尝试将服务注入角度组件时出错“ EXCEPTION：无法解析组件的所有参数”，为什么？ [英] Error when trying to inject a service into an angular component "EXCEPTION: Can't resolve all parameters for component", why?

问题描述

我已经在Angular中构建了一个基本应用程序，但是遇到了一个奇怪的问题，即我无法将服务注入我的组件之一。

I've built a basic app in Angular, but I have encountered a strange issue where I cannot inject a service into one of my components. It injects fine into any of the three other components I have created, however.

对于初学者来说，这是一项服务：

For starters, this is the service:

import { Injectable } from '@angular/core';

@Injectable()
export class MobileService {
  screenWidth: number;
  screenHeight: number;

  constructor() {
    this.screenWidth = window.outerWidth;
    this.screenHeight = window.outerHeight;

    window.addEventListener("resize", this.onWindowResize.bind(this) )
  }
  
  onWindowResize(ev: Event) {
    var win = (ev.currentTarget as Window);
    this.screenWidth = win.outerWidth;
    this.screenHeight = win.outerHeight;
  }
  
}

它拒绝使用的组件：

import { Component, } from '@angular/core';
import { NgClass } from '@angular/common';
import { ROUTER_DIRECTIVES } from '@angular/router';

import {MobileService} from '../';

@Component({
  moduleId: module.id,
  selector: 'pm-header',
  templateUrl: 'header.component.html',
  styleUrls: ['header.component.css'],
  directives: [ROUTER_DIRECTIVES, NgClass],
})
export class HeaderComponent {
  mobileNav: boolean = false;

  constructor(public ms: MobileService) {
    console.log(ms);
  }

}

我在浏览器控制台中看到的错误是：

The error I get in the browser console is this:

例外：无法解析HeaderComponent的所有参数：（？）。

EXCEPTION: Can't resolve all parameters for HeaderComponent: (?).

我在引导功能中有该服务，因此它具有提供程序。而且我似乎可以将其注入到其他任何组件的构造函数中。

I have the service in the bootstrap function so it has a provider. And I seem to be able to inject it into the constructor of any of my other components without issue.

推荐答案

我也遇到了这个问题通过将服务A注入服务B中，反之亦然。

I also encountered this by injecting service A into service B and vice versa.

我认为这很快就失败了是一件好事，因为无论如何应该避免使用 。如果您希望服务更具模块化和可重用性，则最好避免使用循环引用。这篇帖子突出显示了周围的陷阱。

I think it's a good thing that this fails fast as it should probably be avoided anyway. If you want your services to be more modular and re-usable, it's best to avoid circular references as much as possible. This post highlights the pitfalls surrounding that.

因此，我有以下建议：

• 如果您感觉上课了互动太频繁（我在谈论功能嫉妒），您可能需要考虑将2种服务合并为1类。


• 如果上述不适用于您，请考虑使用第三项服务（ EventService ），这两种服务都可以注入该第三项服务来交换消息。

• If you feel the classes are interacting too often (I'm talking about feature envy), you might want to consider merging the 2 services into 1 class.
• If the above doesn't work for you, consider using a 3rd service, (an EventService) which both services can inject in order to exchange messages.

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