Jquery promise implements loading images one by one_jquery

Promise is one of the specifications of CommonJS. It has resolve, reject, done, fail, then and other methods, which can help us control the flow of code and avoid multi-layer nesting of functions. Nowadays, asynchrony is becoming more and more important in web development. For developers, this kind of non-linear execution programming will make it difficult for developers to control. Promise can allow us to better control the execution process of the code. Popular js such as jQuery All libraries have already implemented this object, and ES6, which will be released at the end of the year, will also natively implement Promise.

In the proxy mode of JavaScript design pattern practice - image preloading, the image preloading function is implemented using the proxy mode.

Now let’s go one step further and complete a function that can load continuous pictures one by one.

Function:

1. Load pictures one by one.

2. Loading error, the loading failure image will be displayed after timeout.

Regarding functional requirements, there will definitely be processing of loading status events and callback functions when completed. This will not only cause confusion in the code, but even destroy various principles, so we will no longer use ordinary methods to write it. . In view of the characteristics of this status notification, it is more suitable to use the promise architecture for processing. Promise is essentially a kind of subscription-publish design pattern. Currently, this function is developed using the promise that comes with jquery.

1. Complete a proxy creation function for loading images, which can generate a proxy with loading timeout, failure, success, and cancellation monitoring capabilities.

 function createLoadImgProxy(){
  var imgCache = new Image();
  var dfd = $.Deferred();
  var timeoutTimer;
  //开始加载超时监控，超时后进行reject操作
  function beginTimeoutWatcher(){
   timeoutTimer = setTimeout(function(){
   dfd.reject('timeout');
   }, 10000);
  }
  //结束加载超时监控
  function endTimeoutWatcher(){
   if(!timeoutTimer){
   return;
   }
   clearTimeout(timeoutTimer);
  }
  //加载完成事件处理，加载完成后进行resolve操作
  imgCache.onload = function(){
   dfd.resolve(this.src);
  };
  //加载终止事件处理，终止后进行reject操作
  imgCache.onabort = function(){
   dfd.reject("aborted");
  };
  //加载异常事件处理，异常后进行reject操作
  imgCache.onerror = function(){
   dfd.reject("error");
  };
  return function(eleImg, src){
   dfd.always(function(){
   //加载完成或加载失败都要终止加载超时监控
   endTimeoutWatcher();
   }).done(function(src){
   //加载完成后，往图片元素上设置图片
   loadImg(eleImg, src);
   }).fail(function(msg){
   //加载失败后，往图片元素上设置失败图片
   loadImg(eleImg, 'loadFailed.jpg');
   });
   loadImg(eleImg, 'loading.gif');
   imgCache.src = src;
   //开始进行超时加载监控
   beginTimeoutWatcher();
   return dfd.promise();
  };
  }

Among them, a Deferred object is created in the following way

Copy code The code is as follows:

var dfd = $.Deferred();

The Deferred object triggers the completion event through the resolve method and responds to the completion event using the done method.

Complete event when loading is successful.

Copy code The code is as follows:

imgCache.onload = function(){
                  dfd.resolve(this.src);
                };

and the response processing when loading is completed are to set the image to the element. The following code is a disassembly of the above chain writing method.

Copy code The code is as follows:

dfd.done(function(src){
//After loading is completed, set the image on the image element
LoadImg(eleImg, src);
                     });

The Defferred object triggers a rejection event through the reject method, and uses the fail method to respond to the rejection event, indicating that the loading failed.

Reject events when loading timeout, termination, exception.                                                                       

 //开始加载超时监控，超时后进行reject操作
  function beginTimeoutWatcher(){
   timeoutTimer = setTimeout(function(){
   dfd.reject('timeout');
   }, 10000);
  }
  //加载终止事件处理，终止后进行reject操作
  imgCache.onabort = function(){
   dfd.reject("aborted");
  };
  //加载异常事件处理，异常后进行reject操作
  imgCache.onerror = function(){
   dfd.reject("error");
  };

And response processing when loading fails, setting failure pictures.                                                          

 dfd.fail(function(msg){
   //加载失败后，往图片元素上设置失败图片
   loadImg(eleImg, 'loadFailed.jpg');
   });

At the end of the proxy function, return the deferred promise object, which is used to monitor the completion and failure status of the loading to the calling place to facilitate the loading of the next image.

return dfd.promise();

2. Continuous loading one by one

//一张一张的连续加载图片
  //参数：
  // srcs: 图片路径数组
  function doLoadImgs(srcs){
  var index = 0;
  (function loadOneByOne(){
   //退出条件
   if(!(s = srcs[index++])) {
   return;
   }
   var eleImg = createImgElement();
   document.getElementById('imgContainer').appendChild(eleImg);
   //创建一个加载代理函数
   var loadImgProxy = createLoadImgProxy();
   //在当前图片加载或失败后，递归调用，加载下一张
   loadImgProxy(eleImg, s).always(loadOneByOne);
  })();
  }

Make a recursive loading function of loadOneByOne.

A loading agent is first created internally. After the agent loads the image, regardless of success or failure, the loadOneByOne function is called recursively to load the next image.

The key lies in the promise object returned by the proxy function. Use the .always method to make a recursive call to loadOneByOne to load the next picture after the loading is completed (success or failure).

Copy code The code is as follows:

loadImgProxy(eleImg, s).always(loadOneByOne);

至此完成。

采用了promise模式后，callback函数不见了，维护状态的函数和内部变量也不见了，代码更清晰简单，使得代理函数和本地函数之间的一致性得到保护。

完整代码：

<!DOCTYPE html>
<html>
 <head>
 <meta charset="utf-8">
 </head>
 <body>
 <button id='btnLoadImg'>加载图片</button>
 <br>
 <div id='imgContainer'>
 </div>
 <br>
 <script type='text/javascript' src="./jquery-1.11.3.min.js"></script>
 <script type='text/javascript'>
  var imgSrcs = [
  'http://img.wanchezhijia.com/A/2015/3/20/17/11/de63f77c-f74f-413a-951b-5390101a7d74.jpg',
  'http://www.newbridgemotorsport.com/files/6413/9945/0406/IMG_3630.jpg',
  'http://www.carsceneuk.com/wp-content/uploads/2015/03/88y9989.jpg',
  'http://mfiles.sohu.com/20130223/5ff_403b2e7a_7a1f_7f24_66eb_79e3f27d58cf_1.jpg',
  'http://img1.imgtn.bdimg.com/it/u=2678963350,1378052193&fm=21&gp=0.jpg'
  ];
  $(document).ready(function(){
  $('#btnLoadImg').bind('click', function(){
   doLoadImgs(imgSrcs);
  });
  });
  //创建img标签
  //这里用自执行函数加一个闭包，是为了可以创建多个id不同的img标签。
  var createImgElement = (function(){
  var index = 0;
  return function() {
   var eleImg = document.createElement('img');
   eleImg.setAttribute('width', '200');
   eleImg.setAttribute('heght', '150');
   eleImg.setAttribute('id', 'img' + index++);
   return eleImg;
  };
  })();
  function loadImg(img, src) {
  img.src = src;
  }
  function createLoadImgProxy(){
  var imgCache = new Image();
  var dfd = $.Deferred();
  var timeoutTimer;
  //开始加载超时监控，超时后进行reject操作
  function beginTimeoutWatcher(){
   timeoutTimer = setTimeout(function(){
   dfd.reject('timeout');
   }, 10000);
  }
  //结束加载超时监控
  function endTimeoutWatcher(){
   if(!timeoutTimer){
   return;
   }
   clearTimeout(timeoutTimer);
  }
  //加载完成事件处理，加载完成后进行resolve操作
  imgCache.onload = function(){
   dfd.resolve(this.src);
  };
  //加载终止事件处理，终止后进行reject操作
  imgCache.onabort = function(){
   dfd.reject("aborted");
  };
  //加载异常事件处理，异常后进行reject操作
  imgCache.onerror = function(){
   dfd.reject("error");
  };
  return function(eleImg, src){
   dfd.always(function(){
//   alert('always end');
   //加载完成或加载失败都要终止加载超时监控
   endTimeoutWatcher();
   }).done(function(src){
//   alert('done end');
   //加载完成后，往图片元素上设置图片
   loadImg(eleImg, src);
   }).fail(function(msg){
//   alert('fail end:' + msg);
   //加载失败后，往图片元素上设置失败图片
   loadImg(eleImg, 'loadFailed.jpg');
   });
   loadImg(eleImg, 'loading.gif');
   imgCache.src = src;
   //开始进行超时加载监控
   beginTimeoutWatcher();
   return dfd.promise();
  };
  }
  //一张一张的连续加载图片
  //参数：
  // srcs: 图片路径数组
  function doLoadImgs(srcs){
  var index = 0;
  (function loadOneByOne(){
   //退出条件
   if(!(s = srcs[index++])) {
   return;
   }
   var eleImg = createImgElement();
   document.getElementById('imgContainer').appendChild(eleImg);
   //创建一个加载代理函数
   var loadImgProxy = createLoadImgProxy();
   //在当前图片加载或失败后，递归调用，加载下一张
   loadImgProxy(eleImg, s).always(loadOneByOne);
  })();
  }
 </script>
 </body>
</html>

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