When developing applications for Android, one often facesthe problem of displaying some graphical content from the Internet. So, youshould provide image loading from the Web in an Android app, their processingand displaying
with limited memory again and again. And despite the problemhomogeneity, each new project imposes its own specific requirements on thetask.
You may need to organize caching of downloaded images; ifimages are rather large, it is necessary to ensure effective working with thememory to prevent the disastrous mistake OutOfMemoryError . It is also possiblethat
an image-stub has to be showed during the image loading; and maybe thesame image should be displayed in different size variations, etc.
As a result, time and resources are wasted for codeadaptation to specific needs. It is this problem what urged me on creating alibrary with open source code - Universal Image Loader for image loading in anAndroid app.
Its aim is solution universalization of the above describedproblem in a flexible and configurable tool.
Currently, the library can be used everywhere, where youhave to download and display (and possibly even to cache) an image from theInternet or from the file system of your smartphone. Classic examples forpossibility of
ImageLoader using are various lists, tables, galleries, whereyou need to display images from the Web.
上面一大堆话，是说作者为什么做这个项目
如今这个类库能够用在随意地方。比如。你须要从网络上下载和显示图片。或者从手机本地读取。ImageLoader能够用的地方包含各种lists，tables。galleries
The main features of the ImageLoader for Android are:
主要特性包含：
•asynchronous loading and displaying images from the Internet or the SD-card;
从网络或SD卡异步载入和显示图片
• ability ofcaching loaded images in memory and / or the device's file system;
在本地或内存中缓存图片
• ability tomonitor the loading process by means of "listeners"
通过“listener”监视载入的过程
• effectiveworking with the memory while caching images in the memory;
缓存图片至内存时，更加高效的工作
• wideopportunities to customize the tool to fit it to your needs.
高度可定制化
ImageLoader的可选设置项
What can be configured in the ImageLoader?
全局设置:
• themaximum size of images cached in the memory;
在内存中缓存的图片最大尺寸
• timeoutfor connection establishing and image loading;
连接超时时间和图片载入超时时间
• themaximum number of simultaneously working threads for images loading;
载入图片时使用的同一时候工作线程数量
• threadspriority during downloading and displaying images;
下载和显示图片时的线程优先级
•implementation of disk cache (you can choose from ready-made implementations orcreate your own);
使用已定义本地缓存或自己定义
•implementation of cache in the memory (you can choose from ready-madeimplementations or create your own);
使用已定义内存缓存或自己定义
• default options of image downloading
图片下载的默认选项
Image loading options (applied to each individual callImageLoader.displayImage(...)) provide the ability to specify:
图片载入选项能够进行下面设置：
• whether todisplay the image-stub in the ImageView, while the real image is being loaded(if yes, then you need to specify this "stub");
当真实图片载入成功以后，是否显示image-stub。
（假设是，须要制定stub）
• Whether tocache the downloaded image in the memory.
在内存中是否缓存已下载图片
• Whether tocache the downloaded image in the file system.
在本地是否缓存已下载图片
• Type ofimage decoding (the fastest or the most economical for the memory).
图片解码方式（高速/经济）
As already mentioned, you can implement your own version ofthe disk cache and the cache in memory. But most likely, you will be quitesatisfied with ready solutions, most of which are caches, limited by someparameter (size,
number of files) and having their own logic of self-cleaningby limit excess (FIFO, the oldest object, the largest object, the most seldomused).
如上所述。你能够实现你自己的本地缓存和内存缓存方法，可是通常Jar包中已实现的方法已经能够满足你的需求。
比如。
。。。。
。
There are enough configuration options, but this is not thecase as "the main principle of UNIX": "u can configureEVERYTHING. And you WILL configure everything." :) In theImageLoader
case, you can customize everything, but it is not necessary at all:the default configuration is always available and suitable in the most cases.
虽然UIL已经提供了非常多设置，可是距离“UNIX的核心精神：你能够配置不论什么选项。你能够配置不论什么选项”仍有差距。在UIL中，你能够自己定义不论什么选项。但这并非必须的。由于通常已经有了默认设置，并且这些设置在大多数情况下都是适用的。
Implementationpeculiarities
特定需求的实现
Few words about the project structure. Each task for image loadingand displaying (and that is, looking ahead, the call ImageLoader.displayImage(imageView, imageUrl)) is performed in a separate thread, except if the pictureis
in cache in the memory - then it is just immediately displayed.here is a separate threads queue where tasks get if theneeded image is cached on the file system. If you do not have the rightimage in the cache, then the
task-thread gets in the thread pool. Therefore,there are no obstacles for a fast displaying of cached images.
简单描写叙述一下这个项目的结构。
每个图片的载入和显示任务都执行在独立的线程中，除非这个图片缓存在内存中，这样的情况下图片会马上显示。假设须要的图片缓存在本地，他们会开启一个独立的线程队列。假设在缓存中没有正确的图片，任务线程会从线程池中获取，因此，高速显示缓存图片时不会有明显的障碍。
he algorithm of the taskprocessing isepresented on the scheme:
处理的规则用下图进行简单的描写叙述：
The main actors of the project can relatively be divided:
UIL的主要功能能够分为下面及部分
• the above mentionedqueue and pool of threads;
上面提到的线程队列和线程池
• cache in thememory;
缓存至内存
• disk cache;
缓存至本地
• Image decoder,which decodes image files into Bitmap objects.
图片解析，将图片解析为Bitmap对象
The main class ImageLoadermanages it all; the maininteraction with the user is performed through it.
最重要的类ImageLoader管理了全部的操作。和用户的交互也主要通过该类。