Node.js 模块之Nimble流程控制

NodeJS异步的特性有时候会导致程序非常的难看，回调一层套着一层，这个时候就要用流程控制模块来控制究竟是同步还是异步了。

Nimble是一个轻量、可移植的函数式流程控制模块。经过最小化和压缩后只有837字节，可以运行在Node.js中，也可以用在各种浏览器中。它整合了underscore和async一些最实用的功能，并且API更简单。

nimble有两个流程控制函数，_.parallel和_.series。顾名思义，我们要用的是第二个，可以让一组函数串行执行的_.series。下面这个命令是用来安装Nimble的：

npm install nimble

如果用.series调度执行上面那个解方程的函数，代码应该是这样的：

...
var flow = require('nimble');
(function calculate(i) {
    if(i === l-1) {
        variables[i] = res[i];
        process.exit();
    }else {
        flow.series([
            function (callback) {
                calculateTail(res[i],res[i+1],function(tail) {
                    variables[i] = tail;
                    callback();
                });
            },
            function (callback) {
                calculateHead(res[i],res[i+1],function(head) {
                    res[i+1] = head;
                    callback();
                });
            },
            function(callback){
                calculate(i+1);
            }]);
    }
})(0);
...

.series数组参数中的函数会挨个执行，只是我们的calculateTail和calculateHead都被包在了另一个函数中。尽管这个用流程控制实现的版本代码更多，但通常可读性和可维护性要强一些。

更多实例：

串行执行（非异步）：

var flow = require('nimble');

flow.series([
    function (callback)
    {
        setTimeout(function()
        {
            console.log('I execute first.');
            callback();
            newfunc();
        }, 1000)
    },
    function (callback)
    {
        setTimeout(function()
        {
            console.log('I execute next.');
            callback();
        }, 2000)
    },
    function (callback)
    {
        setTimeout(function()
        {
            console.log('I execute last.');
            callback();
        }, 100)
    }
]);

并行执行（异步）：

var flow = require('nimble');

flow.parallel([
    function (callback) {
        setTimeout(function () {
            console.log('one');
            callback();
        }, 25);
    },
    function (callback) {
        setTimeout(function () {
            console.log('two');
            callback();
        }, 0);
    }
]);

串并行兼顾（流程控制）：

var flow = require('nimble');
var exec = require('child_process').exec;

function downloadNodeVersion(version, destination, callback)
{
    var url      = 'http://nodejs.org/dist/node-v' + version + '.tar.gz';
    var filepath = destination + '/' + version + '.tgz';
    exec('curl ' + url + ' > ' + filepath, callback);
}

flow.series
([
    function(callback)
    {
        flow.parallel
        ([
            function(callback)
            {
                console.log('Downloading Node v0.4.6...');
                downloadNodeVersion('0.4.6', '/tmp', callback);
            },
            function(callback)
            {
                console.log('Downloading Node v0.4.7...');
                downloadNodeVersion('0.4.7', '/tmp', callback);
            }
        ], callback);
    },
    function(callback)
    {
        console.log('Creating archive of downloading files...');
        exec
        (
            'tar cvf node_distros.tar /tmp/0.4.6.tgz /tmp/0.4.7.tgz',
            function(error, stdout, stderr)
            {
                console.log('All Done!');
                callback();
            }
        )
    }
]);

这个稍微解释一下：先异步下载文件，文件下载完成后，再把所有文件打包。

nimble.min.js

(function(a){var b=Object.keys||function(a){var b=[];for(var c in a)a.hasOwnProperty(c)&&b.push(c);return b};var c=function(a,b){var c=Array.prototype[a];return function(d,e,f){var g=d?d[a]:0;return g&&g===c?g.call(d,e,f):b(d,e,f)}};var d=c("forEach",function(a,c){var d=a instanceof Object;var e=d?b(a):a||[];for(var f=0,g=e.length;f<g;f++){var h=d?e[f]:f;c(a[h],h,a)}});var e=function(a,c,e){var f=a.length||b(a).length;if(!f)return e();var g=0;d(a,function(){var a=function(a){a?(e(a),e=function(){}):++g===f&&e()};var b=Array.prototype.slice.call(arguments);c.length?(b=b.slice(0,c.length-1),b[c.length-1]=a):b.push(a),c.apply(this,b)})};var f=function(a,c,d){var e=b(a);if(!e.length)return d();var f=0;var g=function(){var b=e[f];var h=[a[b],b,a].slice(0,c.length-1);h[c.length-1]=function(a){a?(d(a),d=function(){}):++f===e.length?d():g()},c.apply(this,h)};g()};var g=c("map",function(a,b){var c=[];d(a,function(a,d,e){c[c.length]=b(a,d,e)});return c});var h=function(a){return function(b,c,d){var e=[];a(b,function(a,b,d,f){var g=function(a,b){e[e.length]=b,f(a)};var h=[a,b,d];c.length?(h=h.slice(0,c.length-1),h[c.length-1]=g):h.push(g),c.apply(this,h)},function(a){d(a,e)})}};var i=c("filter",function(a,b,c){var e=[];d(a,function(a,c,d){b(a,c,d)&&(e[e.length]=a)});return e});var j=function(a,b,c){var d=[];e(a,function(a,c,e,f){var g=function(b,c){c&&(d[d.length]=a),f(b)};var h=[a,c,e];b.length?(h=h.slice(0,b.length-1),h[b.length-1]=g):h.push(g),b.apply(this,h)},function(a){c(a,d)})};var k=c("reduce",function(a,b,c){d(a,function(a,d,e){c=b(c,a,d,e)});return c});var l=function(a,b,c,d){f(a,function(a,d,e,f){var g=function(a,b){c=b,f(a)};var h=[c,a,d,e];b.length?(h=h.slice(0,b.length-1),h[b.length-1]=g):h.push(g),b.apply(this,h)},function(a){d(a,c)})};a.each=function(a,b,c){return(c?e:d)(a,b,c)},a.map=function(a,b,c){return(c?h(e):g)(a,b,c)},a.filter=function(a,b,c){return(c?j:i)(a,b,c)},a.reduce=function(a,b,c,d){return(d?l:k)(a,b,c,d)},a.parallel=function(a,b){var c=new a.constructor;e(a,function(a,b,d){a(function(a){var e=Array.prototype.slice.call(arguments,1);c[b]=e.length<=1?e[0]:e,d(a)})},function(a){(b||function(){})(a,c)})},a.series=function(a,b){var c=new a.constructor;f(a,function(a,b,d){a(function(a,e){var f=Array.prototype.slice.call(arguments,1);c[b]=f.length<=1?f[0]:f,d(a)})},function(a){(b||function(){})(a,c)})}})(typeof exports==="undefined"?this._=this._||{}:exports)