为什么 Angular HTTPClient 在这种情况下重复其余调用两次？

Question

我有一个使用 Rest API 的 angular 服务，但是当我检查网络和后端时，我发现 API 每次调用两次：

这是我的服务代码：

getAllUsers():Observable<any>{
    return this.http.get(this.mainConfigService.getUsersUrl()).pipe(
        map(this.extractData));
}

private extractData(res: Response) {
        let body = res;
        return body || { };
}

在我的组件中，我调用了这个服务：

  getAllUser(){
    let users : User[] = [];
    this.userService.getAllUsers().subscribe(data=>{
      this.usersList=data;
      data.forEach( (element) => {
        users.push(
               {
                fullName: element.fullName,
                firstName:element.firstName,
                lastName:element.lastName,
                mail:element.mail,
                idNumber:element.idNumber,
                accountExpiresDateTime:element.accountExpiresDateTime,
                role:element.role
             }
             );
      });
      this.dataSource = new MatTableDataSource(users);
      this.dataSource.paginator = this.paginator;
      this.dataSource.sort = this.sort;
    },err=>{
      this.handleError(err)
    })
  }

在控制台中，我看到 API 调用了两次，即使我调用 getAllUser() 的唯一地方是在 Init 方法中

我还是找不到这个问题的原因

Answer 1

这可能是一个 OPTIONS 请求

Preflighted requests

Unlike simple requests (discussed above), "preflighted" requests first send an HTTP OPTIONS request header to the resource on the other domain, in order to determine whether the actual request is safe to send. Cross-site requests are preflighted like this since they may have implications to user data. In particular, a request is preflighted if:

It uses methods other than GET or POST. Also, if POST is used to send request data with a Content-Type other than application/x-www-form-urlencoded, multipart/form-data, or text/plain, e.g. if the POST request sends an XML payload to the server using application/xml or text/xml, then the request is preflighted. It sets custom headers in the request (e.g. the request uses a header such as X-PINGOTHER)

Answer 2

如果您使用 HttpInterceptor，也许就可以了。在某些情况下，这使得。

为什么 Angular HTTPClient 在这种情况下重复其余调用两次？

Why Angular HTTPClient repeat the rest call twice in this case?

javascript

httpclient

rxjs

angular