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）。

不得直接调用 _construct() 方法。 它可以由子类实现，如果是，则只能由内部 Readable 类方法调用。

这个可选函数将由流的构造函数在下一个滴答中调度，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。

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);
    }
  }
}

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

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.

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);
    }
  }
}