Node異步學習

Node異步學習

非阻塞IO

fs.readFile()是異步的

const fs = require('fs');

console.log('1');

fs.readFile('./static/test.jpeg', function (err, data) {
    console.log('2');
});

console.log('3');

1
3
2

外界拿不到異步方法的值

同步情況

const fs = require('fs');


function get() {
    const data = fs.readFileSync('./static/image/test.jpeg');
    return data;
}

console.log(get());

<Buffer ff d8 ff e0 00 10 4a 46 49 46 00 01 01 0

異步

const fs = require('fs');


function get() {
    const data = fs.readFile('./static/image/test.jpeg', function (err, data) {
        return data;
    });
}

console.log(get());

undefined

解決外界拿不到異步方法的值

使用回調函數

const fs = require('fs');


function get(cb) {
    fs.readFile('./static/image/test.jpeg', function (err, data) {
        cb(data);
    });
}

console.log(get(function (data) {
    console.log(data);
}));

undefined
<Buffer ff d8 ff e0 00 10 4a 46 49

events模塊

引入events

const events = require('events');
console.log(events);

{
once: [Function: once],
EventEmitter: [Circular],
usingDomains: false,
defaultMaxListeners: [Getter/Setter],
init: [Function],
listenerCount: [Function]
}

實例化events

const events1 = new events;
console.log(events1);

EventEmitter {
_events: [Object: null prototype] {},
_eventsCount: 0,
_maxListeners: undefined
}

同樣的

const eventEmitter = new events.EventEmitter();
console.log(eventEmitter);

EventEmitter {
_events: [Object: null prototype] {},
_eventsCount: 0,
_maxListeners: undefined
}

events廣播和接收廣播

監聽to的廣播

EventEmitter.on('to', function (data) {
    console.log('接收到了to廣播事件')
});

廣播事件

setTimeout(function () {
    EventEmitter.emit('to', '發送數據');
});

接收到了to廣播事件
發送數據

通過事件驅動獲取異步方法的值

const events = require('events');
const fs = require('fs');

let EventEmitter = new events.EventEmitter();


fs.readFile('./static/image/test.jpeg', function (err, data) {
    EventEmitter.emit('data', data);
});

EventEmitter.on('data', function (data) {
    console.log(data);
});

<Buffer ff d8 ff e0 00 10 4a 46 49 4