Ein Artikel über lesbare Streams in Node

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;
};