一文聊聊Node中的可读流

_readableState: ReadableState {
  objectMode: false, // 操作除了string、Buffer、null之外的其他类型的数据需要把这个模式打开
  highWaterMark: 16384, // 水位限制，1024 \* 16，默认16kb，超过这个限制则会停止调用\_read()读数据到buffer中
  buffer: BufferList { head: null, tail: null, length: 0 }, // Buffer链表，用于保存数据
  length: 0, // 整个可读流数据的大小，如果是objectMode则与buffer.length相等
  pipes: [], // 保存监听了该可读流的所有管道队列
  flowing: null, // 可独流的状态 null、false、true
  ended: false, // 所有数据消费完毕
  endEmitted: false, // 结束事件收否已发送
  reading: false, // 是否正在读取数据
  constructed: true, // 流在构造好之前或者失败之前，不能被销毁
  sync: true, // 是否同步触发'readable'/'data'事件，或是等到下一个tick
  needReadable: false, // 是否需要发送readable事件
  emittedReadable: false, // readable事件发送完毕
  readableListening: false, // 是否有readable监听事件
  resumeScheduled: false, // 是否调用过resume方法
  errorEmitted: false, // 错误事件已发送
  emitClose: true, // 流销毁时，是否发送close事件
  autoDestroy: true, // 自动销毁，在'end'事件触发后被调用
  destroyed: false, // 流是否已经被销毁
  errored: null, // 标识流是否报错
  closed: false, // 流是否已经关闭
  closeEmitted: false, // close事件是否已发送
  defaultEncoding: 'utf8', // 默认字符编码格式
  awaitDrainWriters: null, // 指向监听了'drain'事件的writer引用，类型为null、Writable、Set<Writable>
  multiAwaitDrain: false, // 是否有多个writer等待drain事件 
  readingMore: false, // 是否可以读取更多数据
  dataEmitted: false, // 数据已发送
  decoder: null, // 解码器
  encoding: null, // 编码器
  [Symbol(kPaused)]: null
},

const Stream = require('stream');
const readableStream = new Stream.Readable({
  objectMode: true,
  read() {},
});

readableStream.push({ name: 'lisa'});
console.log(readableStream._readableState.buffer.tail);
readableStream.push(true);
console.log(readableStream._readableState.buffer.tail);
readableStream.push('lisa');
console.log(readableStream._readableState.buffer.tail);
readableStream.push(666);
console.log(readableStream._readableState.buffer.tail);
readableStream.push(() => {});
console.log(readableStream._readableState.buffer.tail);
readableStream.push(Symbol(1));
console.log(readableStream._readableState.buffer.tail);
readableStream.push(BigInt(123));
console.log(readableStream._readableState.buffer.tail);

const Stream = require('stream');

const readableStream = new Stream.Readable({

  objectMode: false,

  read() {},

});

readableStream.push({ name: 'lisa'});

const Stream = require('stream');

const readableStream = new Stream.Readable();

RS._read = function(size) {}

const Stream = require('stream');

const readableStream = new Stream.Readable({
  
  read(size) {}

});

源码地址：BufferList.consume
https://github.com/nodejs/node/blob/d5e94fa7121c9d424588f0e1a388f8c72c784622/lib/internal/streams/buffer_list.js#L80

// Consumes a specified amount of bytes or characters from the buffered data.
consume(n, hasStrings) {
  const data = this.head.data;
  if (n < data.length) {
    // `slice` is the same for buffers and strings.
    const slice = data.slice(0, n);
    this.head.data = data.slice(n);
    return slice;
  }
  if (n === data.length) {
    // First chunk is a perfect match.
    return this.shift();
  }
  // Result spans more than one buffer.
  return hasStrings ? this.\_getString(n) : this.\_getBuffer(n);
}

源码地址：BufferList._getBuffer
https://github.com/nodejs/node/blob/d5e94fa7121c9d424588f0e1a388f8c72c784622/lib/internal/streams/buffer_list.js#L137

// Consumes a specified amount of bytes from the buffered data.
_getBuffer(n) {
  const ret = Buffer.allocUnsafe(n);
  const retLen = n;
  let p = this.head;
  let c = 0;
  do {
    const buf = p.data;
    if (n > buf.length) {
      TypedArrayPrototypeSet(ret, buf, retLen - n);
      n -= buf.length;
    } else {
      if (n === buf.length) {
        TypedArrayPrototypeSet(ret, buf, retLen - n);
        ++c;
        if (p.next)
          this.head = p.next;
        else
          this.head = this.tail = null;
      } else {
       TypedArrayPrototypeSet(ret,
                              new Uint8Array(buf.buffer, buf.byteOffset, n),
                              retLen - n);
        this.head = p;
        p.data = buf.slice(n);
      }
      break;
    }
    ++c;
  } while ((p = p.next) !== null);
  this.length -= c;
  return ret;
}

源码地址：BufferList._getString
https://github.com/nodejs/node/blob/d5e94fa7121c9d424588f0e1a388f8c72c784622/lib/internal/streams/buffer_list.js#L106

// Consumes a specified amount of characters from the buffered data.
_getString(n) {
  let ret = '';
  let p = this.head;
  let c = 0;
  do {
    const str = p.data;
    if (n > str.length) {
    ret += str;
    n -= str.length;
  } else {
    if (n === str.length) {
      ret += str;
      ++c;
      if (p.next)
        this.head = p.next;
      else
        this.head = this.tail = null;
    } else {
      ret += StringPrototypeSlice(str, 0, n);
      this.head = p;
      p.data = StringPrototypeSlice(str, n);
    }
    break;
    }
    ++c;
  } while ((p = p.next) !== null);
  this.length -= c;
  return ret;
}

这部分源码在这儿：stream/legacy
https://github.com/nodejs/node/blob/d5e94fa7121c9d424588f0e1a388f8c72c784622/lib/internal/streams/legacy.js#L10

function Stream(opts) {
  EE.call(this, opts);
}
ObjectSetPrototypeOf(Stream.prototype, EE.prototype);
ObjectSetPrototypeOf(Stream, EE);

这部分源码在这儿：readable
https://github.com/nodejs/node/blob/d5e94fa7121c9d424588f0e1a388f8c72c784622/lib/internal/streams/readable.js#L77

ObjectSetPrototypeOf(Readable.prototype, Stream.prototype);
ObjectSetPrototypeOf(Readable, Stream);

Stream {
  destroy: [Function: destroy],
  _undestroy: [Function: undestroy],
  _destroy: [Function (anonymous)],
  push: [Function (anonymous)],
  unshift: [Function (anonymous)],
  isPaused: [Function (anonymous)],
  setEncoding: [Function (anonymous)],
  read: [Function (anonymous)],
  _read: [Function (anonymous)],
  pipe: [Function (anonymous)],
  unpipe: [Function (anonymous)],
  on: [Function (anonymous)],
  addListener: [Function (anonymous)],
  removeListener: [Function (anonymous)],
  off: [Function (anonymous)],
  removeAllListeners: [Function (anonymous)],
  resume: [Function (anonymous)],
  pause: [Function (anonymous)],
  wrap: [Function (anonymous)],
  iterator: [Function (anonymous)],
  [Symbol(nodejs.rejection)]: [Function (anonymous)],
  [Symbol(Symbol.asyncIterator)]: [Function (anonymous)]
}

Readable.prototype.push = function(chunk, encoding) {
  return readableAddChunk(this, chunk, encoding, false);
};

function readableAddChunk(stream, chunk, encoding, addToFront) {
  const state = stream.\_readableState;
  if (chunk === null) { // push null 流结束信号，之后不能再写入数据
    state.reading = false;
    onEofChunk(stream, state);
  } else if (!state.objectMode) { // 如果不是对象模式
    if (typeof chunk === 'string') {
      chunk = Buffer.from(chunk);
    } else if (chunk instanceof Buffer) { //如果是Buffer
    // 处理一下编码
    } else if (Stream.\_isUint8Array(chunk)) {
      chunk = Stream.\_uint8ArrayToBuffer(chunk);
    } else if (chunk != null) {
      err = new ERR\_INVALID\_ARG\_TYPE('chunk', ['string', 'Buffer', 'Uint8Array'], chunk);
    }
  }

  if (state.objectMode || (chunk && chunk.length > 0)) { // 是对象模式或者chunk是Buffer
    // 这里省略几种数据的插入方式的判断
    addChunk(stream, state, chunk, true);
  }
}

function addChunk(stream, state, chunk, addToFront) {
  if (state.flowing && state.length === 0 && !state.sync &&
    stream.listenerCount('data') > 0) { // 如果处于流动模式，有监听data的订阅者
      stream.emit('data', chunk);
  } else { // 否则保存数据到缓冲区中
    state.length += state.objectMode ? 1 : chunk.length;
    if (addToFront) {
      state.buffer.unshift(chunk);
    } else {
      state.buffer.push(chunk);
    }
  }
  maybeReadMore(stream, state); // 尝试多读一点数据
}

// 如果read的长度大于hwm，则会重新计算hwm
if (n > state.highWaterMark) {
  state.highWaterMark = computeNewHighWaterMark(n);  
}
// 调用用户实现的\_read方法
try {
  const result = this.\_read(state.highWaterMark);
  if (result != null) {
    const then = result.then;
    if (typeof then === 'function') {
      then.call(
        result,
        nop,
        function(err) {
          errorOrDestroy(this, err);
        });
    }
  }
} catch (err) {
  errorOrDestroy(this, err);
}

function fromList(n, state) {
  // nothing buffered.
  if (state.length === 0)
    return null;
  let ret;
  if (state.objectMode)
    ret = state.buffer.shift();
  else if (!n || n >= state.length) { // 处理n为空或者大于缓冲区的长度的情况
    // Read it all, truncate the list.
    if (state.decoder) // 有解码器，则将结果序列化为字符串
      ret = state.buffer.join('');
    else if (state.buffer.length === 1) // 只有一个数据，返回头节点数据
      ret = state.buffer.first();
    else // 将所有数据存储到一个Buffer中
      ret = state.buffer.concat(state.length);
    state.buffer.clear(); // 清空缓冲区
  } else {
    // 处理读取长度小于缓冲区的情况
    ret = state.buffer.consume(n, state.decoder);
  }
  return ret;
}

const Stream = require('stream');
const readableStream = new Stream.Readable({
  read(hwm) {
    this.push(String.fromCharCode(this.currentCharCode++));
    if (this.currentCharCode > 122) {
      this.push(null);
    }
  },
});
readableStream.currentCharCode = 97;
readableStream.pipe(process.stdout);
// abcdefghijklmnopqrstuvwxyz%

Readable.prototype.pipe = function(dest, pipeOpts) {
  const src = this;
  const state = this.\_readableState;
  state.pipes.push(dest); // 收集Writable流

  src.on('data', ondata);
  function ondata(chunk) {
    const ret = dest.write(chunk);
    if (ret === false) {
      pause();
    }
  }
  // Tell the dest that it's being piped to.
  dest.emit('pipe', src);
  // 启动流，如果流处于暂停模式
  if (dest.writableNeedDrain === true) {
    if (state.flowing) {
      pause();
    }
  } else if (!state.flowing) {
    src.resume();
  }
  return dest;
}

Readable.prototype.resume = function() {
  const state = this._readableState;
  if (!state.flowing) {
    state.flowing = !state.readableListening; // 是否处于流动模式取决于是否设置了'readable'监听句柄
    resume(this, state);
  }
};

function resume(stream, state) {
  if (!state.resumeScheduled) { // 开关，使resume_方法仅在同一个Tick中调用一次
    state.resumeScheduled = true;
    process.nextTick(resume_, stream, state);
  }
}

function resume_(stream, state) {
  if (!state.reading) {
    stream.read(0);
  }
  state.resumeScheduled = false;
  stream.emit('resume');
  flow(stream);
}

function flow(stream) { // 当流处于流模式该方法会不断的从buffer中读取数据，直到缓冲区为空
  const state = stream._readableState;
  while (state.flowing && stream.read() !== null); 
  // 因为这里会调用read方法，设置了'readable'事件监听器的stream，也有可能会调用read方法，
  //从而导致数据不连贯（不影响data，仅影响在'readable'事件回调中调用read方法读取数据）
}

Readable.prototype.pause = function() {
  if (this._readableState.flowing !== false) {
    debug('pause');
    this._readableState.flowing = false;
    this.emit('pause');
  }
  return this;
};