在Jasmine单元测试中运行时，承诺未得到解决

Question

在测试在Jasmine单元测试中返回promise的方法时遇到一个奇怪的问题。我有以下课程：

import http = require('http');
import https = require('https');
import fs = require('fs');

export default class ImageDownloader {
    constructor(protected downloadPath:string) {}

    async download(url: string): Promise<string> {
        return new Promise<string>((resolve, reject) => {
            let file = fs.createWriteStream(this.downloadPath, { autoClose: true });
            let schema:any = url.startsWith('https') ? https : http;

            schema.get(url, (response:fs.ReadStream) => {
                response.pipe(file).on('finish', () => resolve(this.downloadPath));
            }).on('error', (error) => reject(error));
        });
    }
}

它将图像下载到构造函数中给出的下载路径，然后在文件写入磁盘后使用该下载路径解析promise。

我写过一个Jasmine单元测试：

import fs = require('fs');
import http = require('http');
import ImageDownloader from '../../src/Image/ImageDownloader';

describe('Image Downloader', () => {
    describe('download()', () => {
        it('returns the path to the downloaded image', done => {
            spyOn(http, 'get').and.callFake((url, cb) => {
                cb(fs.createReadStream(__dirname+'/../assets/elephpant.png'));
            });

            let imageDownloader = new ImageDownloader('/tmp/image');

            imageDownloader.download('http://foo.bar/bazz.jpg').then((path) => {
                expect(path).toEqual('/tmp/image');
                done();
            });
        });
    });
});

在这个测试中，我模拟了Node http模块，以便返回我自己伪造的＃34; HTTP响应＆＃34;。请记住Node的IncomingMessage类（通常作为来自http.get()的响应返回的内容）实现了可读的流接口，所以我只是用{{返回我自己的可读流1}}。

运行此测试时，它只会超时，因为承诺永远不会得到解决：

fs.createReadStream()

然而，奇怪的是，如果我改变解析对此承诺的F Failures: 1) Image Downloader download() returns the path to the downloaded image 1.1) Error: Timeout - Async callback was not invoked within timeout specified by jasmine.DEFAULT_TIMEOUT_INTERVAL. 方法的部分：

download()

我看到了这个输出：

response.pipe(file).on('finish', () => console.log('the promise would normally be resolved here'));

因此，当我们调用诺言时，我们显然已经接触到了这一部分。这就像我打电话给the promise would normally be resolved here F Failures: 1) Image Downloader download() returns the path to the downloaded image 1.1) Error: Timeout - Async callback was not invoked within timeout specified by jasmine.DEFAULT_TIMEOUT_INTERVAL. 但承诺因某种原因无法解决。