@TNII，@Hoang Duc，尝试说您通常是服务公开Observables.订阅Observable时，它位于组件的ngOnInit中.

@TNII, @Hoang Duc, try say you is that generally a service expose Observables. It's in a ngOnInit in your component when you subscribe to the Observable.

//Simple return a get
getAllProductsFromACategory(categoryName: string): any {
   return this.http.get('http://localhost:8080/VespaWebshopAPI
   /api/Article/Category?categoryName=' + categoryName)
}

在组件中，通常当我们订阅时在ngOnInit中

In the component, generally in an ngOnInit when we subscribe to

ngOnInit()
{
    productService.getAllProductsFromACategory('Bremsen').
      subscribe(data => {
         if (data[0])
             this.article=data[0];
      })
}

用html编写的{{article?.id}}是一种简短的说法:如果定义了this.article，请向我显示this.article.id.

the {{article?.id}} wrote in the html is a abreviate way to say: if this.article is defined, show me this.article.id.

检查是否在导航器中编写 http://localhost:8080/VespaWebshopAPI /api/Article/Category?categoryName ='Bremsen'，为您提供一系列文章.检查数组的元素是否具有id，name等属性(或返回具有其他属性的元素)

check if when in a navigator write http://localhost:8080/VespaWebshopAPI /api/Article/Category?categoryName='Bremsen', give you an array of Articles. Check if the elements of the arrays has properties id,name,etc (or return element with another properties)