Typescript 允许对 mixins 使用适当的多重继承，但无法创建声明文件

Question

我正在玩 TypeScript 中的“多重继承”，或者更确切地说是对 mixin 有很好的理解。经过许多弯路，我发现最简单的方法是尽可能少地显式转换，并且能够创建如下所示的内容（完整的示例可以在此gist 中找到）。

我的问题是：为什么 TypeScript 允许我构建它，但是却无法为此创建声明文件？

export function TaggedMixin<Super extends Ctor>(superClass: Super) {
  class Tagged extends superClass {
    public static TAG_PUBLIC: string;
    protected static TAG_PROTECTED: string;
    private static TAG_PRIVATE: string;

    public tag_public!: string;
    protected tag_protected!: number;
    private tag_private!: number;
  }

  return Tagged;
}

const Tagged = TaggedMixin(class {
    public static ANON_PUBLIC: string;
    protected static ANON_PROTECTED: string;
    private static ANON_PRIVATE: string;

    public anon_public!: string;
    protected anon_protected!: number;
    private anon_private!: number;
});

class TaggedClass extends Tagged {
    constructor() {
        super();

        TaggedClass.ANON_PUBLIC;
        TaggedClass.ANON_PROTECTED;
        TaggedClass.TAG_PUBLIC;
        TaggedClass.TAG_PROTECTED;

        this.anon_public;
        this.anon_protected;
        this.tag_public;
        this.tag_protected;
    }
}

编辑：

该TS的错误无法创建声明文件：

Property 'tag_protected' of exported class expression may not be private or protected.ts(4094)
Property 'tag_private' of exported class expression may not be private or protected.ts(4094)
Property 'TAG_PROTECTED' of exported class expression may not be private or protected.ts(4094)
Property 'TAG_PRIVATE' of exported class expression may not be private or protected.ts(4094)

Answer 1

发出声明和混入存在限制。类表达式不能有私有成员或受保护成员，因为正如 here 所述：

如果启用了声明发射，则导出的匿名类不能有私有或受保护成员，因为无法在 .d.ts 文件中表示。

Typescript 会将 mixin 的实例类型表示为对象类型，而对象类型不能有标记为 private 或 protected 的成员，因此会报错。

删除非公开成员将解决问题。

您还可以添加一些手动输入和类声明，这将使您接近目标：

type Ctor = new (...a: any[]) => any
declare class TaggedMixinInstance { // just a dummy, never call new TaggedMixinInstance() or extend this directly 
    public tag_public: string;
    protected tag_protected: number;
    private tag_private: number;
}

export function TaggedMixin<Super extends Ctor>(superClass: Super): {
    new(...a: any[]): TaggedMixinInstance;
    TAG_PUBLIC: string
} & Super {
    class Tagged extends superClass {
        public static TAG_PUBLIC: string;
        protected static TAG_PROTECTED: string;
        private static TAG_PRIVATE: string;

        public tag_public!: string;
        protected tag_protected!: number;
        private tag_private!: number;
    }

    return Tagged;
}

const Tagged = TaggedMixin(class {
    public static ANON_PUBLIC: string;
    protected static ANON_PROTECTED: string;
    private static ANON_PRIVATE: string;

    public anon_public!: string;
    protected anon_protected!: number;
    private anon_private!: number;
});

class TaggedClass extends Tagged {
    constructor() {
        super();

        TaggedClass.ANON_PUBLIC;
        TaggedClass.ANON_PROTECTED;
        TaggedClass.TAG_PUBLIC;
        TaggedClass.TAG_PROTECTED; // still an error

        this.anon_public;
        this.anon_protected;
        this.tag_public;
        this.tag_protected;
    }
}