Volley——网络请求

　　Volley作为当年Google在2013年的Google I/O上的重点，是一个相当给力的框架。它从设计模式上来说，非常具有扩展性，也比较轻巧。关于Volley的使用，网上介绍的很多了，不再赘述。现在，我将记录我阅读Volley源码的过程，来学习Volley的设计思想和其中的一些小技巧。

　　值的一提的是，新版的gradle已经支持：　　

compile 'com.android.volley:volley:1.0.0'

　　这样导入Volley了。

　　从最简单的例子看起：

RequestQueue queue;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        queue = Volley.newRequestQueue(this);
        final StringRequest request = new StringRequest("https://www.baidu.com/",
                new Response.Listener<String>() {
                    @Override
                    public void onResponse(String response) {
                        ((TextView) findViewById(R.id.test_textview_id)).setText(response);
                    }
                },
                new Response.ErrorListener() {
                    @Override
                    public void onErrorResponse(VolleyError error) {

                    }
                });
        findViewById(R.id.test_btn_id).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                queue.add(request);
            }
        });
    }

　　可以看到，一个简单的Volley的http请求，只需要三步，建立Request队列--->建立Request请求--->将请求加入Request队列。

　　今天就只看这三步是怎么运作的。

　　我们先来看看queue = Volley.newRequestQueue(this);

    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;
    }

　　从体来说这个方法，做了以下几件事：获取userAgent、创建HttpStack、创建NetWork、创建并启动RequestQueue。。

　　获取userAgetnt的方法比较简单 userAgent = packageName + "/" + info.versionCode; 一目了然。

　　接着创建HttpStack和NetWork比较复杂，会单独记录。

　　最后会创建一个RequestQueue,作为请求队列。并且在初始化时，请求队列会直接启动。

　　接下来，我们看一下RequestQueue的代码。我们先从构造方法开始阅读：

    /**
     * Creates the worker pool. Processing will not begin until {@link #start()} is called.
     *
     * @param cache A Cache to use for persisting responses to disk
     * @param network A Network interface for performing HTTP requests
     * @param threadPoolSize Number of network dispatcher threads to create
     * @param delivery A ResponseDelivery interface for posting responses and errors
     */
    public RequestQueue(Cache cache, Network network, int threadPoolSize,
            ResponseDelivery delivery) {
        mCache = cache;
        mNetwork = network;
        mDispatchers = new NetworkDispatcher[threadPoolSize];
        mDelivery = delivery;
    }

　　在初始化时，RequestQueue对4个成员赋了值。缓存、NetWork、网络分发线程队列和网络响应分发接口。　

　　下面我们看看在初始化Volley初始化时，第一个会调用的方法start()

    /**
     * Starts the dispatchers in this queue.
     */
    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();
        }
    }

 　　上面的代码比较简洁。首先调用stop();关于stop()：

　　

    public void stop() {
        if (mCacheDispatcher != null) {
            mCacheDispatcher.quit();
        }
        for (int i = 0; i < mDispatchers.length; i++) {
            if (mDispatchers[i] != null) {
                mDispatchers[i].quit();
            }
        }
    }

　　它就是通过遍历，结束了线程中的所有任务。

　　我们可以在NetworkDispatcher.java中找到相关实现

public void quit() {
    mQuit = true;
    interrupt();
}
public void run() {
        Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
        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;
            }
        ......
        }    
}    

　　继续看start方法：

　　

    /**
     * Starts the dispatchers in this queue.
     */
    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();
        }
    }

　　stop之后，就会启动CacheDispatcher和NetworkDispatcher。

　　关于NetworkDispatcher中start()方法的实现，NetworkDispatcher继承了Thread，所以看start()其实是看run方法：

    @Override
    public void run() {
        Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
        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);
            } catch (VolleyError volleyError) {
                volleyError.setNetworkTimeMs(SystemClock.elapsedRealtime() - startTimeMs);
                parseAndDeliverNetworkError(request, volleyError);
            } catch (Exception e) {
                VolleyLog.e(e, "Unhandled exception %s", e.toString());
                VolleyError volleyError = new VolleyError(e);
                volleyError.setNetworkTimeMs(SystemClock.elapsedRealtime() - startTimeMs);
                mDelivery.postError(request, volleyError);
            }
        }
    }

　　代码比较长，我们分段阅读。

        Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
        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;
            }

　　设置线程优先级为标准后台线程，然后进入死循环。虽然这一段代码没有什么实质的实现，但是却很重要，它基本确立了这个线程的工作模式。这个线程在while(true)的死循环中，每次循环有一次停止线程和一次暂停线程的机会，暂停在前。

　　mQueue是个阻塞型队列。每次循环会在这个队列里取一个请求，如果mQueue为空，取不到请求了，线程就会卡死在这里，这就是暂停的机会，等到mQueue加入请求时，线程又会重新跑起来。通过这样的设计，我们后面发起http请求，只需要往mQueue里加入请求就好了。当调用interrupt方法时，循环就会进入catch分支，然后判断是否真的要取消，所以我们在quit()方法中，看到它的实现是先将mQuit置为true，然后调用interrupt来结束这个线程。

            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);

　　如果请求被取消，则调用请求的finish方法，进入下一轮循环。 然后加入流量标记。我们可以看到addTrafficStatsTag

    @TargetApi(Build.VERSION_CODES.ICE_CREAM_SANDWICH)
    private void addTrafficStatsTag(Request<?> request) {
        // Tag the request (if API >= 14)
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.ICE_CREAM_SANDWICH) {
            TrafficStats.setThreadStatsTag(request.getTrafficStatsTag());
        }
    }

　　我们可以对这个网络请求加上标记，方便调式时知晓它的流量使用情况。

                // 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;
                }

　　接着便了Network执行它的performRequest方法，方法的具体实现，将在后面阅读。通过这个方法，我们处理了网络请求，取得了响应——NetworkResponse。下面的一个判断语句，注释解释得很清楚。如果我们的请求返回304，并且我们已经分发过这个响应了。我们将不再处理响应，结束这次循环，开始下一轮。同时执行request.finish的方法。如果没有304，那么我们还要考虑如何处理响应。

                // 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");
                }

 　　这里，进行解析网络响应，获得响应——Response。根据配置，如果需要缓存，则将响应的内容缓存在mCache中。

                // Post the response back.
                request.markDelivered();
                mDelivery.postResponse(request, response);

　　最后，根据请求，分析网络响应。 

　　至此，我们将volley的网络请求及处理流程基本走通了一遍。后面的文章中，将仔细分析一些Volley对于网络请求的一些处理细节。

Done