Node.js的基础知识（一）

一、Buffer类

1.创建缓冲区的三种方式

var buffer = new Buffer(10);
console.log(buffer);

var buffer2 = new Buffer([10,20,30]);
console.log(buffer2);

var buffer3 = new Buffer('hello henanqingyun','utf-8');
console.log(buffer3);

2.缓冲区切片

console.log(buffer3.toString());
buffer2.fill();//缓冲区清零
buffer2 = buffer3.slice(3,5);
console.log(buffer2.toString());
console.log(buffer2.toJSON());

3.复制缓冲区　

var buffer4 = new Buffer(30);
buffer4.fill(); //这个是对buffer4的清零
buffer3.copy(buffer4);
console.log(buffer4.toString());

4.stringDecoder其作用相当于toString（）方法

var stringDecoder = require('string_decoder').StringDecoder;
var decoder = new stringDecoder();
console.log(decoder.write(buffer3));

5.缓冲区的拼接concat

var buffer = new Buffer('hqingyu ');
var buffer2 = new Buffer('node.js');
console.log(Buffer.concat([buffer,buffer2]).toString());

二、流的概念 Stream

1.可读流,可以读取相应的文档，并显示出来

var fs = require('fs');
var data = '';
//创建一个读取流，读取的目标是.txt
var readStream = fs.createReadStream('test.txt');
//添加data事件，会反复的读取，并将读取到的chunk参数赋值给data，直到全部读完
readStream.on('data',function(chunk){
    data += chunk;
});

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

2.可写流可以将数据data写入到指定的txt文档中，例子如下：

var fs = require('fs');

var data = '这是一段用writeable流写入的数据';

var writeStream = fs.createWriteStream('test.txt');

writeStream.write(data);

writeStream.end();

writeStream.on('finish',function(){
    console.log('写入完成');
});

writeStream.on('error',function(){
    console.log(error.stack);
});

console.log('程序执行完毕');

3.管道流,优点是将前面两种方法结合，读的同时，再写。像是在一个管道中

var fs = require('fs');

var readStream = fs.createReadStream('test.txt');

var writeStrame = fs.createWriteStream('destination.txt');

readStream.pipe(writeStrame);

console.log('program is end');

三、事件监听

1.基于http的事件监听（最基本的响应网页请求）

var http = require('http');
var server = http.createServer();
server.on('request',function(req,res){
    res.end('hello henann');
}).listen(8000);

2. 手动触发监听事件

var events = require('events');
var eventEmitter = new events.EventEmitter();
eventEmitter.on('connection',function(){
    console.log('链接可以');
    eventEmitter.emit('data_recived');
});
eventEmitter.on('data_recived',function(){
    console.log('数据接收');
});
eventEmitter.emit('connection');
console.log('程序完成');

3.addListeners这个可以设计自定义的事件监听；emitter.removeListener();移除一个事件；emitter.removeAllListeners();移除多个事件emitter.listenerCount()；获得事件的次数；

例子（

*定义一个自定义的类，账户 account
* 初始金额 =0
* 两个方法，存取钱
* 三个外部函数：通知存款余额
* 当存款小于0
* 当存款大于1000时触发

）如下：

var events = require('events');
function MyAccount(){
    this.money = 0;
    this.drawmoney = function (account) {
        this.money -= account;
        this.emit('change');
    }
    this.savemoney = function (account) {
        this.money += account;
        this.emit('change');
    }
}

MyAccount.prototype = new events.EventEmitter();
function message(){
    console.log('账户还剩',this.money);
}
function less(){
    if(this.money<0){
        console.log('请存钱!');
    }
}
function more(account,goal){
    if (account.money>goal){
        console.log('钱真多');
    }
}

var account = new MyAccount();
/*
 * 证明继承成功
 * */
//console.log(account.constructor.prototype);
account.on('change',message);
account.on('change',less);
account.on('change',function(){
    more(this,1000);
});
account.savemoney(200);
account.savemoney(500);
account.savemoney(301);
account.drawmoney(1200);