volley總結

定義：是一個異步網絡請求框架和圖片加載框架

做用：適合數據量小，通訊頻繁的網絡操做

優勢：一、擴展性強，大多基於接口設計

          二、自動調度網絡

          三、高併發網絡鏈接

整體框架：

volley網絡請求過程：當請求被添加到請求隊列後，volley經過兩種調度線程（）不斷從請求隊列中取出請求，根據是否已緩存調用Cache或Network這兩類數據獲取接口之一，從緩存中或網絡上取得數據。而後交給response delivery去作結果分發及回調處理。

volley請求流程圖：

volley主要類關係圖：

 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 中維護了兩個基於優先級的 Request 隊列，緩存請求隊列和網絡請求隊列。

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.