volley翅溺，應該是Android開源框架中最簡單的框架了饵史，同時它也是最具有代表性的框架。如果你吃透了它嘴拢，那么你看其他的開源框架也會容易很多桩盲。
先上圖：

這里寫圖片描述

從上圖中可以看出，Volley創(chuàng)建了RequestQueue席吴，RequestQueue執(zhí)行Cache和NetWork兩個轉(zhuǎn)發(fā)器赌结，
最終執(zhí)行ExecutorDelivery

下面一步一步開始分析
1.Volley執(zhí)行newRequestQueue

public static RequestQueue newRequestQueue(Context context, HttpStack stack) {
        File cacheDir = new File(context.getCacheDir(), DEFAULT_CACHE_DIR);
        .......
        // 根據(jù)版本不同之行不同當網(wǎng)絡請求
        if (stack == null) {
            if (Build.VERSION.SDK_INT >= 9) {
                stack = new HurlStack();
            } else {
                stack = new HttpClientStack(AndroidHttpClient.newInstance(userAgent));
            }
        }

        Network network = new BasicNetwork(stack);
        RequestQueue queue = new RequestQueue(new DiskBasedCache(cacheDir), network);
        queue.start();
        return queue;
    }

RequestQueue里持有兩個隊列：一個是緩存隊列，一個是網(wǎng)絡隊列

    /** The cache triage queue. */
    private final PriorityBlockingQueue<Request<?>> mCacheQueue =
        new PriorityBlockingQueue<Request<?>>();

    /** The queue of requests that are actually going out to the network. */
    private final PriorityBlockingQueue<Request<?>> mNetworkQueue =
        new PriorityBlockingQueue<Request<?>>();

Volley調(diào)用了start方法孝冒，創(chuàng)建了者兩個隊列柬姚，并同時執(zhí)行

    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();
        }
    }

我們先看緩存轉(zhuǎn)發(fā)器里主要做了什么

    //首先它是繼承Thread
    CacheDispatcher extends Thread

在run方法里執(zhí)行了：

一直在循環(huán)
    while (true) {
            try {
                // Get a request from the cache triage queue, blocking until
                // 不停的從Cache隊列里取出請求
                final Request<?> request = mCacheQueue.take();
                Cache.Entry entry = mCache.get(request.getCacheKey());
                // 如果從緩存里取不到，則把他放到NetWork隊列
                if (entry == null) {
                    request.addMarker("cache-miss");
                    mNetworkQueue.put(request);
                    continue;
                }

                // 如果從緩存里取出則返回
                request.addMarker("cache-hit");
                Response<?> response = request.parseNetworkResponse(
                        new NetworkResponse(entry.data, entry.responseHeaders));
                request.addMarker("cache-hit-parsed");

                if (!entry.refreshNeeded()) {
                    mDelivery.postResponse(request, response);
                } else {
                    request.addMarker("cache-hit-refresh-needed");
                    request.setCacheEntry(entry);

                    // Mark the response as intermediate.
                    mDelivery.postResponse(request, response, new Runnable() {
                        @Override
                        public void run() {
                            try {
                                mNetworkQueue.put(request);
                            } catch (InterruptedException e) {
                                // Not much we can do about this.
                            }
                        }
                    });
                }

NetWorkDispatch,實際就是發(fā)起請求返回結(jié)果

        while (true) {
            long startTimeMs = SystemClock.elapsedRealtime();
            Request<?> request;
            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;
                }

                addTrafficStatsTag(request);

                // 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);

從上面兩個Dispatcher中最后都由mDelivery. postResponse
那么它是什么尼庄涡？

mDelivery ＝ new ExecutorDelivery(new Handler(Looper.getMainLooper()))

        mResponsePoster.execute(new ResponseDeliveryRunnable(request, response, runnable));
    private class ResponseDeliveryRunnable implements Runnable {
        private final Request mRequest;
        private final Response mResponse;
        private final Runnable mRunnable;

        public ResponseDeliveryRunnable(Request request, Response response, Runnable runnable) {
            mRequest = request;
            mResponse = response;
            mRunnable = runnable;
        }

        @SuppressWarnings("unchecked")
        @Override
        public void run() {
            // If this request has canceled, finish it and don't deliver.
            if (mRequest.isCanceled()) {
                mRequest.finish("canceled-at-delivery");
                return;
            }

            // Deliver a normal response or error, depending.
            if (mResponse.isSuccess()) {
                mRequest.deliverResponse(mResponse.result);
            } else {
                mRequest.deliverError(mResponse.error);
            }

            // If this is an intermediate response, add a marker, otherwise we're done
            // and the request can be finished.
            if (mResponse.intermediate) {
                mRequest.addMarker("intermediate-response");
            } else {
                mRequest.finish("done");
            }

            // If we have been provided a post-delivery runnable, run it.
            if (mRunnable != null) {
                mRunnable.run();
            }
       }
    }