Question

您好，这是我的事，我有一个消息服务，我想通过HTTP请求（Rest Api）从服务器收集数据，如下所示：

import { Sensor } from './sensor.model'
import { HttpClient } from '@angular/common/http';

export class MessageService{
  mqttMessageData : JSON[]=[];
  coapMessageData : JSON[]=[];
  xmppMessageData : JSON[]=[];

  constructor(private httpClient: HttpClient) {

  }
getMqttMessages() {
    this.httpClient.get<JSON>('http://127.0.0.1:5002/messages/mqtt').subscribe(data => {
      this.mqttMessageData.push(data);
      console.log(data);
    });
  }

但是我的浏览器控制台上出现此错误（ng serve中没有错误）：

Uncaught Error: Can't resolve all parameters for MessageService: (?).
    at syntaxError (compiler.js:1016)
    at CompileMetadataResolver.push../node_modules/@angular/compiler/fesm5/compiler.js.CompileMetadataResolver._getDependenciesMetadata (compiler.js:10917)
    at CompileMetadataResolver.push../node_modules/@angular/compiler/fesm5/compiler.js.CompileMetadataResolver._getTypeMetadata (compiler.js:10810)
    at CompileMetadataResolver.push../node_modules/@angular/compiler/fesm5/compiler.js.CompileMetadataResolver._getInjectableTypeMetadata (compiler.js:11032)
    at CompileMetadataResolver.push../node_modules/@angular/compiler/fesm5/compiler.js.CompileMetadataResolver.getProviderMetadata (compiler.js:11041)
    at compiler.js:10979
    at Array.forEach (<anonymous>)
    at CompileMetadataResolver.push../node_modules/@angular/compiler/fesm5/compiler.js.CompileMetadataResolver._getProvidersMetadata (compiler.js:10939)
    at CompileMetadataResolver.push../node_modules/@angular/compiler/fesm5/compiler.js.CompileMetadataResolver.getNonNormalizedDirectiveMetadata (compiler.js:10416)
    at CompileMetadataResolver.push../node_modules/@angular/compiler/fesm5/compiler.js.CompileMetadataResolver._getEntryComponentMetadata (compiler.js:11013)

所以我认为httpClient存在问题，但我不知道如何通过其他方式进行操作。

Answer 1

您需要在服务上添加@Injectable装饰器

import { Injectable } from '@angular/core'

@Injectable()
export class MessageService {
   ...
}

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