三种状态

Readable 流的操作的"两种模式"是对 Readable 流实现中发生的更复杂的内部状态管理的简化抽象。

具体来说，在任何给定的时间点，每个 Readable 都处于三种可能的状态之一：

• readable.readableFlowing === null
• readable.readableFlowing === false
• readable.readableFlowing === true

当 readable.readableFlowing 为 null 时，则不提供消费流数据的机制。 因此，流不会生成数据。 在此状态下，为 'data' 事件绑定监听器、调用 readable.pipe() 方法、或调用 readable.resume() 方法会将 readable.readableFlowing 切换到 true，从而使 Readable 在生成数据时开始主动触发事件。

调用readable.pause()、readable.unpipe()、或者接收背压都会导致 readable.readableFlowing 被设置为 false，暂时停止事件的流动，但不会停止数据的生成。 在此状态下，为 'data' 事件绑定监听器不会将 readable.readableFlowing 切换到 true。

const { PassThrough, Writable } = require('node:stream');
const pass = new PassThrough();
const writable = new Writable();

pass.pipe(writable);
pass.unpipe(writable);
// readableFlowing 现在为 false。

pass.on('data', (chunk) => { console.log(chunk.toString()); });
// readableFlowing 仍然是 false。
pass.write('ok');  // 不会触发 'data'。
pass.resume();     // 必须调用才能使流触发 'data'。
// readableFlowing 现在是 true。

虽然 readable.readableFlowing 是 false，但数据可能会在流的内部缓冲区中累积。

The "two modes" of operation for a Readable stream are a simplified abstraction for the more complicated internal state management that is happening within the Readable stream implementation.

Specifically, at any given point in time, every Readable is in one of three possible states:

• readable.readableFlowing === null
• readable.readableFlowing === false
• readable.readableFlowing === true

When readable.readableFlowing is null, no mechanism for consuming the stream's data is provided. Therefore, the stream will not generate data. While in this state, attaching a listener for the 'data' event, calling the readable.pipe() method, or calling the readable.resume() method will switch readable.readableFlowing to true, causing the Readable to begin actively emitting events as data is generated.

Calling readable.pause(), readable.unpipe(), or receiving backpressure will cause the readable.readableFlowing to be set as false, temporarily halting the flowing of events but not halting the generation of data. While in this state, attaching a listener for the 'data' event will not switch readable.readableFlowing to true.

const { PassThrough, Writable } = require('node:stream');
const pass = new PassThrough();
const writable = new Writable();

pass.pipe(writable);
pass.unpipe(writable);
// readableFlowing is now false.

pass.on('data', (chunk) => { console.log(chunk.toString()); });
// readableFlowing is still false.
pass.write('ok');  // Will not emit 'data'.
pass.resume();     // Must be called to make stream emit 'data'.
// readableFlowing is now true.

While readable.readableFlowing is false, data may be accumulating within the stream's internal buffer.