延期承诺 [英] Deferred that extends Promise

问题描述

如何实现扩展Promise的Deferred Promise？重要的是，将Promise扩展到期望典型Promise的类型安全使用。

How to implement a Deferred promise that extends Promise? It's important to extend Promise for type-safe usage where a typical Promise is expected.

后续实施

export class Deferred<T> extends Promise<T> {                                   
  public _resolveSelf;
  public _rejectSelf;                                                           
  constructor() {
    super(
      (resolve, reject) =>
      {
        this._resolveSelf = resolve
        this._rejectSelf = reject
      }
    )
  }                                                                             
  public resolve(val:T) { this._resolveSelf(val) }
  public reject(reason:any) { this._rejectSelf(reason) }                        
}

抛出 TypeError：_this undefined 。

在此Typescript游乐场，您可以看到编译的javascript很有趣。在第15行中，在声明 _this 时，已经分配了其属性。

In this Typescript playground, one can see that the compiled javascript is funny. In line 15, during declaration of _this already its properties are being assigned.

推荐答案

export class Deferred<T> implements Promise<T> {

  private _resolveSelf;
  private _rejectSelf;
  private promise: Promise<T>

  constructor() {
    this.promise = new Promise( (resolve, reject) =>
      {
        this._resolveSelf = resolve
        this._rejectSelf = reject

      }
    )
  }

  public then<TResult1 = T, TResult2 = never>(
    onfulfilled?: ((value: T) =>
      TResult1 | PromiseLike<TResult1>) | undefined | null,
    onrejected?: ((reason: any) =>
      TResult2 | PromiseLike<TResult2>) | undefined | null
    ): Promise<TResult1 | TResult2> {
      return this.promise.then(onfulfilled, onrejected)
    }

  public catch<TResult = never>(
    onrejected?: ((reason: any) =>
      TResult | PromiseLike<TResult>) | undefined | null
    ): Promise<T | TResult> {
      return this.promise.then(onrejected)
    }

  public resolve(val:T) { this._resolveSelf(val) }
  public reject(reason:any) { this._rejectSelf(reason) }

  [Symbol.toStringTag]: 'Promise'

}

然后和 catch 的方法签名-从 Typescript的Promise接口粘贴并化妆品清理。 [Symbol.toStringTag] 行对于Typescript来说也是必须的，尽管您只能从编译器中找到。

Method signatures of then and catch copy-pasted from Typescript's Promise interface and cosmetically cleaned up. The [Symbol.toStringTag] line is also necessary for Typescript to consider this a Promise, though you only find that out from the compiler.

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