readable._construct(callback)

• callback <Function> 当流初始化完成后调用此函数（可选择传入一个错误参数）。

_construct() 方法不能被直接调用。它可以由子类实现，如果实现了，也只会被内部的 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.】

此可选函数将由流构造函数在下一个事件循环时调度，延迟任何 _read() 和 _destroy() 调用，直到 callback 被调用。这对于在流可以使用之前初始化状态或异步初始化资源非常有用。

【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