ReadableStream.cancel()

ReadableStream 接口的 cancel() 方法返回一个 Promise,这个 promise 会在流被取消的时候兑现。

cancel 用于在不再需要来自它的任何数据的情况下(即使仍有排队等待的数据块)完全结束一个流。调用 cancel 后该数据丢失,并且流不再可读。为了仍然可以读这些数据块而不完全结束这个流,你应该使用 ReadableStreamDefaultController.close()

语法

js

cancel()
cancel(reason)

参数

reason 可选

人类可读的取消原因。底层源代码可能会使用它。

返回值

一个 Promise,会在结束时使用给定的 reason 参数兑现。

异常

TypeError

它尝试取消的流不是 ReadableStream,或者它已被锁定。

示例

在 Jake Archibald 的取消一个 fetch 示例中,流用于逐块获取 WHATWG HTML 规范;并在每个块中搜索“service workers”字符串。当找到搜索词时,使用 cancel() 取消流——作业已被完成,因此不再需要它。

js

var searchTerm = "service workers";
// Chars to show either side of the result in the match
var contextBefore = 30;
var contextAfter = 30;
var caseInsensitive = true;
var url = "https://html.spec.whatwg.org/";
console.log(`Searching '${url}' for '${searchTerm}'`);
fetch(url)
  .then((response) => {
    console.log("Received headers");
    var decoder = new TextDecoder();
    var reader = response.body.getReader();
    var toMatch = caseInsensitive ? searchTerm.toLowerCase() : searchTerm;
    var bufferSize = Math.max(toMatch.length - 1, contextBefore);
    var bytesReceived = 0;
    var buffer = "";
    var matchFoundAt = -1;
    return reader.read().then(function process(result) {
      if (result.done) {
        console.log("Failed to find match");
        return;
      }
      bytesReceived += result.value.length;
      console.log(`Received ${bytesReceived} bytes of data so far`);
      buffer += decoder.decode(result.value, { stream: true });
      // already found match & just context-gathering?
      if (matchFoundAt === -1) {
        matchFoundAt = (
          caseInsensitive ? buffer.toLowerCase() : buffer
        ).indexOf(toMatch);
      }
      if (matchFoundAt === -1) {
        buffer = buffer.slice(-bufferSize);
      } else if (
        buffer.slice(matchFoundAt + toMatch.length).length >= contextAfter
      ) {
        console.log("Here's the match:");
        console.log(
          buffer.slice(
            Math.max(0, matchFoundAt - contextBefore),
            matchFoundAt + toMatch.length + contextAfter,
          ),
        );
        console.log("Cancelling fetch");
        reader.cancel();
        return;
      } else {
        console.log("Found match, but need more context…");
      }
      // 保持读取
      return reader.read().then(process);
    });
  })
  .catch((err) => {
    console.log(
      "Something went wrong. See devtools for details. Does the response lack CORS headers?",
    );
    throw err;
  });

规范

Specification
Streams Standard
# ref-for-rs-cancel③

浏览器兼容性

BCD tables only load in the browser

参见