參考連結：
http://www.codekk.com/open-source-project-analysis/detail/Android/grumoon/Volley%20%E6%BA%90%E7%A0%81%E8%A7%A3%E6%9E%90
使用Volley很簡單，過程分為兩步：
1. 建立請求隊列RequestQueue queue = new Volley.newRequestQueue()
2. 建立請求XXRequest，然後将請求加入隊列queue中：queue.add(XXRequest);
一、Volley.newRequestQueue()
建立queue之後隻要将各類請求放入該隊列即可，系統會自動對其進行處理。現在先看一下Volley.newRequestQueue做了些什麼：
public static RequestQueue newRequestQueue(Context context, HttpStack stack) {
File cacheDir = new File(context.getCacheDir(), DEFAULT_CACHE_DIR);
String userAgent = "volley/0";
try {
String packageName = context.getPackageName();
PackageInfo info = context.getPackageManager().getPackageInfo(packageName, 0);
userAgent = packageName + "/" + info.versionCode;
} catch (NameNotFoundException e) {
}
if (stack == null) {
if (Build.VERSION.SDK_INT >= 9) {
stack = new HurlStack();
} else {
// Prior to Gingerbread, HttpUrlConnection was unreliable.
// See: http://android-developers.blogspot.com/2011/09/androids-http-clients.html
stack = new HttpClientStack(AndroidHttpClient.newInstance(userAgent));
}
}
Network network = new BasicNetwork(stack);
RequestQueue queue = new RequestQueue(new DiskBasedCache(cacheDir), network);
queue.start();
return queue;
}
該函數中的核心代碼為：
RequestQueue queue = new RequestQueue(new DiskBasedCache(cacheDir), network);
queue.start();
newRequestQueue中的兩個參數分别用于處理緩存請求和網絡請求。這個一會再講。先看後面一句~queue.start();
public void start() {
stop(); // Make sure any currently running dispatchers are stopped.
// Create the cache dispatcher and start it.
mCacheDispatcher = new CacheDispatcher(mCacheQueue, mNetworkQueue, mCache, mDelivery);
mCacheDispatcher.start();
// Create network dispatchers (and corresponding threads) up to the pool size.
for (int i = 0; i < mDispatchers.length; i++) {
NetworkDispatcher networkDispatcher = new NetworkDispatcher(mNetworkQueue, mNetwork,
mCache, mDelivery);
mDispatchers[i] = networkDispatcher;
networkDispatcher.start();
}
}
從代碼中可看到，queue.start()的執行開啟了其中的mCacheDispatcher和networkDispatcher，也就是負責處理Cache請求及Network請求的線程。
綜上，Volley.newRequestQueue建立了一個請求隊列，并開啟了處理緩存請求的線程mCacheDispatcher和處理網絡請求的線程networkDispatcher。
二、NetworkDispatcher& CacheDispatcher
NetworkDispatcher用于處理網絡請求。其構造函數為：
public NetworkDispatcher(BlockingQueue<Request> queue,
Network network, Cache cache,
ResponseDelivery delivery) {
mQueue = queue;
mNetwork = network;
mCache = cache;
mDelivery = delivery;
}
現在再回過頭看RequestQueue的start函數，其中有~RequestQueue.java/start():
NetworkDispatchernetworkDispatcher = new NetworkDispatcher(mNetworkQueue, mNetwork,mCache,mDelivery);
這裡的mNetworkQueue 是RequestQueue中定義&配置設定的~RequestQueue.java：
private final PriorityBlockingQueue<Request> mNetworkQueue =
new PriorityBlockingQueue<Request>();
這個由Request組成的BlockingQueue用于存放網絡Request。(RequestQueue中還聲明&定義了一個mCacheQueue，和mNetworkQueue類似，用于存放緩存請求)
現在看下NetworkDispatcher的run函數中執行的功能：
public void run() {
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
Request request;
while (true) {
try {
// Take a request from the queue.
request = mQueue.take();
} catch (InterruptedException e) {
// We may have been interrupted because it was time to quit.
if (mQuit) {
return;
}
continue;
}
try {
request.addMarker("network-queue-take");
// If the request was cancelled already, do not perform the
// network request.
if (request.isCanceled()) {
request.finish("network-discard-cancelled");
continue;
}
// Tag the request (if API >= 14)
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.ICE_CREAM_SANDWICH) {
TrafficStats.setThreadStatsTag(request.getTrafficStatsTag());
}
// Perform the network request.
NetworkResponse networkResponse = mNetwork.performRequest(request);
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.
if (networkResponse.notModified && request.hasHadResponseDelivered()) {
request.finish("not-modified");
continue;
}
// Parse the response here on the worker thread.
Response<?> response = request.parseNetworkResponse(networkResponse);
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) {
mCache.put(request.getCacheKey(), response.cacheEntry);
request.addMarker("network-cache-written");
}
// Post the response back.
request.markDelivered();
mDelivery.postResponse(request, response);
} catch (VolleyError volleyError) {
parseAndDeliverNetworkError(request, volleyError);
} catch (Exception e) {
VolleyLog.e(e, "Unhandled exception %s", e.toString());
mDelivery.postError(request, new VolleyError(e));
}
}
}
其循環執行while(true)中的内容，大緻流程為：從mQueue(即RequestQueue中的mNetworkQueue)中取出一個request(注意，這裡mNetworkQueue的類型是PriorityBlockingQueue<Request>，帶優先級的阻塞隊列，即在當mNetworkQueue隊列為空時，本線程為一直阻塞在mQueue.take()這一步，等待直到新的請求到來。)接下來是：if(mQuit) {return; }，當線程執行quit時會将mQuit指派為true，線程退出。這個在這裡不重要，可以先不糾結。之後判斷請求是否被取消，若被取消則重新在mNetworkQueue中取出一個新的Request進行處理；若未被取消，執行NetworkResponse networkResponse = mNetwork.performRequest(request);(之前有段API>=14的判斷，以及一些其他的東西，這裡都先不管，看懂邏輯先)mNetwork是一個Network接口，Network中的performRequest函數負責執行request，并傳回一個NetworkResponse。其原型為(Network.java)：
public NetworkResponse performRequest(Request<?> request) throws VolleyError;
mNetwork在構造函數中被初始化(RequestQueue.start())：
NetworkDispatcher networkDispatcher = new NetworkDispatcher(mNetworkQueue, mNetwork，mCache, mDelivery);
RequestQueue中的mNetwork在其構造函數中被初始化(Volley.newRequestQueue())：
Network network = new BasicNetwork(stack);
RequestQueue queue = new RequestQueue(new DiskBasedCache(cacheDir), network);
接着用request解析傳回的NetworkResponse，傳回一個類型為Response<?>的response。之後判斷是否要緩存至cache。最後執行：
request.markDelivered();//将mResponseDelivered置為true，先忽略掉，重點是下面一句
mDelivery.postResponse(request,response);
将結果傳遞到主線程。
CacheDispatcher用于處理緩存請求，這裡先假設使用無緩存的類NoCache，不分析這個東西
綜上，這裡了解了RequestQueue和NetworkDispatcher：
1. Volley.newRequestQueue執行的大緻工作：初始化了RequestQueue queue，之後執行的queue.start啟動了mCacheDispatcher線程和X個NetworkDispatcher.線程。
2. 簡述了NetworkDispatcher類：本線程啟動後，線上程停止之前，會不斷從網絡請求隊列mQueue中取出Request，并利用接口Network完成該請求。Network執行請求後傳回NetworkResponse，利用request将NetworkResponse解析成Response，最後利用mDelivery傳送結果。類中用到的、未分析的類有：Network、NetworkResponse、Response、mDelivery。留到下次繼續。
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