有效：

let viewFruit: OnViewFood = (fruit: Fruit) => `This is ${fruit}`; // we assign a function that can only handle Fruit.

viewFruit(Vegetable.Cucumber); // the declared signature allows us to use a Vegetable, possibly causing an error 

您可以将OnViewFood 声明为函数的联合，但是调用会成为一个问题，因为您不能调用函数的联合并且您不能轻松地进行类型保护：

type OnViewFood = ((food: Fruit) => void) | ((food: Vegetable) => void);
let viewFruit: OnViewFood = (fruit: Fruit) => `This is ${fruit}`; // ok now can be either function signature 
viewFruit(Vegetable.Cucumber); // now an error 

在任何一种情况下，我们都可以使用类型断言来解决类型错误，您可以选择任一选项，具体取决于您更信任哪一方，函数创建方或调用方：

// Type assertion on call 
type OnViewFood = ((food: Fruit) => void) | ((food: Vegetable) => void);
let viewFruit: OnViewFood = (fruit: Fruit) => `This is ${fruit}`;

(viewFruit as any as ((food: Vegetable) => void))(Vegetable.Cucumber);

// Type assertion on declaration 
type OnViewFood = (food: Food) => void
let viewFruit: OnViewFood = ((fruit: Fruit) => `This is ${fruit}`) as (food: Food) => void;
viewFruit(Vegetable.Cucumber);