volley总结

定义：是一个异步网络请求框架和图片加载框架

做用：适合数据量小，通讯频繁的网络操做

优势：一、扩展性强，大多基于接口设计

          二、自动调度网络

          三、高并发网络链接

整体框架：

volley网络请求过程：当请求被添加到请求队列后，volley经过两种调度线程（）不断从请求队列中取出请求，根据是否已缓存调用Cache或Network这两类数据获取接口之一，从缓存中或网络上取得数据。而后交给response delivery去作结果分发及回调处理。

volley请求流程图：

volley主要类关系图：

 RequestQueue：A request dispatch queue with a thread pool of dispatchers.Calling {@link  #add(Request)} will enqueue the given Request for dispatch, resolving from either cache or network on a worker thread, and then delivering a parsed response on the main thread.

RequestQueue 中维护了两个基于优先级的 Request 队列，缓存请求队列和网络请求队列。

CacheDispatcher：Provides a thread for performing cache triage on a queue of requests.Requests added to the specified cache queue are resolved from cache.Any deliverable response is posted back to the caller via a{@link  ResponseDelivery}.  Cache misses and responses that require refresh are enqueued on the specified network queue for processing by a { @link  NetworkDispatcher}.

NetworkDispatcher：Provides a thread for performing network dispatch from a queue of requests.Requests added to the specified queue are processed from the network via a specified {@link  Network} interface. Responses are committed to cache, if eligible, using a specified {@link  Cache} interface. Valid responses and errors are posted back to the caller via a {@link ResponseDelivery}.

Request<T>：Base class for all network requests.@param <T> The type of parsed response this request expects.

ExecutorDelivery：Delivers responses and errors.

DiskBasedCache：Cache implementation that caches files directly onto the hard disk in the specified directory. The default disk usage size is 5MB, but is configurable.