1. 流的概念
- 流是一组有序的，有起点和终点的字节数据传输手段
- 它不关心文件的整体内容，只关注是否从文件中读到了数据，以及读到数据之后的处理
- 流是一个抽象接口，被 Node 中的很多对象所实现。比如HTTP 服务器request和response对象都是流。
2.可读流createReadStream
实现了stream.Readable接口的对象,将对象数据读取为流数据,当监听data事件后,开始发射数据
fs.createReadStream = function(path, options) {
return new ReadStream(path, options);
};
util.inherits(ReadStream, Readable);
2.1 创建可读流
var rs = fs.createReadStream(path,[options]);
- path读取文件的路径
- options
- flags打开文件要做的操作,默认为'r'
- encoding默认为null
- start开始读取的索引位置
- end结束读取的索引位置(包括结束位置)
- highWaterMark读取缓存区默认的大小64kb
如果指定utf8编码highWaterMark要大于3个字节
2.2 监听data事件
流切换到流动模式,数据会被尽可能快的读出
rs.on('data', function (data) {
console.log(data);
});
2.3 监听end事件
该事件会在读完数据后被触发
rs.on('end', function () {
console.log('读取完成');
});
2.4 监听error事件
rs.on('error', function (err) {
console.log(err);
});
2.5 监听open事件
rs.on('open', function () {
console.log(err);
});
2.6 监听close事件
rs.on('close', function () {
console.log(err);
});
2.7 设置编码
与指定{encoding:'utf8'}效果相同，设置编码
rs.setEncoding('utf8');
2.8 暂停和恢复触发data
通过pause()方法和resume()方法
rs.on('data', function (data) {
rs.pause();
console.log(data);
});
setTimeout(function () {
rs.resume();
},2000);
3.可写流createWriteStream
实现了stream.Writable接口的对象来将流数据写入到对象中
fs.createWriteStream = function(path, options) {
return new WriteStream(path, options);
};
util.inherits(WriteStream, Writable);
3.1 创建可写流
var ws = fs.createWriteStream(path,[options]);
- path写入的文件路径
- options
- flags打开文件要做的操作,默认为'w'
- encoding默认为utf8
- highWaterMark写入缓存区的默认大小16kb
3.2 write方法
ws.write(chunk,[encoding],[callback]);
- chunk写入的数据buffer/string
- encoding编码格式chunk为字符串时有用，可选
- callback 写入成功后的回调
返回值为布尔值，系统缓存区满时为false,未满时为true
3.3 end方法
ws.end(chunk,[encoding],[callback]);
表明接下来没有数据要被写入 Writable 通过传入可选的 chunk 和 encoding 参数，可以在关闭流之前再写入一段数据 如果传入了可选的 callback 函数，它将作为 'finish' 事件的回调函数
3.4 drain方法
当一个流不处在 drain 的状态， 对 write() 的调用会缓存数据块， 并且返回 false。 一旦所有当前所有缓存的数据块都排空了（被操作系统接受来进行输出）， 那么 'drain' 事件就会被触发
-
建议， 一旦 write() 返回 false， 在 'drain' 事件触发前， 不能写入任何数据块
let fs = require('fs'); let ws = fs.createWriteStream('./2.txt',{ flags:'w', encoding:'utf8', highWaterMark:3 }); let i = 10; function write(){ let flag = true; while(i&&flag){ flag = ws.write("1"); i--; console.log(flag); } } write(); ws.on('drain',()=>{ console.log("drain"); write(); });
3.5 finish方法
在调用了 stream.end() 方法，且缓冲区数据都已经传给底层系统之后， 'finish' 事件将被触发。
var writer = fs.createWriteStream('./2.txt');
for (let i = 0; i < 100; i++) {
writer.write(`hello, ${i}!\n`);
}
writer.end('结束\n');
writer.on('finish', () => {
console.error('所有的写入已经完成!');
});
4.pipe方法
4.1 pipe方法的原理
var fs = require('fs');
var ws = fs.createWriteStream('./2.txt');
var rs = fs.createReadStream('./1.txt');
rs.on('data', function (data) {
var flag = ws.write(data);
if(!flag)
rs.pause();
});
ws.on('drain', function () {
rs.resume();
});
rs.on('end', function () {
ws.end();
});
4.2 pipe用法
readStream.pipe(writeStream);
var from = fs.createReadStream('./1.txt');
var to = fs.createWriteStream('./2.txt');
from.pipe(to);
将数据的滞留量限制到一个可接受的水平，以使得不同速度的来源和目标不会淹没可用内存。
4.3 unpipe用法
readable.unpipe()方法将之前通过stream.pipe()方法绑定的流分离
-
如果 destination 没有传入, 则所有绑定的流都会被分离.
let fs = require('fs'); var from = fs.createReadStream('./1.txt'); var to = fs.createWriteStream('./2.txt'); from.pipe(to); setTimeout(() => { console.log('关闭向2.txt的写入'); from.unpipe(writable); console.log('手工关闭文件流'); to.end(); }, 1000);
4.4 cork
调用 writable.cork() 方法将强制所有写入数据都存放到内存中的缓冲区里。 直到调用 stream.uncork() 或 stream.end() 方法时，缓冲区里的数据才会被输出。
4.5 uncork
writable.uncork()将输出在stream.cork()方法被调用之后缓冲在内存中的所有数据。
stream.cork();
stream.write('1');
stream.write('2');
process.nextTick(() => stream.uncork());
5. 简单实现
5.1 可读流的简单实现 [#](#t235.1 可读流的简单实现)
let fs = require('fs');
let ReadStream = require('./ReadStream');
let rs = ReadStream('./1.txt', {
flags: 'r',
encoding: 'utf8',
start: 3,
end: 7,
highWaterMark: 3
});
rs.on('open', function () {
console.log("open");
});
rs.on('data', function (data) {
console.log(data);
});
rs.on('end', function () {
console.log("end");
});
rs.on('close', function () {
console.log("close");
});
/**
open
456
789
end
close
**/
let fs = require('fs');
let EventEmitter = require('events');
class WriteStream extends EventEmitter {
constructor(path, options) {
super(path, options);
this.path = path;
this.fd = options.fd;
this.flags = options.flags || 'r';
this.encoding = options.encoding;
this.start = options.start || 0;
this.pos = this.start;
this.end = options.end;
this.flowing = false;
this.autoClose = true;
this.highWaterMark = options.highWaterMark || 64 * 1024;
this.buffer = Buffer.alloc(this.highWaterMark);
this.length = 0;
this.on('newListener', (type, listener) => {
if (type == 'data') {
this.flowing = true;
this.read();
}
});
this.on('end', () => {
if (this.autoClose) {
this.destroy();
}
});
this.open();
}
read() {
if (typeof this.fd != 'number') {
return this.once('open', () => this.read());
}
let n = this.end ? Math.min(this.end - this.pos, this.highWaterMark) : this.highWaterMark;
fs.read(this.fd,this.buffer,0,n,this.pos,(err,bytesRead)=>{
if(err){
return;
}
if(bytesRead){
let data = this.buffer.slice(0,bytesRead);
data = this.encoding?data.toString(this.encoding):data;
this.emit('data',data);
this.pos += bytesRead;
if(this.end && this.pos > this.end){
return this.emit('end');
}
if(this.flowing)
this.read();
}else{
this.emit('end');
}
})
}
open() {
fs.open(this.path, this.flags, this.mode, (err, fd) => {
if (err) return this.emit('error', err);
this.fd = fd;
this.emit('open', fd);
})
}
end() {
if (this.autoClose) {
this.destroy();
}
}
destroy() {
fs.close(this.fd, () => {
this.emit('close');
})
}
}
module.exports = WriteStream;
5.2 可写流的简单实现
let fs = require('fs');
let FileWriteStream = require('./FileWriteStream');
let ws = FileWriteStream('./2.txt',{
flags:'w',
encoding:'utf8',
highWaterMark:3
});
let i = 10;
function write(){
let flag = true;
while(i&&flag){
flag = ws.write("1",'utf8',(function(i){
return function(){
console.log(i);
}
})(i));
i--;
console.log(flag);
}
}
write();
ws.on('drain',()=>{
console.log("drain");
write();
});
/**
10
9
8
drain
7
6
5
drain
4
3
2
drain
1
**/
let fs = require('fs');
let EventEmitter = require('events');
class WriteStream extends EventEmitter{
constructor(path, options) {
super(path, options);
this.path = path;
this.fd = options.fd;
this.flags = options.flags || 'w';
this.mode = options.mode || 0o666;
this.encoding = options.encoding;
this.start = options.start || 0;
this.pos = this.start;
this.writing = false;
this.autoClose = true;
this.highWaterMark = options.highWaterMark || 16 * 1024;
this.buffers = [];
this.length = 0;
this.open();
}
open() {
fs.open(this.path, this.flags, this.mode, (err, fd) => {
if (err) return this.emit('error', err);
this.fd = fd;
this.emit('open', fd);
})
}
write(chunk, encoding, cb) {
if (typeof encoding == 'function') {
cb = encoding;
encoding = null;
}
chunk = Buffer.isBuffer(chunk) ? chunk : Buffer.from(chunk, this.encoding || 'utf8');
let len = chunk.length;
this.length += len;
let ret = this.length < this.highWaterMark;
if (this.writing) {
this.buffers.push({
chunk,
encoding,
cb,
});
} else {
this.writing = true;
this._write(chunk, encoding,this.clearBuffer.bind(this));
}
return ret;
}
_write(chunk, encoding, cb) {
if (typeof this.fd != 'number') {
return this.once('open', () => this._write(chunk, encoding, cb));
}
fs.write(this.fd, chunk, 0, chunk.length, this.pos, (err, written) => {
if (err) {
if (this.autoClose) {
this.destroy();
}
return this.emit('error', err);
}
this.length -= written;
this.pos += written;
cb && cb();
});
}
clearBuffer() {
let data = this.buffers.shift();
if (data) {
this._write(data.chunk, data.encoding, this.clearBuffer.bind(this))
} else {
this.writing = false;
this.emit('drain');
}
}
end() {
if (this.autoClose) {
this.emit('end');
this.destroy();
}
}
destroy() {
fs.close(this.fd, () => {
this.emit('close');
})
}
}
module.exports = WriteStream;
5.3 pipe [#](#t255.3 pipe)
let fs = require('fs');
let ReadStream = require('./ReadStream');
let rs = ReadStream('./1.txt', {
flags: 'r',
encoding: 'utf8',
highWaterMark: 3
});
let FileWriteStream = require('./WriteStream');
let ws = FileWriteStream('./2.txt',{
flags:'w',
encoding:'utf8',
highWaterMark:3
});
rs.pipe(ws);
ReadStream.prototype.pipe = function (dest) {
this.on('data', (data)=>{
let flag = dest.write(data);
if(!flag){
this.pause();
}
});
dest.on('drain', ()=>{
this.resume();
});
this.on('end', ()=>{
dest.end();
});
}
ReadStream.prototype.pause = function(){
this.flowing = false;
}
ReadStream.prototype.resume = function(){
this.flowing = true;
this.read();
}
5.4 暂停模式
let fs =require('fs');
let ReadStream2 = require('./ReadStream2');
let rs = new ReadStream2('./1.txt',{
start:3,
end:8,
encoding:'utf8',
highWaterMark:3
});
rs.on('readable',function () {
console.log('readable');
console.log('rs.buffer.length',rs.length);
let d = rs.read(1);
console.log(d);
console.log('rs.buffer.length',rs.length);
setTimeout(()=>{
console.log('rs.buffer.length',rs.length);
},500)
});
open() {
fs.open(this.path, this.flags, this.mode, (err, fd) => {
if (err) {
if (this.autoClose) {
this.destroy();
return this.emit('error', err);
}
}
this.fd = fd;
this.emit('open');
});
}
read(n) {
if (typeof this.fd != 'number') {
return this.once('open', () => this.read());
}
n = parseInt(n,10);
if(n != n){
n = this.length;
}
if(this.length ==0)
this.needReadable = true;
let ret;
if (0 {
let m = this.end ? Math.min(this.end - this.pos + 1, this.highWaterMark) : this.highWaterMark;
fs.read(this.fd, this.buffer, 0, m, this.pos, (err, bytesRead) => {
if (err) {
return
}
let data;
if (bytesRead > 0) {
data = this.buffer.slice(0, bytesRead);
this.pos += bytesRead;
this.length += bytesRead;
if (this.end && this.pos > this.end) {
if(this.needReadable){
this.emit('readable');
}
this.emit('end');
} else {
this.buffers.push(data);
if(this.needReadable){
this.emit('readable');
this.needReadable = false;
}
}
} else {
if(this.needReadable) {
this.emit('readable');
}
return this.emit('end');
}
})
}
if (this.length == 0 || (this.length < this.highWaterMark)) {
_read(0);
}
return ret;
}
destroy() {
fs.close(this.fd, (err) => {
this.emit('close');
});
}
pause() {
this.flowing = false;
}
resume() {
this.flowing = true;
this.read();
}
pipe(dest) {
this.on('data', (data) => {
let flag = dest.write(data);
if (!flag) this.pause();
});
dest.on('drain', () => {
this.resume();
});
this.on('end', () => {
dest.end();
});
}
}
module.exports = ReadStream; /**
-
if (n !== 0) state.emittedReadable = false; 只要要读的字节数不是0就需要触发readable事件 如果传入的NaN,则将n赋为缓区的长度，第一次就是0
缓存区为0就开始读吧 如果n等于0就返回null,state.needReadable = true; 如果缓存区为0，是 state.needReadable = true; 需要触发readable
- streaming_in_node
- stream-handbook