Observable.of对图像src中的管道不返回任何内容

我在应用程序中为图像src制作了一个自定义管道：

它正像这样在选择器上使用：

<img [src]="myobject?.URL | secure" />

管道的代码是：

    import { Pipe, PipeTransform } from '@angular/core';
    import { Http, ResponseContentType } from '@angular/http';
    import { DomSanitizer, SafeUrl } from '@angular/platform-browser';
    import { Observable } from 'rxjs/Observable';
    import 'rxjs/add/observable/of';

    @Pipe({
      name: 'secure'
    })
    export class SecurePipe implements PipeTransform {

      constructor(private http: Http, private sanitizer: DomSanitizer) { }

      transform(url): Observable<SafeUrl> {

        if (//myboolcondition) {
          return this.http
            .get(url, { responseType: ResponseContentType.Blob })
            .catch(err => Observable.throw(err))
            .map(val => this.sanitizer.bypassSecurityTrustUrl(URL.createObjectURL(val)));
        }
        else {
          // This is not working and the image src are blank 
// My goal in this else loop is just to return what was there originally // in [src]="myobject?.URL in selector

          return Observable.of(url);

        }
      }
    }

我面临的问题是管道代码中if循环的else部分无法正常工作，并且返回空响应，而else部分则无法呈现图像。

当我的代码中满足条件的else循环时，您能否让我知道如何返回相同的url，即“ [src] =“ myobject？.URL”一样。