Question

var fs = require('fs');

var file = fs.createReadStream('./zeros.txt');
var dataSize = 0;

file.on('readable', function () {
    var data = file.read(10);
    console.log('readable size = ', data.length);
    console.log(data.toString());
});

你的文件“zeros.txt”包含700个字符“0”

据我所知，在调用read（10）之后，流必须停止并等待新的read（）调用。但是，调用的结果是：

readable size =  10
0000000000
readable size =  10
0000000000

Answer 1

在Node.js将文件（整个或只是一部分，取决于文件本身的大小）加载到缓冲区（使用push（）方法）之后，它将发出readable event以指示某些文件数据已被读入缓冲区并可以使用。然后在调用read(10)之后，您将释放缓冲区，然后Node.js将再次自动填充缓冲区并再次发出readable event以指示仍有一些数据要从缓冲区中读取。如果您致电read(700)，则不会再次发出下一个readable event。

流动和非流动模式

与监听data event时不同，流将保持所谓的非流动模式。这意味着开发人员将负责释放流（从流中读取）。另一方面，当监听data event时，流将自动进入所谓的流动模式，这意味着流本身将负责释放自身，即流将填充并清空自己直到他的底层系统（在在这种情况下，zero.txt将被完全读取。请注意，缓冲区将在任一模式下自动填充数据。

流动模式

非流动模式的一个示例，我们必须手动清空缓冲区（使用read()方法）：

var fs = require('fs'),
util = require('util');

// I have increased the file size to 19 MB (about 19 mln characters);
// Cause of the buffer capicity.
var file = fs.createReadStream('./zeros.txt'); 
var dataSize = 0;

// Readable will be called when the buffer has been filled with data.
// Initially Node.js will fill the buffer with data automatically,
// so this event will be called automatically aswell of course.
// Once the buffer will be free again after the first fill, Node.js
// will fill the buffer automatically again. Node.js just watches this stream
// and makes sure to fill it, when there is still some unread data in the zero.txt file.
file.on('readable', function() {
var i = 0; // we will count how many times did while loop, for fun

// If the buffer will be empty Node will write data to the buffer
// automatically, we don't have to care about that. However
// you can specify the buffer capicty manually if you want.
console.log('loading more data from the underlying system');

// This will make the stream read 1000 bytes
// it will also return a value NULL if there is not enough 
// data to read from the buffer (meaning buffer has been fully read 
// or there is still some data but you are trying to read 1000 bytes 
// and there is less than 1000 bytes left)
while(file.read(1000) !== null) {
    i++;
}
// At this moment while loop has read everything from the buffer.
// The buffer is now empty. After this comment console.log will execute
// Node.js will fill the buffer again with new data automatically.
// And then the 'readable' event will fire again.
console.log("had to loop: " + i + " times before the buffer was empty");
})

控制台的最后几项结果：

loading more data from the underlying system
had to loop: 66 times before the buffer was empty
loading more data from the underlying system
had to loop: 65 times before the buffer was empty
loading more data from the underlying system
had to loop: 66 times before the buffer was empty
loading more data from the underlying system
had to loop: 46 times before the buffer was empty
loading more data from the underlying system
had to loop: 1 times before the buffer was empty

非流动模式

这是非流动模式，因为我们必须手动释放缓冲区。现在我们将进入流动模式。在data event listener上设置Readable Stream会将流从最初的flowing mode切换为non-flowing mode。这意味着缓冲区将自动清空。 Node.js会将数据作为参数传递给data event listener，一旦该函数执行，缓冲区将再次清空，如果底层源缓冲区中仍有一些数据将自动填充新数据，然后数据事件将再次发出。注意：如果您正在倾听data事件并且readable event两者都将触发，但data event listener将首先清空缓冲区，然后readable event将触发，因此您read() 1}}将始终返回NULL。

var fs = require('fs'),
util = require('util');

var file = fs.createReadStream('./zeros.txt');
var dataSize = 0;

file.on('data', function() {
    // Once this listener will stop executing new data will be read
    // into the buffer and then the 'data' event will be emitted
    // again.
    console.log('data has been loaded and emptied!')
})

file.on('readable', function () {
    // Notice we want to try to read 1 byte from the buffer
    // but in the console we see that the read() method
    // resulted in NULL, which means that the buffer is empty.
    // That's of course because we enterd the flowing mode
    // by setting up the 'data' event. (In flowing mode)
    // after the execution of the 'data' event all data
    // from the buffer will be read, but the execution
    // of listeners will continue. After all the event listeners
    // attached to this stream will execute, Node.js will fill
    // the buffer automatically again.
    console.log('readable ' + file.read(1))
});

控制台的最后几项结果：

data has been loaded and emptied!
readable null
data has been loaded and emptied!
readable null
data has been loaded and emptied!
readable null
data has been loaded and emptied!
readable null
data has been loaded and emptied!
readable null

Answer 2

我的答案是基于0.12.4的版本。

1：从read(n)扩展的每个Stream.Readable函数将在当前内部缓冲区长度为0或小于 {{的值时触发内部_read(n)函数1}} 属性。

2：仅当当前内部缓冲区长度为0或从内部缓冲区读取的数据为空或发生空指示符时，才会触发highWaterMark事件。

让我们以您的代码为例，看看发生了什么。

readable

file.on('readable', function () {事件处理程序寄存器将触发readable函数将数据从文件加载到内部缓冲区。如果您不覆盖 read(0) 的值，则最多会加载64 * 1024 = 65535个块。在您的代码中，它加载了文件＆＃34; zeros.txt＆＃34;中的所有数据。之后，它将触发highWaterMark事件，因为在调用readable函数之前内部缓冲区长度为0。

read(0)

在处理程序中，您再次调用了var data = file.read(10);函数。这也会触发从文件到缓冲区的加载过程。但是，此时不会加载任何数据。因此， read(10) 会被推送以表明阅读过程已经完成。第二个null事件已被触发。这就是您应该看到并且只看到两个readable事件的原因。

如果您读取的文件大小超过65535字节（几乎66KB），您应该只会看到一个readable事件被触发。

你不应该像这样编写readable事件处理程序，你应该参考以下内容：

readable

如果你想以自己的方式处理大块做一些特别的事情，请注意规则;否则，程序将停留在＆＃34;暂停＆＃34;状态，不再读取数据，也不会检索更多数据。

请参阅fs.ReadStream和stream.Readable。

“可读”事件发生两次

2 个答案: