Angular2:如何在渲染组件之前加载数据？

Question

我试图在组件呈现之前从我的API加载一个事件.目前我正在使用我的API服务,我从组件的ngOnInit函数调用它.

我的EventRegister组件:

import {Component, OnInit, ElementRef} from "angular2/core";
import {ApiService} from "../../services/api.service";
import {EventModel} from '../../models/EventModel';
import {Router, ROUTER_DIRECTIVES, ROUTER_PROVIDERS, RouteConfig, RouteParams, RouterLink} from 'angular2/router';
import {FORM_PROVIDERS, FORM_DIRECTIVES, Control} from 'angular2/common';

@Component({
    selector: "register",
    templateUrl: "/events/register"
    // provider array is added in parent component
})

export class EventRegister implements OnInit {
    eventId: string;
    ev: EventModel;

    constructor(private _apiService: ApiService, 
                        params: RouteParams) {
        this.eventId = params.get('id');
    }

    fetchEvent(): void {
        this._apiService.get.event(this.eventId).then(event => {
            this.ev = event;
            console.log(event); // Has a value
            console.log(this.ev); // Has a value
        });
    }

    ngOnInit() {
        this.fetchEvent();
        console.log(this.ev); // Has NO value
    }
}

我的EventRegister模板

<register>
    Hi this sentence is always visible, even if `ev property` is not loaded yet    
    <div *ngIf="ev">
        I should be visible as soon the `ev property` is loaded. Currently I am never shown.
        <span>{{event.id }}</span>
    </div>
</register>

我的API服务

import "rxjs/Rx"
import {Http} from "angular2/http";
import {Injectable} from "angular2/core";
import {EventModel} from '../models/EventModel';

@Injectable()
export class ApiService {
    constructor(private http: Http) { }
    get = {
        event: (eventId: string): Promise<EventModel> => {
            return this.http.get("api/events/" + eventId).map(response => {
                return response.json(); // Has a value
            }).toPromise();
        }     
    }     
}

在ngOnInit函数中的API调用完成获取数据之前,组件将被呈现.所以我永远不会在视图模板中看到事件ID.所以看起来这是一个ASYNC问题.我希望绑定ev(EventRegister组件)ev在设置属性后做一些工作.可悲的是,它没有显示属性设置时的div标记*ngIf="ev".

问题:我是否采用了良好的方法？如果不; 在组件开始渲染之前加载数据的最佳方法是什么？

注意:该ngOnInit方法用于此angular2教程.

编辑:

两种可能的方案.首先是放弃fetchEvent并在ngOnInit函数中使用API​​服务.

ngOnInit() {
    this._apiService.get.event(this.eventId).then(event => this.ev = event);
}

二解决方案.就像给出的答案一样.

fetchEvent(): Promise<EventModel> {
    return this._apiService.get.event(this.eventId);
}

ngOnInit() {
    this.fetchEvent().then(event => this.ev = event);
}

Answer 1

更新

• 如果您使用路由器,您可以使用生命周期挂钩或解析器来延迟导航,直到数据到达. https://angular.io/guide/router#milestone-5-route-guards

• 在APP_INITIALIZER可以使用根组件的初始渲染之前加载数据如何将从后端渲染的参数传递给angular2 bootstrap方法

原版的

当console.log(this.ev)后执行this.fetchEvent();,这并不意味着fetchEvent()调用完成,这只是意味着它计划.当console.log(this.ev)执行,调用服务器还没做过,当然还没有返回值.

改变fetchEvent()返回一个Promise

 fetchEvent(){
    return  this._apiService.get.event(this.eventId).then(event => {
        this.ev = event;
        console.log(event); // Has a value
        console.log(this.ev); // Has a value
    });
 }

更改ngOnInit()等待Promise完成

ngOnInit() {
    this.fetchEvent().then(() =>
    console.log(this.ev)); // Now has value;
}

这实际上不会为你的用例买多少.

我的建议:将整个模板包裹起来 <div *ngIf="isDataAvailable"> (template content) </div>

并在 ngOnInit()

isDataAvailable:boolean = false;

ngOnInit() {
    this.fetchEvent().then(() =>
    this.isDataAvailable = true); // Now has value;
}

Answer 2

我发现一个很好的解决方案是在UI上执行以下操作:

<div *ngIf="isDataLoaded">
 ...Your page...
</div

仅当:isDataLoaded为true时才会呈现页面.

Answer 3

您可以使用Angular2 +中的解析器预取数据,解析器会在您的Component完全加载之前处理您的数据.

在许多情况下,只有在发生某些事情时才要加载组件,例如,仅当已登录的人员导航到仪表板时,在这种情况下,解析器非常方便.

查看我为您创建的简单图表,其中一种方法是使用解析器将数据发送到组件.

将Resolver应用于您的代码非常简单,我为您创建了片段,以便了解如何创建解析器:

import { Injectable } from '@angular/core';
import { Router, Resolve, RouterStateSnapshot, ActivatedRouteSnapshot } from '@angular/router';
import { MyData, MyService } from './my.service';

@Injectable()
export class MyResolver implements Resolve<MyData> {
  constructor(private ms: MyService, private router: Router) {}

  resolve(route: ActivatedRouteSnapshot, state: RouterStateSnapshot): Promise<MyData> {
    let id = route.params['id'];

    return this.ms.getId(id).then(data => {
      if (data) {
        return data;
      } else {
        this.router.navigate(['/login']);
        return;
      }
    });
  }
}

在模块中:

import { MyResolver } from './my-resolver.service';

@NgModule({
  imports: [
    RouterModule.forChild(myRoutes)
  ],
  exports: [
    RouterModule
  ],
  providers: [
    MyResolver
  ]
})
export class MyModule { }

你可以在你的组件中访问它,如下所示:

/////
 ngOnInit() {
    this.route.data
      .subscribe((data: { mydata: myData }) => {
        this.id = data.mydata.id;
      });
  }
/////

在路线中这样的东西(通常在app.routing.ts文件中):

////
{path: 'yourpath/:id', component: YourComponent, resolve: { myData: MyResolver}}
////