NodeJS - emitter.setMaxListeners() 方法

emitter.setMaxListeners() 方法允許我們修改特定 EventEmitter 例項的監聽器限制。預設情況下，EventEmitter 不會允許單個事件超過 10 個監聽器，如果新增超過 10 個監聽器，EventEmitter 將發出警告訊息。

要允許向特定事件新增無限數量的監聽器，我們可以將值設定為 Infinity 或 0。

此方法屬於EventEmitter 類，它是 node:events 模組的內建類。

語法

以下是 NodeJs emitter.setMaxListeners() 方法的語法：

emitter.setMaxListeners(n)

引數

此方法只接受一個引數，如下所述。

n: 此引數將儲存為特定 EventEmitter 例項設定的最大值。

返回值

此方法返回對 EventEmitter 的引用，以便可以連結呼叫。

示例 1

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

首先，我們匯入了 node:events 模組。然後，我們使用 eventName ‘myEvent’ 將五個監聽器函式傳遞給 emitter.addListener() 方法。這裡我們沒有為特定事件設定任何限制。因此，如果我們編譯並執行程式，將列印“10”到輸出，因為預設情況下，最多可以新增 10 個監聽器。

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'));

輸出

上述程式產生以下輸出：

示例 2

在這個程式中，我們呼叫了 emitter.setMaxListener() 方法，但是我們沒有向其傳遞任何引數。如果我們編譯並執行程式，它將向輸出丟擲一個錯誤訊息。

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'));

輸出

上述程式產生以下輸出：

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

我們呼叫了 emitter.setMaxListener() 方法，但是我們傳遞了一個負數作為引數。它將向輸出丟擲一個錯誤訊息。

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'));

輸出

上述程式產生以下輸出：

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

在這個程式中，我們向特定事件“myEvent”添加了 11 個監聽器。預設情況下，EventEmitter 對於特定事件最多隻接受 10 個監聽器。當我們編譯並執行下面的程式時，EventEmitter 將向輸出發出警告訊息。

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'));

輸出

上述程式產生以下輸出：

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

在下面的程式中，我們向特定事件“myEvent”添加了 3 個監聽器。我們呼叫了 emitter.setMaxListener() 方法，並向其傳遞了一個非負數作為引數。我們添加了 3 個監聽器，我們傳遞的限制也是 3。它將返回“3”作為輸出，因為最大限制是 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'));

輸出

上述程式產生以下輸出：

nodejs_events.htm

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