mixin 类的类型是什么

Question

我无法弄清楚如何在不求助于一些 hack 的情况下获取 typescript mixin 类的类型（如下所示）

type Constructor<T = {}> = new(...args: any[]) => T;

function MyMixin<T extends Constructor>(BaseClass: T) {
  return class extends BaseClass {doY() {}}
}

// Option A: Ugly/wasteful
const MixedA = MyMixin(class {doX() {}});
const dummy = new MixedA();
type MixedA = typeof dummy;

class OtherA {
  field: MixedA = new MixedA();
  a() {this.field.doX(); this.field.doY();}
}

// Option B: Verbose
class Cls {doX() {}}
interface MixinInterface {doY(): void}

const MixedB = MyMixin(Cls);
type MixedB = Cls & MixinInterface;

class OtherB {
  field: MixedB = new MixedB();
  a() {this.field.doX(); this.field.doY();}
}

typescript 不支持诚实的 mixins/traits 让我感到非常难过，但是有没有其他方法可以声明 field 的类型，而无需使用 typeof 实例或不必在接口中重新声明签名（我试过typeof(new MixedBaseA())，但 typeof 不接受任意表达式）？

Answer 1

可能不是您想要的，但这里有一个不那么浪费的选择。给定定义：

type Constructor<T = {}> = new(...args: any[]) => T;

function MyMixin<T extends Constructor>(BaseClass: T) {
    return class extends BaseClass {
        doY() { }
    }
}

const MixedA = MyMixin(class { doX() {} });

您可以使用以下方法获取类型：

function getReturnType<R>(fn: (new(...args: any[]) => R)): R {
  return {} as R;
}

const dummy = getReturnType(MixedA);
type MixedAType = typeof dummy;

const mixedA : MixedAType = new MixedA();
mixedA.doX();
mixedA.doY();

Playground

获取任何表达式类型的建议仍在讨论中：https://github.com/Microsoft/TypeScript/issues/6606。这将摆脱虚拟变量和函数。

或者，为了得到一个干净的type MixedAType = MyMixinY & X，你可以选择在mixin中返回正确的构造函数类型：

type Constructor<T = {}> = new(...args: any[]) => T;

interface MyMixinY {
    doY()
} 

function MixinY<T extends Constructor>(BaseClass: T)
    : Constructor<MyMixinY> & T {

    return <any> class Y extends BaseClass implements MyMixinY {
        doY() { 
            console.log("in Y");
        }
    }
}

const MixedA = MixinY(class X {
    doX() {
        console.log("in X");
    }
});

function getReturnType<R>(fn: (new(...args: any[]) => R)): R {
  return {} as R;
}

const dummy = getReturnType(MixedA);
type MixedAType = typeof dummy; // now is `type MixedAType = MyMixinY & X`

const mixedA: MixedAType = new MixedA();
mixedA.doX();
mixedA.doY();