1. 在 Angular 项目中引入 RxJs 库：

import { Observable } from 'rxjs/Observable';
import { BehaviorSubject } from 'rxjs/BehaviorSubject';
import { Subscription } from 'rxjs/Subscription';
import { ajax } from 'rxjs/observable/dom/ajax';
import 'rxjs/add/operator/map';
import 'rxjs/add/operator/scan';
import 'rxjs/add/operator/takeUntil';
import 'rxjs/add/operator/startWith';
import 'rxjs/add/operator/catch';

2. 创建一个 Service 来下载文件，并且带有进度监测功能：

@Injectable()
export class DownloadService {
    progress$: BehaviorSubject = new BehaviorSubject(0);
    progress: number = 0;
    subscription: Subscription;

    constructor() { }

    downloadFile(url: string, fileName: string): Observable {
        return Observable.create(observer => {
            this.subscription = ajax({
                url: url,
                responseType: 'blob',
                progressSubscriber: progress => {
                    this.updateProgress(progress.loaded, progress.total);
                }
            }).map(xhr => xhr.response)
                .subscribe(data => {
                    observer.next(data);
                    observer.complete();
                }, error => {
                    observer.error(error);
                });
        });
    }

    updateProgress(loaded: number, total: number) {
        if (total === 0) {
            this.progress = 0;
        } else {
            this.progress = Math.round(loaded / total * 100);
        }
        this.progress$.next(this.progress);
    }

    cancelDownload() {
        if (this.subscription) {
            this.subscription.unsubscribe();
        }
    }
}

3. 在组件的代码中使用下载服务，并且监听服务的进度：

export class DownloadComponent {
    // 初始化变量
    isDownloading: boolean = false;
    fileName = 'example.pdf';

    constructor(private downlaodService: DownloadService) { }

    downloadFile() {
        this.isDownloading = true;
        const url = 'http://localhost:8080/api/download';
        this.downlaodService.downloadFile(url, this.fileName)
            .takeUntil(this.cancelDownload)
            .subscribe(
                data => {
                    saveAs(data, this.fileName);
                },
                error => {
                    console.log(error);
                },
                ()