在 Typescript 中混合接口

Question

I have a class with over 80 methods, and each method accepts an object containing some defined interface.

class Stuff {
   /* many more */
   getAccount(req: IAccount, callback: ICallback) {
      return this._call('getAccount', req, callback);
   }

   getIds(req: IIDs, callback: ICallback) {
      return this._call('getIds', req, callback);
   }
   /* many more */
}

pretty 'boring' stuff, since it's just mapping to the underlaying _call method and making it type safe for each of the methods.

But sometimes these req param objects are made up from 2 interfaces or more, and instead of creating another interface for each time there's an "awkward", like this:

export interface ILoled extends IAccount {
   loled: boolean;
}

export interface IRofloled extends ILoled {
   rofled: boolean;
}

class Stuff {
  getLols(req: ILoled){
  }

  getRofls(req: IRofloled){
  }
}

is there any way I can just put it as an "inline" mixin of interfaces inside the method parameter list? like (which obviously don't work):

class Stuff {
  getMoreStuff(req: <{} extends IAccount, ITime>) {
  }
}

Answer 1

Yes you can, as of Typescript 1.6 . Called Intersection types , use the & operator to combine types.

function extend<T, U>(first: T, second: U): T & U {
  let result = <T & U> {};
  for (let id in first) {
    result[id] = first[id];
  }

  for (let id in second) {
    if (!result.hasOwnProperty(id)) {
      result[id] = second[id];
    }
  }
  return result;
}

var x = extend({ a: "hello" }, { b: 42 });
x.a; // works
x.b; // works 

Answer 2

is there any way I can just put it as an "inline" mixin of interfaces inside the method parameter list

No. You cannot extend an interface inline

Answer 3

I'm not sure if this can help, but did you know about Kotlin's interface implementation delegation with the by keyword?
In Kotlin, you can basically create a class implementing one interface and use it to delegate the implementation of the same interface for another class, like this:

interface Abc {
    val foo: Int
}

class HelloAbc : Abc {
    val foo = 5
}

class MyClass : Abc by HelloAbc() {
    val bar = "Hello world"
}

The above code creates an interface named Abc , a class HelloAbc implementing that interface, and one more class, MyClass , which uses HelloAbc to implement Abc .

I just realized it can be useful in TypeScript as well, and I could come up with a type–safe, mixin implementation, which follows:

type UnionToIntersectionUnchecked<T> =
    (T extends any
        ? (k: T) => void
        : never
        ) extends ((k: infer I) => void)
            ? I
            : never

type IsUnion<T> = [T] extends [UnionToIntersectionUnchecked<T>] ? false : true

type UnionToIntersection<T> =
    IsUnion<T> extends true
        ? UnionToIntersectionUnchecked<T>
        : T

function deepCopy<T, U>(target: T, source: U): T & U {

    const chain = []

    for (let proto = source; source !== Object.prototype; source = Object.getPrototypeOf(source))
        chain.unshift(proto)

    for (const proto of chain)
        Object.defineProperties(target, Object.getOwnPropertyDescriptors(proto))

    return target as T & U
}

function mixin<
    TBase extends object,
    T extends object[]
    >(
        baseClass: { new (...args: any[]): TBase },
        ...objects: T
    ): { new (): TBase & UnionToIntersection<T[number]> } {

    const proto = Object.assign(Object.create(baseClass.prototype), ...objects)

    const ctor = (function(this: TBase, ...args: any[]) {

        const thisProto = Object.getPrototypeOf(this)
        const instance = new baseClass(...args)

        Object.setPrototypeOf(instance, deepCopy(proto, thisProto))

        return instance
    }) as any

    Object.setPrototypeOf(ctor, baseClass)

    return ctor
}

The demo code below illustrates how to use this technique to delegate interface implementation:

interface Abc {
    readonly foo: number
}

interface Def {
    readonly bar: string
}

class HelloAbc implements Abc {
    readonly foo = 5
}

class HelloDef implements Def {
    readonly bar = "Hello world"
}

class GreetingBase {

    sayHello() {
        console.log("Hello!")
    }
}

class Greeting extends mixin(GreetingBase, new HelloAbc(), new HelloDef()) implements Abc, Def {

    printTest() {
        console.log("test")
    }
}

const o = new Greeting()

o.sayHello()
o.printTest()
console.log(o.foo)
console.log(o.bar)

You can even test it on the playground .

You'll want to avoid this in performance–critical code, since creating all these prototypes can be expensive, but I don't expect it to affect that much overall.