初识Promise

Promise对象

曾经用seajs开发后台管理的时候,矫情的PHPER非要JS内联到HTML文件中,方便他调用内容,还指定了jQueryFileUpload作为上传插件.
当时看到jQueryFileUpload的时候,我是懵逼的,竟然要引入这么多JS!还要按照顺序去引入,不能顺序错误!整个人都崩溃了,于是就写出了一下的回调金字塔.

seajs.use('__STATIC__/admin/js/other/jquery/jquery.js',function($){
	seajs.use('__STATIC__/admin/js/plugins/jQueryFileUpload/load-image.all.js',function(){
		seajs.use('__STATIC__/admin/js/plugins/jQueryFileUpload/canvas-to-blob.js',function(){
			seajs.use('__STATIC__/admin/js/plugins/jQueryFileUpload/vendor/jquery.ui.widget.js',function(){
				seajs.use('__STATIC__/admin/js/plugins/jQueryFileUpload/jquery.iframe-transport.js',function(){
					seajs.use('__STATIC__/admin/js/plugins/jQueryFileUpload/jquery.fileupload.js',function(){
						seajs.use('__STATIC__/admin/js/plugins/jQueryFileUpload/jquery.fileupload-process.js',function(){
							seajs.use('__STATIC__/admin/js/plugins/jQueryFileUpload/jquery.fileupload-image.js',function(){
								seajs.use('__STATIC__/admin/js/plugins/jQueryFileUpload/jquery.fileupload-validate.js',function(){
									......
								});
							});
						});
					});
				});
			});
		});
	});
});

回调金字塔,一层一层的回调,很容易出现问题,导致整个都崩溃掉.为了解决异步编程,ES6提供了Promise对象.
Promise对象代表一个异步操作,三种状态: Pending(进行中), Resolved(已完成)和 Rejected(已失败).
Promise对象的状态变化只有两种,Pending => Resolved 或者 Pending 和 Rejected

Promise新建后,其内部的代码将直接运行.

var promise = new Promise(function(resolve, reject){
	//	创建后 以下代码直接执行.
	var value = Math.round(Math.random());
	console.log('将要输出: 值为' + value);
	//	setTimeout 模拟延迟操作
	setTimeout(function(){
		if (value === 1) {
			resolve(value);
		} else {
			reject(value);
		}
	}, 2000);
});

promise.then(function(value){
	console.log('值为1');
}, function(){
	console.log('值为0');
});

(以后待补充)