angular - 为什么大多数 HttpClient API 类都定义不可变对象(immutable对象)？

Question

documentation for HttpClient声明以下关于不变性的内容:

Interceptors exist to examine and mutate outgoing requests and incoming responses. However, it may be surprising to learn that the HttpRequest and HttpResponse classes are largely immutable.

This is for a reason: because the app may retry requests, the interceptor chain may process an individual request multiple times. If requests were mutable, a retried request would be different than the original request. Immutability ensures the interceptors see the same request for each try.

我觉得很难理解这个解释。有人可以提供解释吗？

Answer 1

这个解释不懂就很难看懂the source code .在文章 Insider’s guide into interceptors and HttpClient mechanics in Angular 中对此进行了深入解释.

当你调用 http 的任何方法时，比如 get，Angular 会创建一个请求。然后这个请求用于启动一个可观察的序列，当订阅这个请求时通过拦截器链传递。这是作为序列处理的一部分完成的(非常简化的代码):

function get() {
    let req: HttpRequest<any> = new HttpRequest<any>();
    const events$ = of(req).pipe(concatMap((req: HttpRequest<any>) => {
        // running request through interceptors chain
        this.handler.handle(req);
    }));
    return $events;
}

这是来自消息来源的评论:

Start with an Observable.of() the initial request, and run the handler (which includes all interceptors) inside a concatMap(). This way, the handler runs inside an Observable chain, which causes interceptors to be re-run on every subscription (this also makes retries re-run the handler, including interceptors).

因此 $events 流是从 http 请求方法返回的并且可以重试。拦截器应始终从原始请求开始。如果请求是可变的并且可以在之前的拦截器运行期间修改，则不能满足此条件。因此请求及其所有组成部分应该是不可变的。