angular - 如何在 Angular 2 中使用 "chain"两个单独的可观察对象

Question

我正在使用 Angular2 开发 Web 应用程序，但在从服务器获取数据时遇到了一些问题。

 import ...

 @Component({
 ...

 })
 export class EmployeeManagementTableComponent implements OnInit, OnDestroy{

private employees: Employee[];
private departments: SelectItem[] = [];
private selectedDepartment: string;
private columns: any[];
private paramSub: any;
private employeesSub: any;
private departmentSub: any;


constructor(private employeeManagementService: EmployeeManagementService, 
            private route: ActivatedRoute, 
            private router: Router,
            private ccs: ComponentCommunicatorService,
            private logger: Logger) { }

ngOnInit(){

    this.columns = [
     ...
    ];

    //ccs is just a service for storing/getting app wide infomation

    this.selectedDepartment = this.ccs.getSelectedDepartment();
    this.getDepartments();
    this.getEmployees(this.selectedDepartment);

    ...
}

ngOnDestroy(){
    /*this.employeesSub.unsubscribe();
    this.departmentDub.unsubscribe();*/
}

getDepartments(){

    this.departments.push({label: 'Alle', value: 'all'});

    this.departmentSub = this.employeeManagementService.getDepartments().subscribe(
          data =>  {data.forEach((item, index) => {
                        this.departments.push({label: item, value: index.toString()});
                    });    
                   },
          err => this.logger.error(err),
          () => {this.logger.log('done loading');
                 this.departmentSub.unsubscribe()}
    );
}
getEmployees(department: any){

this.employeesSub = this.employeeManagementService.getEmployees(department).subscribe(
          data => {this.employees = data},
          err => this.logger.error(err),
          () => {this.logger.log('done loading');
                 this.employeesSub.unsubscribe()}
    );
}

如您所见，当组件初始化时，它会调用两种方法来获取数据。这些方法从我的服务中获取可观察值并订阅它们。
问题是顺序就像 call1, call2, result1, result2, ... 我认为有些地方不对。它应该是 call1, result1, call2, result2, ... 还是我错了？我尝试在 observable1 的 onComplete 中订阅 observable2，但我认为专用方法将毫无用处。我已经研究并找到了一些通过 concat 同时订阅两个可观察对象的解决方案，但我只希望代码在所有数据流量完成时在 getDepartments() 之后继续。

我应该在 OnDestroy() 中取消订阅，还是在 subscribe 函数的 OnComplete 中取消订阅我真的不明白其中的区别？

Answer 1

如果你想控制 observable 的执行顺序，你需要构建一个异步数据流，利用 flatMap(串行执行)或 Observable.forkJoin(并行执行)

示例如下:

// Series
someObservable.flatMap(result1 => {
  return someOtherObservable;
}).subscribe(result2 => {
  (...)
  (...)
});

// Parallel
Observable.forkJoin([ someObservable, someOtherObservable ])
  .subscribe(results => {
    let result1 = results[0];
    let result2 = results[1];
  });