Node.js flow control

Node.js Flow Control
MiCloud - Simon

首先，今天
都是程式碼....

● node.js
● text editor
● command line tool
● coffee… (可以清醒一點>.<)
請先準備好你的環境...

● modules?
● callback?
● closure?
● json?
測試一下你對Node.js的認識

Modules
[test.js]
var say = require(./say’);
say.hello(‘simon’)
[say.js]
exports.hello = function(word) {
console.log( ‘Hello ’ + word);
}

Callback
[say.js]
exports.hello = function(word, fn) {
fn(‘Hello ’ + word);
}
[test.js]
var say = require(./say’);
say.hello(‘simon’, function(word){
console.log(word);
})

Closure
[say.js]
var f = function(x,y){
return x + y;
}
function foo(callback) {
var a = 1; var b = 2; callback(a,b);
}
foo(f);

JSON
[test.js]
var a = {name:’simon’};
a.sex = ‘man’;
a.say = function(word) {
console.log(‘Hello’ + word);
}
console.log(a.name);
a.say(a.name);
delete a.sex;

大綱
● Write a function with callback
● 基本的流程控制作法
○ array queue流程控制
○ process.nextTick() + callee
● 流程控制套件介紹
○ step
○ q
○ node-promise
● Web流程控制
○ expressjs

Write a function with callback
var request = require('request')
, util = require('util');
var url_ds = 'http://odf.micloud.tw/odf/datasets';
var url_field = 'http://odf.micloud.tw/odf/%s/field';
request.get(url_ds, function(err, res, body){
if(err) console.log(err);
var data = JSON.parse(body);
for(var i = 0 ; i < data.length ; i++){
var ds = data[i];
var url = util.format(url_field, ds);
request.get(url, function(e,r,b){
if(e) console.log(e);
console.log('Dataset: %s', ds);
console.log(b);
});
}
});
沒有做好流程控制，你會
有更多Callback...

插播一下：for 與 .forEach的差別
var arr = [0,1,2,3,4,5,6,7,8,9];
arr.forEach(function(v){
if(v == 3) return;
console.log(v);
})
var arr = [0,1,2,3,4,5,6,7,8,9];
for(var i = 0; i< arr.length ; i++){
var v = arr[i];
if(v == 3) return;
console.log(v);
}
結果是：0,1,2 結果是：0,1,2,4,5,6,7,8,9

解法(2)：process.nextTick + callee
var queue = ['http://www.google.com','http://micloud.tw','http://tw.yahoo.com'];
process.nextTick(function fn1(){
var url = queue.pop();
console.log('Processing %s...', url);
var _callee = arguments.callee;
request.get(url, function(e,r,d){
console.log('[%s] word count: %s', url, d.length);
if(queue.length > 0)
process.nextTick(_callee);
});
});
如果queue還有值，才會
call下一個request...
因為Scope的關係，callee
需要先指定給另一個變
數，後面才能取用...

3’rd party modules
Step, q, node-promise

● Github: http://github.com/creationix/step.git
Step模組

Step的用法
var Step = require('step');
Step(
function step1() {
console.log('Step1...');
throw 'error..'; //這個會掉到step2的arguments[0]
return 123; //有return才會往下走
},
function step2() {
console.log(arguments); //可以觀察接到的參數
return 223;
},
function step3() {
console.log(arguments);
}
);

其他功能 - parallel()
Step(
// Loads two files in parallel
function loadStuff() {
fs.readFile(__filename, this.parallel());
fs.readFile("/etc/passwd", this.parallel());
},
// Show the result when done
function showStuff(err, code, users) {
if (err) throw err;
console.log(code);
console.log(users);
}
)

其他功能 - group()
Step(
function readDir() { fs.readdir(__dirname, this); },
function readFiles(err, results) {
if (err) throw err;
var group = this.group(); // Create a new group
results.forEach(function (filename) {
fs.readFile(__dirname + "/" + filename, 'utf8', group());
});
},
function showAll(err , files) {
if (err) throw err;
console.dir(files);
}
);

Step注意事項
● Step的操作是擷取callback作為下一個
function的input
● 如果沒有callback的step，必須要return
● 中間step(ex: step2)若沒有return，則程式有可
能會卡住不動(ex: web app)
● 每個step function中的return不代表中斷整個
流程

CommonJS - Promises/A
■ Q Works in both NodeJS and browsers,
compatible with jQuery, thenable, and
usable as remote objects via message
passing
■ RSVP.js A lightweight library that provides
tools for organizing asynchronous code
■ when.js Compact promises with joining
and chaining
■ node-promise Promises for NodeJS
■ jQuery 1.5 is said to be 'based on the
CommonJS Promises/A design'.
■ ForbesLindesay/promises-a A bare bones
implementation of Promises/A intended to
pass https://github.com/domenic/promise-
tests while being as small as possible
■ WinJS / Windows 8 / Metro

q模組
● Github: https://github.com/kriskowal/q
● 範例: https://github.com/kriskowal/q/wiki/Examples-Gallery

安裝q模組
npm install q --save

q提供的功能
● Q.delay
● Q.defer
● Q.nfcall
● Q.nfapply
● Q.ninvoke
● Q.npost
● ...

var Q = require('q')
, request = require('request')
, queue = ['http://www.google.com','http://micloud.tw','http://tw.yahoo.com'];
var fn = function(url) {
var deferred = Q.defer();
console.log('[%s] word count:%s', url, d.length);
deferred.resolve();
});
return deferred.promise;
};
Q.allResolved( [ fn(queue[0]), fn(queue[1]), fn(queue[2]) ] )
.then(function(){
console.log(‘end...’)
}).done();
使用q操做爬蟲
q只
保
證
開
始
順
序

延續剛剛的範例
…(skip)
var out = function (x, y, z) {
var d = Q.defer();
console.log('x:%s, y:%s, z:%s', x, y, z);
d.resolve();
return d.promise;
};
Q.allResolved([fn(queue[0]), fn(queue[1]), fn(queue[2])])
.spread(out)
.then(function(){
console.log('end...');
})
.done();
使用spread來接收執
行結果值

node-promise模組
● Github: https://github.com/kriszyp/node-promise

安裝node-promise模組
npm install node-promise --save

, util = require('util')
var site = ‘http://odf.micloud.tw’
var url = site + '/odf/datasets'
var url_detail = site + '/odf/%s/field';
function step1(url){
var p = new Promise();
request({
url : url,
method : "GET"
},
function(e,r,d){
p.resolve(JSON.parse(d));
});
return p;
}
延續剛剛的範例
step1(url)
.then(function(d){
for(var i = 0 ; i< d.length ; i++){
step1(util.format(url_detail, d[i]))
.then(function(d){
console.log(d);
});
}
})
使用return promise的方式，串連then操作
使用function scope限制promise存續區域

ExpressJS中的流程控制
● next()

透過流程控制增加authfilter
//增加req.headers.auth認證
function authfilter(req, res, next){
if(req.headers.auth == 'demo')
next(); //next代表通過此filter
else
//認證錯誤，則回傳statusCode與錯誤訊息
res.send(401, 'Auth Error!');
}
//則在routing中可以安插在真正執行route之前
app.get('/users', authfilter, user.list);

Reference
● http://opennodes.arecord.us
● http://calculist.org/blog/2011/12/14/why-
coroutines-wont-work-on-the-web/
● http://wiki.commonjs.org/wiki/Promises/A
● https://github.com/basicallydan/q-
examples/blob/master/wait-for-multiple-
promises.js
● https://github.com/kriskowal/q/wiki/API-
Reference

Node.js flow control

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