javascript - 如何解决 Can't resolve all parameters error on angular

Question

刚开始使用 angular 2，遇到了我的第一个错误。我正在尝试创建用于从服务器获取数据的服务，但出现此错误

Error: Can't resolve all parameters for CourseService: (?). at SyntaxError.ZoneAwareError (http:……}

这是抛出错误的服务代码 app/courses/courses.service.ts

import { Http } from '@angular/http';

export class CourseService {

    private  _courses : Object[];

    public get courses() : Object[] {
        return this._courses;
    }

    public set courses(v : Object[]) {
        this._courses = v;
    }

    constructor(private http: Http) {
        http.get("https://jsonplaceholder.typicode.com/posts").subscribe(data => {
            this._courses = data.json();
        });
    }
}

这个服务应该在 app/courses/courses.component.ts 中使用

import { Component } from '@angular/core'
import { CourseService } from "app/courses/course.service";

@Component({
    selector: 'courses',
    providers: [CourseService],
    templateUrl: './courses.component.html',
    styleUrls: ['./courses.component.css'] 
})

export class CoursesComponent {
    title : string = "The title of the Course";
    courses : Object[];

    constructor(private courseService: CourseService) {
        console.log("hello");
        this.courses = courseService.courses;
    }
}

和 app.module 包含

....
....
import { AppComponent } from './app.component';
import { CoursesComponent }  from './courses/courses.component';
import { CourseService } from "./courses/course.service";

@NgModule({
     declarations: [
       AppComponent,
       CoursesComponent,
    ],
    imports: [
      BrowserModule,
      FormsModule,
      HttpModule
    ],
    providers: [CourseService],
    bootstrap: [AppComponent, CoursesComponent]
})
 export class AppModule { }

Answer 1

将@Injectable() 放在导出类之前。
@Injectable() lets Angular know that a class can be used with the dependency injector.