@Inject工作时@Injectable不工作

Question

我遇到了一个奇怪的问题。

我得到了非常简单的服务：

import { Inject, Injectable } from '@angular/core';
import { HttpClient } from '@angular/common/http';
import { Observable } from 'rxjs/Observable';

@Injectable()
export class EchoService {
   constructor(private httpClient: HttpClient) {}

   public makeCall(): Observable<any> {
       return this.httpClient.get<any>('https://jsonplaceholder.typicode.com/posts/1');
   }
}

现在我正尝试在我的组件中重用它：

import { Component, Inject, OnInit } from '@angular/core';
import { EchoService } from './services/echo.service';
import { Observable } from 'rxjs/Observable';

@Component({
 selector: 'my-app',
 templateUrl: `app.component.html`,
 styleUrls: ['./app.component.css']
})
export class AppComponent implements OnInit {
   public response: Observable<any>;

   constructor(private echoService: EchoService) {}

   public ngOnInit(): void {
       this.response = this.echoService.makeCall();
   }
}

还有..。我遇到了Uncaught Error: Can't resolve all parameters for EchoService: (?).

当我从@Injectable切换到@Inject时，一切都正常：

 constructor(@Inject(HttpClient) private httpClient: HttpClient) {}

在组件中：

 constructor(@Inject(EchoService) private echoService: EchoService) {}

我的tsconfig.json是：

{
  "compilerOptions": {
    "emitDecoratorMetadata": true,
    "experimentalDecorators": true,
    "lib": [ "dom", "esnext" ]
  }
}

是什么情况？为什么@Inject可以工作，而@Injectable却不能工作？

Answer 1

同样的情况。当我将项目从Angular 5迁移到7版本时。对于我的作品：

add to tsconfig.json - "emitDecoratorMetadata"：true

完整的tsconfig.json：

​

{
  "compilerOptions": {
    "baseUrl": "./src",
    "experimentalDecorators": true,
    "emitDecoratorMetadata": true,
    "sourceMap": true,
    "moduleResolution": "node",
    "target": "es2015",
    "rootDir": "./src",
    "typeRoots": [
      "node_modules/@types"
    ],
    "lib": [
      "es2018",
      "dom"
    ],
    "skipLibCheck": true,
    "types": []
  }
}

​

Answer 2

请检查您的@NgModule声明，也许您错过了imports列表中的HttpClientModule。

Answer 3

如果我没理解错的话，你应该添加反射元数据库。https://www.typescriptlang.org/docs/handbook/decorators.html#metadata