我需要"暂停"一个可读的流以一定数量的秒数,然后再次恢复。可读的流被管道输送到变换流,因此我无法使用常规的pause
和resume
方法,我必须使用unpipe
和pipe
。在"变换流"中,我能够检测到pipe
事件,然后在可读的流上进行unpipe
,然后在数秒之后,再次进行pipe
进行恢复(我希望(。
这是代码:
main.ts
import {Transform, Readable} from 'stream';
const alphaTransform = new class extends Transform {
constructor() {
super({
objectMode: true,
transform: (chunk: string | Buffer, encoding: string, callback: Function) => {
let transformed: IterableIterator<string>;
if (Buffer.isBuffer(chunk)) {
transformed = function* () {
for (const val of chunk) {
yield String.fromCharCode(val);
}
}();
} else {
transformed = chunk[Symbol.iterator]();
}
callback(null,
Array.from(transformed).map(s => s.toUpperCase()).join(''));
}
});
}
}
const spyingAlphaTransformStream = new class extends Transform {
private oncePaused = false;
constructor() {
super({
transform: (chunk: string | Buffer, encoding: string, callback: Function) => {
console.log('Before transform:');
if (Buffer.isBuffer(chunk)) {
console.log(chunk.toString('utf-8'));
alphaTransform.write(chunk);
} else {
console.log(chunk);
alphaTransform.write(chunk, encoding);
}
callback(null, alphaTransform.read());
}
});
this.on('pipe', (src: Readable) => {
if (!this.oncePaused) {
src.unpipe(this); // Here I unpipe the readable stream
console.log(`Data event listeners count: ${src.listeners('data').length}`);
console.log(`Readable state of reader: ${src.readable}`);
console.log("We paused the reader!!");
setTimeout(() => {
this.oncePaused = true;
src.pipe(this); // Here I resume it...hopefully?
src.resume();
console.log("We unpaused the reader!!");
console.log(`Data event listeners count: ${src.listeners('data').length}`);
console.log(`Readable state of reader: ${src.readable}`);
}, 1000);
}
});
this.on('data', (transformed) => {
console.log('After transform:n', transformed);
});
}
}
const reader = new class extends Readable {
constructor(private content?: string | Buffer) {
super({
read: (size?: number) => {
if (!this.content) {
this.push(null);
} else {
this.push(this.content.slice(0, size));
this.content = this.content.slice(size);
}
}
});
}
} (new Buffer('The quick brown fox jumps over the lazy dog.n'));
reader.pipe(spyingAlphaTransformStream)
.pipe(process.stdout);
问题是中间流spyingAlphaTransformStream
。这是听管道事件的聆听,然后暂停并在1 second
之后恢复可读的流。问题在于,在它解开可读的流之后,然后再次从其管道上进行管道,没有写入标准输出,这意味着spyingAlphaTransformStream
的transform
方法永远不会被调用,这意味着流中的某些东西被损坏了。
我希望输出看起来像:
Data event listeners count: 0
Readable state of reader: true
We paused the reader!!
We unpaused the reader!!
Data event listeners count: 1
Readable state of reader: true
Before transform:
The quick brown fox jumps over the lazy dog.
After transform:
THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG.
THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG.
但实际上看起来像:
Data event listeners count: 0
Readable state of reader: true
We paused the reader!!
We unpaused the reader!!
Data event listeners count: 1
Readable state of reader: true
基本上什么都没从中填充可读性是我可以从中得出的。
我该如何修复?
package.json
{
"name": "hello-stream",
"version": "1.0.0",
"main": "main.ts",
"scripts": {
"start": "npm run build:live",
"build:live": "nodemon"
},
"keywords": [
"typescript",
"nodejs",
"ts-node",
"cli",
"node",
"hello"
],
"license": "WTFPL",
"devDependencies": {
"@types/node": "^7.0.21",
"nodemon": "^1.11.0",
"ts-node": "^3.0.4",
"typescript": "^2.3.2"
},
"dependencies": {}
}
nodemon.json
{
"ignore": ["node_modules"],
"delay": "2000ms",
"execMap": {
"ts": "ts-node"
},
"runOnChangeOnly": false,
"verbose": true
}
tsconfig.json
{
"compilerOptions": {
"target": "es2015",
"module": "commonjs",
"typeRoots": ["node_modules/@types"],
"lib": ["es6", "dom"],
"strict": true,
"noUnusedLocals": true,
"types": ["node"]
}
}
该解决方案比我预期的要简单。我要做的是找到一种方法来推迟transform
方法中完成的所有回调,然后等到流到"准备好"之前,请拨打初始回调。
基本上,在spyingAlphaTransformStream
构造函数中,我有一个布尔值检查流是否准备就绪,如果还没有,我在同类中存储了一个回调,该回调将执行我在transform
方法中收到的第一个回调。现在,由于没有执行第一个回调,因此流未接收其他呼叫 即,只有一个待定的回调可以担心;因此,现在只是一个等待游戏,直到流表示已准备就绪(这是用简单的setTimeout
完成的(。
当流"准备就绪"时,我将准备就绪的布尔值设置为true,然后我调用待定回调(如果设置(,此时,流程在整个流中继续。
我有一个更长的例子来显示它的工作方式:
import {Transform, Readable} from 'stream';
const alphaTransform = new class extends Transform {
constructor() {
super({
objectMode: true,
transform: (chunk: string | Buffer, encoding: string, callback: Function) => {
let transformed: IterableIterator<string>;
if (Buffer.isBuffer(chunk)) {
transformed = function* () {
for (const val of chunk) {
yield String.fromCharCode(val);
}
}();
} else {
transformed = chunk[Symbol.iterator]();
}
callback(null,
Array.from(transformed).map(s => s.toUpperCase()).join(''));
}
});
}
}
class LoggingStream extends Transform {
private pending: () => void;
private isReady = false;
constructor(message: string) {
super({
objectMode: true,
transform: (chunk: string | Buffer, encoding: string, callback: Function) => {
if (!this.isReady) { // ready flag
this.pending = () => { // create a pending callback
console.log(message);
if (Buffer.isBuffer(chunk)) {
console.log(`[${new Date().toTimeString()}]: ${chunk.toString('utf-8')}`);
} else {
console.log(`[${new Date().toTimeString()}]: ${chunk}`);
}
callback(null, chunk);
}
} else {
console.log(message);
if (Buffer.isBuffer(chunk)) {
console.log(`[${new Date().toTimeString()}]: ${chunk.toString('utf-8')}`);
} else {
console.log(`[${new Date().toTimeString()}]: ${chunk}`);
}
callback(null, chunk);
}
}
});
this.on('pipe', this.pauseOnPipe);
}
private pauseOnPipe() {
this.removeListener('pipe', this.pauseOnPipe);
setTimeout(() => {
this.isReady = true; // set ready flag to true
if (this.pending) { // execute pending callbacks (if any)
this.pending();
}
}, 3000); // wait three seconds
}
}
const reader = new class extends Readable {
constructor(private content?: string | Buffer) {
super({
read: (size?: number) => {
if (!this.content) {
this.push(null);
} else {
this.push(this.content.slice(0, size));
this.content = this.content.slice(size);
}
}
});
}
} (new Buffer('The quick brown fox jumps over the lazy dog.n'));
reader.pipe(new LoggingStream("Before transformation:"))
.pipe(alphaTransform)
.pipe(new LoggingStream("After transformation:"))
.pipe(process.stdout);
输出
<Waits about 3 seconds...>
Before transformation:
[11:13:53 GMT-0600 (CST)]: The quick brown fox jumps over the lazy dog.
After transformation:
[11:13:53 GMT-0600 (CST)]: THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG.
THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG.
注意,由于JS是单线螺纹,因此两个详细的流都等待相同的时间,然后继续