read()
方法在
ReadableStreamDefaultReader
interface returns a promise providing access to the next chunk in the stream's internal queue.
var promise = readableStreamDefaultReader.read();
None.
A
Promise
, which fulfills/rejects with a result depending on the state of the stream. The different possibilities are as follows:
{ value: theChunk, done: false }
.
{ value: undefined, done: true }
.
ReadableStreamDefaultReader
, or the stream has no owner.
This example shows the basic API usage, but doesn't try to deal with complications like stream chunks not ending on line boundaries for example.
In this example
stream
is a previously-created custom
ReadableStream
. It is read using a
ReadableStreamDefaultReader
created using
getReader()
. (see our
Simple random stream example
for the full code). Each chunk is read sequentially and output to the UI as an array of UTF-8 bytes, until the stream has finished being read, at which point we return out of the recursive function and print the entire stream to another part of the UI.
function fetchStream() {
const reader = stream.getReader();
let charsReceived = 0;
// read() returns a promise that resolves
// when a value has been received
reader.read().then(function processText({ done, value }) {
// Result objects contain two properties:
// done - true if the stream has already given you all its data.
// value - some data. Always undefined when done is true.
if (done) {
console.log("Stream complete");
para.textContent = result;
return;
}
// value for fetch streams is a Uint8Array
charsReceived += value.length;
const chunk = value;
let listItem = document.createElement('li');
listItem.textContent = 'Received ' + charsReceived + ' characters so far. Current chunk = ' + chunk;
list2.appendChild(listItem);
result += chunk;
// Read some more, and call this function again
return reader.read().then(processText);
});
}
This example shows how you might fetch a text file and handle it as a stream of text lines. It deals with stream chunks not ending on line boundaries and converting from Uint8Array to strings.
async function* makeTextFileLineIterator(fileURL) {
const utf8Decoder = new TextDecoder("utf-8");
let response = await fetch(fileURL);
let reader = response.body.getReader();
let {value: chunk, done: readerDone} = await reader.read();
chunk = chunk ? utf8Decoder.decode(chunk) : "";
let re = /\r\n|\n|\r/gm;
let startIndex = 0;
let result;
for (;;) {
let result = re.exec(chunk);
if (!result) {
if (readerDone) {
break;
}
let remainder = chunk.substr(startIndex);
({value: chunk, done: readerDone} = await reader.read());
chunk = remainder + (chunk ? utf8Decoder.decode(chunk) : "");
startIndex = re.lastIndex = 0;
continue;
}
yield chunk.substring(startIndex, result.index);
startIndex = re.lastIndex;
}
if (startIndex < chunk.length) {
// last line didn't end in a newline char
yield chunk.substr(startIndex);
}
}
for await (let line of makeTextFileLineIterator(urlOfFile)) {
processLine(line);
}
| 规范 | 状态 | 注释 |
|---|---|---|
|
流
The definition of 'read()' in that specification. |
实时标准 | 初始定义。 |
| 桌面 | 移动 | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
read
|
Chrome ? | Edge ? |
Firefox
65
|
IE ? | Opera ? | Safari ? | WebView Android ? | Chrome Android ? |
Firefox Android
65
|
Opera Android ? | Safari iOS ? | Samsung Internet Android ? |
完整支持
兼容性未知
实验。期望将来行为有所改变。
用户必须明确启用此特征。
ReadableStreamDefaultReader
cancel()
read()
releaseLock()