Node 深入Stream（1）

lio-mengxiang發表於2019-03-07

1. 流的概念

  • 流是一組有序的，有起點和終點的位元組資料傳輸手段
  • 它不關心檔案的整體內容，只關注是否從檔案中讀到了資料，以及讀到資料之後的處理
  • 流是一個抽象介面，被 Node 中的很多物件所實現。比如HTTP 伺服器request和response物件都是流。

2.可讀流createReadStream

實現了stream.Readable介面的物件,將物件資料讀取為流資料,當監聽data事件後,開始發射資料

fs.createReadStream = function(path, options) {
  return new ReadStream(path, options);
};
util.inherits(ReadStream, Readable);
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2.1 建立可讀流

var rs = fs.createReadStream(path,[options]);
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  1. path讀取檔案的路徑
  2. options
    • flags開啟檔案要做的操作,預設為'r'
    • encoding預設為null
    • start開始讀取的索引位置
    • end結束讀取的索引位置(包括結束位置)
    • highWaterMark讀取快取區預設的大小64kb

如果指定utf8編碼highWaterMark要大於3個位元組

2.2 監聽data事件

流切換到流動模式,資料會被儘可能快的讀出

rs.on('data', function (data) {
    console.log(data);
});
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2.3 監聽end事件

該事件會在讀完資料後被觸發

rs.on('end', function () {
    console.log('讀取完成');
});
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2.4 監聽error事件

rs.on('error', function (err) {
    console.log(err);
});
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2.5 監聽open事件

rs.on('open', function () {
    console.log(err);
});
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2.6 監聽close事件

rs.on('close', function () {
    console.log(err);
});
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2.7 設定編碼

與指定{encoding:'utf8'}效果相同，設定編碼

rs.setEncoding('utf8');
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2.8 暫停和恢復觸發data

通過pause()方法和resume()方法

rs.on('data', function (data) {
    rs.pause();
    console.log(data);
});
setTimeout(function () {
    rs.resume();
},2000);
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3.可寫流createWriteStream

實現了stream.Writable介面的物件來將流資料寫入到物件中

fs.createWriteStream = function(path, options) {
  return new WriteStream(path, options);
};

util.inherits(WriteStream, Writable);
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3.1 建立可寫流

var ws = fs.createWriteStream(path,[options]);
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  1. path寫入的檔案路徑
  2. options
    • flags開啟檔案要做的操作,預設為'w'
    • encoding預設為utf8
    • highWaterMark寫入快取區的預設大小16kb

3.2 write方法

ws.write(chunk,[encoding],[callback]);
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  1. chunk寫入的資料buffer/string
  2. encoding編碼格式chunk為字串時有用，可選
  3. callback 寫入成功後的回撥

返回值為布林值，系統快取區滿時為false,未滿時為true

3.3 end方法

ws.end(chunk,[encoding],[callback]);
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表明接下來沒有資料要被寫入 Writable 通過傳入可選的 chunk 和 encoding 引數，可以在關閉流之前再寫入一段資料 如果傳入了可選的 callback 函式，它將作為 'finish' 事件的回撥函式

3.4 drain方法

  • 當一個流不處在 drain 的狀態， 對 write() 的呼叫會快取資料塊， 並且返回 false。 一旦所有當前所有快取的資料塊都排空了（被作業系統接受來進行輸出）， 那麼 'drain' 事件就會被觸發
  • 建議， 一旦 write() 返回 false， 在 'drain' 事件觸發前， 不能寫入任何資料塊
    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();
});
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3.5 finish方法

在呼叫了 stream.end() 方法，且緩衝區資料都已經傳給底層系統之後， 'finish' 事件將被觸發。

var writer = fs.createWriteStream('./2.txt');
for (let i = 0; i < 100; i++) {
  writer.write(`hello, ${i}!\n`);
}
writer.end('結束\n');
writer.on('finish', () => {
  console.error('所有的寫入已經完成!');
});
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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();
});
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4.2 pipe用法

readStream.pipe(writeStream);
var from = fs.createReadStream('./1.txt');
var to = fs.createWriteStream('./2.txt');
from.pipe(to);
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將資料的滯留量限制到一個可接受的水平，以使得不同速度的來源和目標不會淹沒可用記憶體。

4.3 unpipe用法

  • readable.unpipe()方法將之前通過stream.pipe()方法繫結的流分離
  • 如果 destination 沒有傳入, 則所有繫結的流都會被分離.
    let fs = require('fs');
var from = fs.createReadStream('./1.txt');
var to = fs.createWriteStream('./2.txt');
from.pipe(to);
setTimeout(() => {
console.log('關閉向2.txt的寫入');
from.unpipe(writable);
console.log('手工關閉檔案流');
to.end();
}, 1000);
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4.4 cork

呼叫 writable.cork() 方法將強制所有寫入資料都存放到記憶體中的緩衝區裡。 直到呼叫 stream.uncork() 或 stream.end() 方法時，緩衝區裡的資料才會被輸出。

4.5 uncork

writable.uncork()將輸出在stream.cork()方法被呼叫之後緩衝在記憶體中的所有資料。

stream.cork();
stream.write('1');
stream.write('2');
process.nextTick(() => stream.uncork());
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5. 簡單實現

5.1 可讀流的簡單實現

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
 **/
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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;
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5.2 可寫流的簡單實現

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
  **/
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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;
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5.3 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);
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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();
}
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5.4 暫停模式

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)
});
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` let fs = require('fs'); let EventEmitter = require('events'); class ReadStream extends EventEmitter { constructor(path, options) { super(path, options); this.path = path; this.highWaterMark = options.highWaterMark || 64 * 1024; this.buffer = Buffer.alloc(this.highWaterMark); this.flags = options.flags || 'r'; this.encoding = options.encoding; this.mode = options.mode || 0o666; this.start = options.start || 0; this.end = options.end; this.pos = this.start; this.autoClose = options.autoClose || true; this.bytesRead = 0; this.closed = false; this.flowing; this.needReadable = false; this.length = 0; this.buffers = []; this.on('end', function () { if (this.autoClose) { this.destroy(); } }); this.on('newListener', (type) => { if (type == 'data') { this.flowing = true; this.read(); } if (type == 'readable') { this.read(0); } }); this.open(); }

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<n < this.length) {
        ret = Buffer.alloc(n);
        let b ;
        let index = 0;
        while(null != (b = this.buffers.shift())){
            for(let i=0;i<b.length;i++){
                ret[index++] = b[i];
                if(index == ret.length){
                    this.length -= n;
                    b = b.slice(i+1);
                    this.buffers.unshift(b);
                    break;
                }
            }
        }
        if (this.encoding) ret = ret.toString(this.encoding);
    }

    let _read = () => {
        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();
    });
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