前面几篇通过源码分析了Volley是怎样进行请求调度及请求是如何被实际执行的，这篇最后来看下请求结果是如何交付给请求者的（一般是Android的UI主线程）。
类图：
请求结果的交付是通过ResponseDelivery接口完成的，它有一个实现类ExecutorDelivery, 主要有postResponse()与postError()两个方法，分别在请求成功或失败时将结果提交给请求发起者。
1. 首先，在NetworkDispatcher的run()方法中，当服务器返回响应并解析完后，会调用mDelivery.postResponse(request, response);来提交请求响应。
1 @Override
2 public void run() {
3 Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
4 Request<?> request;
5 while (true) {
6 try {
7 // Take a request from the queue.
8 request = mQueue.take();
9 } catch (InterruptedException e) {
10 // We may have been interrupted because it was time to quit.
11 if (mQuit) {
12 return;
13 }
14 continue;
15 }
16
17 try {
18 request.addMarker("network-queue-take");
19
20 // If the request was cancelled already, do not perform the
21 // network request.
22 if (request.isCanceled()) {
23 request.finish("network-discard-cancelled");
24 continue;
25 }
26
27 addTrafficStatsTag(request);
28
29 // Perform the network request.
30 NetworkResponse networkResponse = mNetwork.performRequest(request);
31 request.addMarker("network-http-complete");
32
33 // If the server returned 304 AND we delivered a response already,
34 // we‘re done -- don‘t deliver a second identical response.
35 if (networkResponse.notModified && request.hasHadResponseDelivered()) {
36 request.finish("not-modified");
37 continue;
38 }
39
40 // Parse the response here on the worker thread.
41 Response<?> response = request.parseNetworkResponse(networkResponse);
42 request.addMarker("network-parse-complete");
43
44 // Write to cache if applicable.
45 // TODO: Only update cache metadata instead of entire record for 304s.
46 if (request.shouldCache() && response.cacheEntry != null) {
47 mCache.put(request.getCacheKey(), response.cacheEntry);
48 request.addMarker("network-cache-written");
49 }
50
51 // Post the response back.
52 request.markDelivered();
53 mDelivery.postResponse(request, response);
54 } catch (VolleyError volleyError) {
55 parseAndDeliverNetworkError(request, volleyError);
56 } catch (Exception e) {
57 VolleyLog.e(e, "Unhandled exception %s", e.toString());
58 mDelivery.postError(request, new VolleyError(e));
59 }
60 }
61 }
2. 看ExecutorDelivery中postResponse()方法的具体实现。其中mResponsePoster是一个Executor。每post一个response，都会调用ResponseDeliveryRunnable的run()方法。在这个run()方法中，会通过mRequest.deliverResponse(mResponse.result)来传递response的result，这个result其实就是已经解析好的响应结果，比如一个表示处理结果的字符串或一个User对象。
1 @Override
2 public void postResponse(Request<?> request, Response<?> response) {
3 postResponse(request, response, null);
4 }
5
6 @Override
7 public void postResponse(Request<?> request, Response<?> response, Runnable runnable) {
8 request.markDelivered();
9 request.addMarker("post-response");
10 mResponsePoster.execute(new ResponseDeliveryRunnable(request, response, runnable));
11 }
12
13 /**
14 * A Runnable used for delivering network responses to a listener on the
15 * main thread.
16 */
17 @SuppressWarnings("rawtypes")
18 private class ResponseDeliveryRunnable implements Runnable {
19 private final Request mRequest;
20 private final Response mResponse;
21 private final Runnable mRunnable;
22
23 public ResponseDeliveryRunnable(Request request, Response response, Runnable runnable) {
24 mRequest = request;
25 mResponse = response;
26 mRunnable = runnable;
27 }
28
29 @SuppressWarnings("unchecked")
30 @Override
31 public void run() {
32 // If this request has canceled, finish it and don‘t deliver.
33 if (mRequest.isCanceled()) {
34 mRequest.finish("canceled-at-delivery");
35 return;
36 }
37
38 // Deliver a normal response or error, depending.
39 if (mResponse.isSuccess()) {
40 mRequest.deliverResponse(mResponse.result);
41 } else {
42 mRequest.deliverError(mResponse.error);
43 }
44
45 // If this is an intermediate response, add a marker, otherwise we‘re done
46 // and the request can be finished.
47 if (mResponse.intermediate) {
48 mRequest.addMarker("intermediate-response");
49 } else {
50 mRequest.finish("done");
51 }
52
53 // If we have been provided a post-delivery runnable, run it.
54 if (mRunnable != null) {
55 mRunnable.run();
56 }
57 }
58 }
3. 既然是通过Request的deliverResponse()来传递响应结果，就来看下这个方法， 第二篇中已经知道这个方法是个抽象函数，由它子类来实现。以第一篇中的MyGsonRequest为例，其实现很简单，就是调用了mListener的onResponse方法。
1 @Override
2 protected void deliverResponse(T response) {
3 mListener.onResponse(response);
4 }
这个mListener就是在主线程实例化MyGsonRequest的时候，传过来的一个Response.Listener<T>实例，这是MyGsonRequest的构造函数：
1 public MyGsonRequest(int method
2 , String url
3 , Object requestBody
4 , Class<T> responseClass
5 , Listener<T> listener
6 , ErrorListener errorListener) {
7
8 super(method, url, errorListener);
9 this.mRequestBody = requestBody;
10 this.mResponseClass = responseClass;
11 this.mListener = listener;
12 mGson = new Gson();
13
14 }
这里mListener也就是第一篇中在主线程中通过createRegisterSuccessListener函数返回的监听器实例，如下代码所示。 所以最终会调到这里的onResponse()方法，来做一些更新UI或提示用户请求成功之类的操作。请求失败时，响应错误结果的提交与之类似。这样，Volley就完成了响应结果的交付。
1 private Listener<String> createRegisterSuccessListener() {
2 return new Listener<String>() {
3 @Override
4 public void onResponse(String response) {
5 if (mProgressDialog != null) {
6 mProgressDialog.dismiss();
7 }
8 Toast.makeText(
9 RegisterActivity.this,
10 getString(R.string.msg_register_success),
11 Toast.LENGTH_SHORT).show();
12
13 }
14 };
15 }
这里还有一个问题， 因为更新UI的操作只能在主线程中进行，那么ResponseDeliveryRunnable的run()方法不能再新起一个线程来执行，而应该在主线程中执行，这个是如何做到的？
其实还是用的Handler,Looper，MessageQueue的那套机制。 在Volley初始化一个RequestQueue的时候，会调用RequestQueue的如下构造函数，它构建了一个ExecutorDelivery对象，并把一个与主线程的Looper关联的一个Handler，
1 public RequestQueue(Cache cache, Network network, int threadPoolSize) {
2 this(cache, network, threadPoolSize,
3 new ExecutorDelivery(new Handler(Looper.getMainLooper())));
4 }
然后再看下ExecutorDelivery的构造方法， 通过handler的post方法，把ResponseDeliveryRunnable 这个runnable加到了主线程的消息队列中，所以它的run()方法是在主线程中执行的。
1 public ExecutorDelivery(final Handler handler) {
2 // Make an Executor that just wraps the handler.
3 mResponsePoster = new Executor() {
4 @Override
5 public void execute(Runnable command) {
6 handler.post(command);
7 }
8 };
9 }
到这里，Volley的源码基本上看完了。 还有一些诸如NetworkImageView控件（这个挺好用，直接可以取代ImageView），以及Cache的一些实现，有时间再具体看看。
[Android]Volley源码分析（五）