Volley源码学习3-NetworkDispatcher类

先看看google给的注释:

/**
 * 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}.
 */

在RequestQueue 中构造了此类的对象并且调用了其start方法，下来我们来看看它是怎么执行的。因为继承了Thread，来看一看其run方法 。

 @Override
    public void run() {
        //将线程设置为最高级
        Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
        while (true) { //如果调用RequestQueue的add方法，这里是while(true)的，将会在这马上收到请求。
            //获取系统时间
            long startTimeMs = SystemClock.elapsedRealtime();
            Request<?> request;
            try {
                // Take a request from the queue.从队列中取出请求
                request = mQueue.take();
            } catch (InterruptedException e) {
                // We may have been interrupted because it was time to quit.
                if (mQuit) {
                    return;
                }
                continue;
            }

            try {
                //添加状态标记
                request.addMarker("network-queue-take");

                // If the request was cancelled already, do not perform the
                // network request.
                //如果放弃请求，则销毁请求
                if (request.isCanceled()) {
                    request.finish("network-discard-cancelled");
                    request.notifyListenerResponseNotUsable();
                    continue;
                }
                //????
                addTrafficStatsTag(request);

                // Perform the network request.//请求网络 用mNetwork请求网络得到NetworkResponse
                NetworkResponse networkResponse = mNetwork.performRequest(request);
                //添加状态标记
                request.addMarker("network-http-complete");

                // If the server returned 304 AND we delivered a response already,
                // we're done -- don't deliver a second identical response. 
                //304的状态 以及处理
                if (networkResponse.notModified && request.hasHadResponseDelivered()) {
                    request.finish("not-modified");
                    request.notifyListenerResponseNotUsable();
                    continue;
                }

                // Parse the response here on the worker thread. 在工作线程对响应进行parse
                Response<?> response = request.parseNetworkResponse(networkResponse);
               //添加状态标记
                request.addMarker("network-parse-complete");

                // Write to cache if applicable.
                // TODO: Only update cache metadata instead of entire record for 304s.
                if (request.shouldCache() && response.cacheEntry != null) {
                    //将响应结果存入缓存
                    mCache.put(request.getCacheKey(), response.cacheEntry);
                    request.addMarker("network-cache-written");
                }        
               // Post the response back.
                request.markDelivered();
                //通过mDelivery 将request和response 发送回主线程
                mDelivery.postResponse(request, response);
                request.notifyListenerResponseReceived(response);
            } catch (VolleyError volleyError) {
                //错误处理
                volleyError.setNetworkTimeMs(SystemClock.elapsedRealtime() - startTimeMs);
                parseAndDeliverNetworkError(request, volleyError);
                request.notifyListenerResponseNotUsable();
            } catch (Exception e) {
                //错误处理
                VolleyLog.e(e, "Unhandled exception %s", e.toString());
                VolleyError volleyError = new VolleyError(e);
                volleyError.setNetworkTimeMs(SystemClock.elapsedRealtime() - startTimeMs);
                mDelivery.postError(request, volleyError);
                request.notifyListenerResponseNotUsable();
            }
        }
    }


    private void parseAndDeliverNetworkError(Request<?> request, VolleyError error) {
        error = request.parseNetworkError(error);
        mDelivery.postError(request, error);
    }

这就是NetworkDispatcher 分发的过程，接下来将分析BasicNetwork 类。