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

现在这个类库可以用在任意地方，例如，你需要从网络上下载和显示图片，或者从手机本地读取。ImageLoader可以用的地方包括各种lists，tables，galleries

The main features of the ImageLoader for Android are:

主要特性包括：

?asynchronous loading and displaying images from the Internet or the SD-card;

从网络或SD卡异步加载和显示图片

? 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”监视加载的过程

? 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加载超时时间

? themaximum number of simultaneously working threads for images loading;

加载图片时使用的同时工作线程数量

? 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:

图片加载选项可以进行以下设置：

? whether todisplay the image-stub in the ImageView, while the real image is being loaded(if yes, then you need to specify this "stub");

当真实图片加载成功以后，是否显示image-stub。（如果是，需要制定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（快速/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，你可以实现你自己的本地缓存和内存缓存方法，但是通常Jar包中已实现的方法已经可以满足你的需求。例如。。。。。。

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.

尽管UIL已经提供了很多设置，但是距离“UNIX的核心精神：你可以配置任何选项，你能够配置任何选项”仍有差距。在UIL中，你可以自定义任何选项，但这并不是必须的，因为通常已经有了默认设置，而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.

简单描述一下这个项目的结构。每一个图片的加载和显示任务都运行在独立的线程中，除非这个图片缓存在内存中，这种情况下图片会立即显示。如果需要的图片缓存在本地，他们会开启一个独立的线程队列。如果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，快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述：

The main actors of the project can relatively be divided:

UIL的主要功能可以分为以下及部分

? 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 ImageLoader manages it all; the maininteraction with the user is performed through it.

最重要的类ImageLoader管理了所有的操作，和用户的交互也主要通过该类。