Android進階-Volley-2.RequestQueue&NetworkDispatcher

Android進階-Volley-2.RequestQueue&NetworkDispatcher

 

 

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，然後將請求排入佇列queue中：queue.add(XXRequest);

一、Volley.newRequestQueue()

建立queue之後只要將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中的兩個參數分別用於處理緩衝請求和網路請求。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，並開啟了處理緩衝請求的線程mCacheDispatcher和處理網路請求的線程networkDispatcher。

 

二、NetworkDispatcher& CacheDispatcher

NetworkDispatcher用於處理網路請求。其建構函式為：

public NetworkDispatcher(BlockingQueue 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&分配的~RequestQueue.java：

private final PriorityBlockingQueue mNetworkQueue =        new PriorityBlockingQueue();

這個由Request組成的BlockingQueue用於存放網路Request。(RequestQueue中還聲明&定義了一個mCacheQueue，和mNetworkQueue類似，用於存放緩衝請求)

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));            }        }}

其迴圈執行while(true)中的內容，大致流程為：從mQueue(即RequestQueue中的mNetworkQueue)中取出一個request(注意，這裡mNetworkQueue的類型是PriorityBlockingQueue，帶優先順序的阻塞隊列，即在當mNetworkQueue隊列為空白時，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時會將mQuit賦值為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介面，Network中的performRequest函數負責執行request，並返回一個NetworkResponse。其原型為(Network.java)：

public NetworkResponse performRequest(Request request) throws VolleyError;

mNetwork在建構函式中被初始化(RequestQueue.start())：

NetworkDispatcher networkDispatcher = new NetworkDispatcher(mNetworkQueue, mNetwork，mCache, mDelivery);

RequestQueue中的mNetwork在其建構函式中被初始化(Volley.newRequestQueue())：

Network network = new BasicNetwork(stack);RequestQueue queue = new RequestQueue(new DiskBasedCache(cacheDir), network);

接著用request解析返回的NetworkResponse，返回一個類型為Response的response。之後判斷是否要緩衝至cache。最後執行：

request.markDelivered();//將mResponseDelivered置為true，先忽略掉，重點是下面一句

mDelivery.postResponse(request,response);

將結果傳遞到主線程。

CacheDispatcher用於處理緩衝請求，這裡先假設使用無緩衝的類NoCache，不分析這個東西

 

 

 

綜上，這裡瞭解了RequestQueue和NetworkDispatcher：

1. Volley.newRequestQueue執行的大致工作：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，會不斷從網路請求隊列mQueue中取出Request，並利用介面Network完成該請求。Network執行請求後返回NetworkResponse，利用request將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。