如何在角度6服務中發出Http請求

Question

這是我的事，我有一個消息服務，我想通過HTTP請求（Rest Api）從我的服務器收集數據，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);
    });
  }

但我在瀏覽器控制台上出現此錯誤（ng服務中沒有錯誤）：

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)

所以我假設httpClient存在問題，但我不知道如何以其他方式執行此操作。

Answer 1

您需要在服務上添加@Injectable裝飾器

import { Injectable } from '@angular/core'

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