在DB Firestore上写入可观察值

Question

我正在使用Firestore构建一个项目，并且只停留其中一部分。我创建了一项上传文件的服务，效果很好，并且我将pipe与last()和concatMap结合使用，以从Firestore返回getDownloadURL()。

我遇到的问题是因为这是一个可观察到的问题，我无法获取该值并将其分配给我创建的要推送到数据库的对象。有人可以帮我吗？

以下是文件：

export class AddComponent { 
    uploadPercentage: Observable<number>;
    logoURL: Observable<string>;

onLogoChanged(event) {
        this.venueID = this.venueService.createID();
        const logo: File = event.target.files[0];
        const path = 'venues/logo/' + this.venueID + '/' + logo.name;
        const task = this.uploadService.uploadFile(logo, path);
        this.uploadPercentage = task.percentageChanges();
        this.logoURL = task.snapshotChanges().pipe(
            last(),
            concatMap(() => task.task.snapshot.ref.storage.ref(path).getDownloadURL())
        );
    }

    onFormSubmit() {
        const venueObject = { creationDate: Date.now(), isVerified: false, logo: this.logoURL, ...this.venueForm.value };
        console.log(venueObject);
        this.venueService.createNewVenue(venueObject)
            .then(
                () => {
                    this.dismiss();
                    this.venueForm.reset();
                },
                (e) => console.error(e)
            );
    }

}

upload.service.ts

export class UploadService {
    uploadPercent: Observable<number>;
    constructor(private uploadDB: AngularFireStorage) {}

    uploadFile(file: File, path: string) {
        return this.uploadDB.upload(path, file);
    }
}