readable._construct(callback)

• callback <Function> 当流完成初始化时调用此函数（可选地带有错误参数）。

  ¥callback <Function> Call this function (optionally with an error argument) when the stream has finished initializing.

不得直接调用 _construct() 方法。它可能由子类实现，如果是这样，将仅由内部 Readable 类方法调用。

¥The _construct() method MUST NOT be called directly. It may be implemented by child classes, and if so, will be called by the internal Readable class methods only.

这个可选函数将被流构造函数安排在下一个时钟周期，延迟任何 _read() 和 _destroy() 调用，直到调用 callback。这对于在使用流之前初始化状态或异步初始化资源很有用。

¥This optional function will be scheduled in the next tick by the stream constructor, delaying any _read() and _destroy() calls until callback is called. This is useful to initialize state or asynchronously initialize resources before the stream can be used.

const { Readable } = require('node:stream');
const fs = require('node:fs');

class ReadStream extends Readable {
  constructor(filename) {
    super();
    this.filename = filename;
    this.fd = null;
  }
  _construct(callback) {
    fs.open(this.filename, (err, fd) => {
      if (err) {
        callback(err);
      } else {
        this.fd = fd;
        callback();
      }
    });
  }
  _read(n) {
    const buf = Buffer.alloc(n);
    fs.read(this.fd, buf, 0, n, null, (err, bytesRead) => {
      if (err) {
        this.destroy(err);
      } else {
        this.push(bytesRead > 0 ? buf.slice(0, bytesRead) : null);
      }
    });
  }
  _destroy(err, callback) {
    if (this.fd) {
      fs.close(this.fd, (er) => callback(er || err));
    } else {
      callback(err);
    }
  }
} 拷贝