[功能介紹-20] HttpClient

使用HttpClientModule

官方文件：https://angular.io/api/common/http/HttpClientModule

下面是一個簡單的使用範例，假使我們要讀取這樣格式的JSON資料：
{
“results”: [
“Item 1”,
“Item 2”,
]
}

@Component(...)
export class MyComponent implements OnInit {

  results: string[];

  // Inject HttpClient into your component or service.
  constructor(private http: HttpClient) {}

  ngOnInit(): void {
    // Make the HTTP request:
    this.http.get('/api/items').subscribe(data => {
      // Read the result field from the JSON response.
      this.results = data['results'];
    });
  }
}

this.results即為一個含有["item1","item2"]的陣列

定義回傳格式

首先先建立一個要回傳的物件介面

interface ItemsResponse {
  results: string[];
}

在get後面設定要回傳時可取得的物件或介面

http.get<itemsResponse>('/api/items').subscribe(data => {
  // data is now an instance of type ItemsResponse, so you can do this:
  this.results = data.results;
});

取得完整的HTTP Response

很多時候我們也會需要Http header的資料，而不光是回傳的body，這時候可以更改observe的值為response

http
  .get<myJsonData>('/data.json', {observe: 'response'})
  .subscribe(resp => {
    // Here, resp is of type HttpResponse<myJsonData>.
    // You can inspect its headers:
    console.log(resp.headers.get('X-Custom-Header'));
    // And access the body directly, which is typed as MyJsonData as requested.
    console.log(resp.body.someField);
  });

處理錯誤訊息

可以在.subscribe()下面增加err函數來處理HTTP錯誤的狀況

http
  .get<itemsResponse>('/api/items')
  .subscribe(
    // Successful responses call the first callback.
    data => {...},
    // Errors will call this callback instead:
    err => {
      console.log('Something went wrong!');
    }
  );

err這個含數可以有傳入更詳細的HTTP錯誤訊息物件HttpErrorResponse

http
  .get<itemsResponse>('/api/items')
  .subscribe(
    data => {...},
    (err: HttpErrorResponse) => {
      if (err.error instanceof Error) {
        // A client-side or network error occurred. Handle it accordingly.
        console.log('An error occurred:', err.error.message);
      } else {
        // The backend returned an unsuccessful response code.
        // The response body may contain clues as to what went wrong,
        console.log(`Backend returned code ${err.status}, body was: ${err.error}`);
      }
    }
  );

遇到錯誤時自動重試

RxJS有一個有用的運算符.retry()，可以在遇到錯誤時自動重新嘗試，使用範例如下：
先導入需要的類別

import 'rxjs/add/operator/retry';

使用的方式就是直接.retry()並傳入重試次數

http
  .get<itemsResponse>('/api/items')
  // Retry this request up to 3 times.
  .retry(3)
  // Any errors after the 3rd retry will fall through to the app.
  .subscribe(...);

接收非JSON格式的資料

傳入responseType可以設定預期會接收到的資料格式，angular預設值是json，如果是其他格式則需要設定這個欄位。
可接受的項目有『arraybuffer』、『blob』、『json』、『text』。
下面是使用方式：

http
  .get('/textfile.txt', {responseType: 'text'})
  // The Observable returned by get() is of type Observable<string>
  // because a text response was specified. There's no need to pass
  // a <string> type parameter to get().
  .subscribe(data => console.log(data));

發送POST請求

下面是發送POST請求的範例：

const body = {name: 'Brad'};

http
  .post('/api/developers/add', body)
  // See below - subscribe() is still necessary when using post().
  .subscribe(...);

所有rxjs的動作都會在有人subscribe後才會呼叫，因此如果忽略subscribe()，http將不會做任何動作。

const req = http.post('/api/items/add', body);
// 0 requests made - .subscribe() not called.
req.subscribe();
// 1 request made.
req.subscribe();
// 2 requests made.

設置request的header

http
  .post('/api/items/add', body, {
    headers: new HttpHeaders().set('Authorization', 'my-auth-token'),
  })
  .subscribe();

該HttpHeaders的內容是不變的，每次set()時會返回一個新的實體，並套用所設定的更改。

設置URL參數

如果想要發送請求至/api/items/add?id=3，可以用以下的寫法

http
  .post('/api/items/add', body, {
    params: new HttpParams().set('id', '3'),
  })
  .subscribe();

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