NodeJS - emitter.setMaxListeners() 方法

---

emitter.setMaxListeners() 方法允许我们修改特定 EventEmitter 实例的监听器限制。默认情况下，EventEmitter 不会允许单个事件超过 10 个监听器，如果添加超过 10 个监听器，EventEmitter 将发出警告消息。

要允许向特定事件添加无限数量的监听器，我们可以将值设置为 Infinity 或 0。

此方法属于EventEmitter 类，它是 node:events 模块的内置类。

语法

以下是 NodeJs emitter.setMaxListeners() 方法的语法：

emitter.setMaxListeners(n)

参数

此方法只接受一个参数，如下所述。

• n: 此参数将保存为特定 EventEmitter 实例设置的最大值。

返回值

此方法返回对 EventEmitter 的引用，以便可以链接调用。

示例 1

以下是 NodeJs emitter.setMaxListeners() 方法的基本示例。

首先，我们导入了 node:events 模块。然后，我们使用 eventName ‘myEvent’ 将五个监听器函数传递给 emitter.addListener() 方法。这里我们没有为特定事件设置任何限制。因此，如果我们编译并运行程序，将打印“10”到输出，因为默认情况下，最多可以添加 10 个监听器。

const EventEmitter = require('node:events');
const myEmitter = new EventEmitter();

myEmitter.addListener('myEvent', function one() {
});
myEmitter.addListener('myEvent', function two() {
});
myEmitter.addListener('myEvent', function three() {
});
myEmitter.addListener('myEvent', function four() {
});
myEmitter.addListener('myEvent', function five() {
});

console.log(myEmitter.getMaxListeners('myEvent'));

输出

上述程序产生以下输出：

10

示例 2

在这个程序中，我们调用了 emitter.setMaxListener() 方法，但是我们没有向其传递任何参数。如果我们编译并运行程序，它将向输出抛出一个错误消息。

const EventEmitter = require('node:events');
const myEmitter = new EventEmitter();

myEmitter.setMaxListeners();

myEmitter.addListener('myEvent', function one() {
});
myEmitter.addListener('myEvent', function two() {
});
myEmitter.addListener('myEvent', function three() {
});
myEmitter.addListener('myEvent', function four() {
});
myEmitter.addListener('myEvent', function five() {
});

console.log(myEmitter.getMaxListeners('myEvent'));

输出

上述程序产生以下输出：

node:events:397
    throw new ERR_OUT_OF_RANGE('n', 'a non-negative number', n);
    ^

RangeError [ERR_OUT_OF_RANGE]: The value of "n" is out of range. It must be a non-negative number. Received undefined
    at new NodeError (node:internal/errors:371:5)
    at EventEmitter.setMaxListeners (node:events:397:11)
    at Object. (/index.js:4:11)
    at Module._compile (node:internal/modules/cjs/loader:1103:14)
    at Object.Module._extensions..js (node:internal/modules/cjs/loader:1157:10)
    at Module.load (node:internal/modules/cjs/loader:981:32)
    at Function.Module._load (node:internal/modules/cjs/loader:822:12)
    at Function.executeUserEntryPoint [as runMain] (node:internal/modules/run_main:77:12)
    at node:internal/main/run_main_module:17:47 {
  code: 'ERR_OUT_OF_RANGE'
}

示例 3

我们调用了 emitter.setMaxListener() 方法，但是我们传递了一个负数作为参数。它将向输出抛出一个错误消息。

const EventEmitter = require('node:events');
const myEmitter = new EventEmitter();

myEmitter.setMaxListeners(-3);

myEmitter.addListener('myEvent', function one() {
});
myEmitter.addListener('myEvent', function two() {
});
myEmitter.addListener('myEvent', function three() {
});
myEmitter.addListener('myEvent', function four() {
});
myEmitter.addListener('myEvent', function five() {
});

console.log(myEmitter.getMaxListeners('myEvent'));

输出

上述程序产生以下输出：

node:events:300
      throw new ERR_OUT_OF_RANGE('n', 'a non-negative number', n);
      ^

RangeError [ERR_OUT_OF_RANGE]: The value of "n" is out of range. It must be a non-negative number. Received -2
    at new NodeError (node:internal/errors:371:5)
    at EventEmitter.setMaxListeners (node:events:300:13)
    at Object. (/index.js:6:1)
    at Module._compile (node:internal/modules/cjs/loader:1103:14)
    at Object.Module._extensions..js (node:internal/modules/cjs/loader:1157:10)
    at Module.load (node:internal/modules/cjs/loader:981:32)
    at Function.Module._load (node:internal/modules/cjs/loader:822:12)
    at Function.executeUserEntryPoint [as runMain] (node:internal/modules/run_main:77:12)
    at node:internal/main/run_main_module:17:47 {
  code: 'ERR_OUT_OF_RANGE'
}

示例 4

在这个程序中，我们向特定事件“myEvent”添加了 11 个监听器。默认情况下，EventEmitter 对于特定事件最多只接受 10 个监听器。当我们编译并运行下面的程序时，EventEmitter 将向输出发出警告消息。

const EventEmitter = require('node:events');
const myEmitter = new EventEmitter();

myEmitter.addListener('myEvent', function one() {
});
myEmitter.addListener('myEvent', function two() {
});
myEmitter.addListener('myEvent', function three() {
});
myEmitter.addListener('myEvent', function four() {
});
myEmitter.addListener('myEvent', function five() {
});
myEmitter.addListener('myEvent', function six() {
});
myEmitter.addListener('myEvent', function seven() {
});
myEmitter.addListener('myEvent', function eight() {
});
myEmitter.addListener('myEvent', function nine() {
});
myEmitter.addListener('myEvent', function ten() {
});
myEmitter.addListener('myEvent', function eleven() {
});

console.log(myEmitter.getMaxListeners('myEvent'));

输出

上述程序产生以下输出：

10

(node:11610) MaxListenersExceededWarning: Possible EventEmitter memory leak detected. 11 myEvent listeners added to [EventEmitter]. Use emitter.setMaxListeners() to increase limit
(Use `node --trace-warnings ...` to show where the warning was created)

示例 5

在下面的程序中，我们向特定事件“myEvent”添加了 3 个监听器。我们调用了 emitter.setMaxListener() 方法，并向其传递了一个非负数作为参数。我们添加了 3 个监听器，我们传递的限制也是 3。它将返回“3”作为输出，因为最大限制是 3。

const EventEmitter = require('node:events');
const myEmitter = new EventEmitter();

myEmitter.setMaxListeners(3);

myEmitter.addListener('myEvent', function one() {
});
myEmitter.addListener('myEvent', function two() {
});
myEmitter.addListener('myEvent', function three() {
});

console.log(myEmitter.getMaxListeners('myEvent'));

输出

上述程序产生以下输出：

3