强制 typescript 类方法是具有特定类型参数的一元方法

Question

为了好玩，我想看看我是否可以想出一种方法，只使用类型，它会强制类中的所有函数成为一个一元函数，只要该对象符合指定的接口（在这种情况下，我想确保提供 ContextObj 作为参数）

我是这样尝试的：

interface BaseContext {}
interface SomeContext { foo: 'lish'; };
interface ContextObj<T> { context: T }

// a unary function that takes an argument that conforms to the ContextObj interface
// and returns a value of type U
type ContextFn<T, U> = (x: ContextObj<T>)  => U;

// a type to represent any function
type AnyFunction = (...args: any[]) => any;

// we use mapped types and conditional types here to determine if
// T[K], the value of the property K on type T, is a function
// if it's not a function we return the T[K] unaltered
// if it is a function we make sure it conforms to our ContextFn type
// otherwise we return never which I was hoping would result in an error
type ContextService<T, U extends BaseContext> = {
  [K in keyof T]: T[K] extends AnyFunction
  ? T[K] extends ContextFn<U, ReturnType<T[K]>>
    ? T[K]
    : never
  : T[K]
};

class BarService implements ContextService<BarService, SomeContext> {
  test:string = 'test';
  // expected error: not assignable to type never
  updateBar(): string {
    return '';
  }
}

事实证明这并没有按预期工作，因为它是 TypeScript 的限制（或功能？）：https://github.com/Microsoft/TypeScript/wiki/FAQ#why-are-functions-with-fewer-parameters-assignable-to-functions-that-take-more-parameters

我很好奇是否有办法在类或接口级别做这样的事情（强制方法参数与特定类型签名匹配）？是否可以通过装饰器在单个方法级别完成（此时授予您最好使用适当的类型签名）。

只是测试极限:)

Answer 1

您只需要为参数的长度添加另一个条件。您可以使用Parameters 将参数的类型提取为元组。元组将具有一个数字字面量类型的长度属性，因此您可以检查 length 是否扩展了 1：

type ContextService<T, U extends BaseContext> = {
    [K in keyof T]: T[K] extends AnyFunction ?
        T[K] extends ContextFn<U, ReturnType<T[K]>> ?
            Parameters<T[K]>["length"] extends 1 ? T[K]
        : never : never
    : T[K]
};

class BarService implements ContextService<BarService, SomeContext> {
    test: string = 'test';
    // error: not assignable to type never
    updateBar(): string {
        return '';
    }
    updateBar2(p: ContextObj<SomeContext>): string { // ok
        return '';
    }
}

向用户提示问题可能很有用，尽管不支持自定义错误，但这将非常接近：

type ContextService<T, U extends BaseContext> = {
    [K in keyof T]: T[K] extends AnyFunction ?
        T[K] extends ContextFn<U, ReturnType<T[K]>> ?
            Parameters<T[K]>["length"] extends 1 ? T[K]
        : ["Method must have exactly one parameter of type ", ContextObj<U>, "Found Parameters:", Parameters<T[K]>] 
        : ["Parameters types not macthed, expected  [", ContextObj<U>, "] Found Parameters:", Parameters<T[K]>]
    : T[K]
};

class BarService implements ContextService<BarService, SomeContext> {
    test: string = 'test';
    // error: not assignable to type never
    updateBar(): string {// Type '() => string' is missing the following properties from type '["Method must have exactly one parameter of type ", ContextObj<SomeContext>, "Found Parameters:", []]'
        return '';
    }
    updateBar2(p: ContextObj<SomeContext>): string {
        return '';
    }

    updateBar3(p: SomeContext): string { // Type '(p: SomeContext) => string' is missing the following properties from type '["Parameters types not macthed, expected  [", ContextObj<SomeContext>, "] Found Parameters:", [SomeContext]]'
        return '';
    }
}

编辑

回答评论问题：这可以用装饰器完成吗？

答案是肯定的，我们可以捕获应用装饰器的类以及应用它的键作为类型参数，并使用与我们之前对属性键所做的相同逻辑。只是这次我们将错误附加到传递给装饰器的键上：

type Check<TSig extends (...a: any[]) => any, T, K extends keyof T> = 
    T[K] extends (...a: any[]) => any ?
    T[K] extends TSig ?
            Parameters<T[K]>["length"] extends 1 ? unknown
        : ["Method must have exactly one parameter of type ", Parameters<TSig>, "Found Parameters:", Parameters<T[K]>] 
        : ["Parameters types not macthed, expected  [", Parameters<TSig>, "] Found Parameters:", Parameters<T[K]>]
    : unknown
function ensureSignatire<TSig extends (...a: any[]) => any>() {
    return function <TTarget, TKey extends keyof TTarget>(target: TTarget, key: TKey & Check<TSig, TTarget, TKey>) {

    }
}

class BarService {
    test: string = 'test';

    @ensureSignatire<ContextFn<SomeContext, any>>() // Type '"updateBar"' is not assignable to type '["Method must have exactly one parameter of type ", [ContextObj<SomeContext>], "Found Parameters:", []]'.
    updateBar(): string {
        return '';
    }

    @ensureSignatire<ContextFn<SomeContext, any>>() //ok
    updateBar2(p: ContextObj<SomeContext>): string {
        return '';
    }
    @ensureSignatire<ContextFn<SomeContext, any>>() //  Type '"updateBar3"' is not assignable to type '["Parameters types not macthed, expected  [", [ContextObj<SomeContext>], "] Found Parameters:", [SomeContext]]'
    updateBar3(p: SomeContext): string { 
        return '';
    }
}