非常感謝你能夠堅持看到第四篇，同時這也是這個Volley系列教程的最後一篇了。經過前三節的學習，相信你也已經懂得如何運用Volley提供的Request以及自定義Request了，這一節我將從源碼的角度帶領大家理解Volley的工作流程。 ...
歡迎訪問我的個人博客轉發請註明出處：http://www.wensibo.top/2017/02/17/一口一口吃掉Volley（四）/
非常感謝你能夠堅持看到第四篇，同時這也是這個Volley系列教程的最後一篇了。經過前三節的學習，相信你也已經懂得如何運用Volley提供的Request以及自定義Request了，這一節我將從源碼的角度帶領大家理解Volley的工作流程。
從newRequestQueue()看起
我們都知道，使用Volley最開始要做的就是使用newRequestQueue()獲取一個RequestQueue對象，仔細看一下這個方法
- newRequestQueue()
public static RequestQueue newRequestQueue(Context context, HttpStack stack, int maxDiskCacheBytes) {
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;
if (maxDiskCacheBytes <= -1)
{
// No maximum size specified
queue = new RequestQueue(new DiskBasedCache(cacheDir), network);
}
else
{
// Disk cache size specified
queue = new RequestQueue(new DiskBasedCache(cacheDir, maxDiskCacheBytes), network);
}
queue.start();
return queue;
}在方法內部我們可以看到在api等級大於9的時候，使用HurlStack實例來進行主要的網路請求工作，到這裡已經很明顯了，Volley底層是使用HttpUrlConnection進行的；而對於小於9的API則創建否則就創建一個HttpClientStack的實例，也就是對於9之前的API使用HttpClient進行網路通訊。最後被包裝為一個BasicNetwork對象。
接著根據得到的BasicNetwork對象和一個DiskBasedCache對象（磁碟緩存）來構造一個RequestQueue，並且調用了它的start方法來啟動這個線程。
接著看start()
- 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();
}
}首先先創建CacheDispatcher對象，接著進入for迴圈這個for迴圈遍歷了mCacheDispatcher，這個mCacheDispatcher其實相當於一個線程池，這個線程池的大小預設是4。然後分別讓這裡面的線程運行起來（調用了它們的start方法）。這裡為什麼要有多個線程來處理呢？原因很簡單，因為我們每一個請求都不一定會馬上處理完畢，多個線程進行同時處理的話效率會提高。 所以最終這裡會有5個線程，4個是網路線程NetworkDispatcher，1個是緩存線程CacheDispatcher。
得到了RequestQueue之後，我們只需要構建出相應的Request，然後調用RequestQueue的add()方法將Request傳入就可以完成網路請求操作了，那就先來看一下add()吧！
add()方法
- add()
public <T> Request<T> add(Request<T> request) {
// Tag the request as belonging to this queue and add it to the set of current requests.
request.setRequestQueue(this);
synchronized (mCurrentRequests) {
mCurrentRequests.add(request);
}
// Process requests in the order they are added.
request.setSequence(getSequenceNumber());
request.addMarker("add-to-queue");
// If the request is uncacheable, skip the cache queue and go straight to the network.
if (!request.shouldCache()) {
mNetworkQueue.add(request);
return request;
}
// Insert request into stage if there's already a request with the same cache key in flight.
synchronized (mWaitingRequests) {
String cacheKey = request.getCacheKey();
if (mWaitingRequests.containsKey(cacheKey)) {
// There is already a request in flight. Queue up.
Queue<Request<?>> stagedRequests = mWaitingRequests.get(cacheKey);
if (stagedRequests == null) {
stagedRequests = new LinkedList<Request<?>>();
}
stagedRequests.add(request);
mWaitingRequests.put(cacheKey, stagedRequests);
if (VolleyLog.DEBUG) {
VolleyLog.v("Request for cacheKey=%s is in flight, putting on hold.", cacheKey);
}
} else {
// Insert 'null' queue for this cacheKey, indicating there is now a request in
// flight.
mWaitingRequests.put(cacheKey, null);
mCacheQueue.add(request);
}
return request;
}
}可以看到，在第13行的時候會判斷當前的請求是否可以緩存，如果不能緩存則在第14行直接將這條請求加入網路請求隊列，可以緩存的話則在第36行將這條請求加入緩存隊列。在預設情況下，每條請求都是可以緩存的，當然我們也可以調用Request的setShouldCache(false)方法來改變這一預設行為。
那就先來看看NetworkDispatcher的run()吧!
- run()
@Override
public void run() {
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
Request<?> request;
while (true) {
long startTimeMs = SystemClock.elapsedRealtime();
// release previous request object to avoid leaking request object when mQueue is drained.
request = null;
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);
}
}
}第4行設置了這些線程的優先順序，這個優先順序比較低，目的是為了儘量減少對UI線程的影響保證流暢度。
接著第12行，調用mQueue的take方法取出隊列頭的一個請求進行處理，這個mQueue就是我們在上面add方法中添加進去的一個請求。
直接看到第34行，如果請求沒有被取消，也就是正常的情況下，我們會調用mNetwork的performRequest方法進行請求的處理。不知道你還記的這個mNetwork不，它其實就是我們上面提到的那個由HttpUrlConnection層層包裝的網路請求對象。
如果請求得到了結果，我們會看到55行調用了mDelivery的postResponose方法來回傳我們的請求結果。
先來看performRequest()
因為Network是一個介面，這裡具體的實現是BasicNetwork，所以我們可以看到其中重寫的performRequest()如下：
- performRequest()
@Override
public NetworkResponse performRequest(Request<?> request) throws VolleyError {
long requestStart = SystemClock.elapsedRealtime();
while (true) {
HttpResponse httpResponse = null;
byte[] responseContents = null;
Map<String, String> responseHeaders = Collections.emptyMap();
try {
// Gather headers.
Map<String, String> headers = new HashMap<String, String>();
addCacheHeaders(headers, request.getCacheEntry());
httpResponse = mHttpStack.performRequest(request, headers);
StatusLine statusLine = httpResponse.getStatusLine();
int statusCode = statusLine.getStatusCode();
responseHeaders = convertHeaders(httpResponse.getAllHeaders());
// Handle cache validation.
if (statusCode == HttpStatus.SC_NOT_MODIFIED) {
Entry entry = request.getCacheEntry();
if (entry == null) {
return new NetworkResponse(HttpStatus.SC_NOT_MODIFIED, null,
responseHeaders, true,
SystemClock.elapsedRealtime() - requestStart);
}
// A HTTP 304 response does not have all header fields. We
// have to use the header fields from the cache entry plus
// the new ones from the response.
// http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.3.5
entry.responseHeaders.putAll(responseHeaders);
return new NetworkResponse(HttpStatus.SC_NOT_MODIFIED, entry.data,
entry.responseHeaders, true,
SystemClock.elapsedRealtime() - requestStart);
}
// Handle moved resources
if (statusCode == HttpStatus.SC_MOVED_PERMANENTLY || statusCode == HttpStatus.SC_MOVED_TEMPORARILY) {
String newUrl = responseHeaders.get("Location");
request.setRedirectUrl(newUrl);
}
// Some responses such as 204s do not have content. We must check.
if (httpResponse.getEntity() != null) {
responseContents = entityToBytes(httpResponse.getEntity());
} else {
// Add 0 byte response as a way of honestly representing a
// no-content request.
responseContents = new byte[0];
}
// if the request is slow, log it.
long requestLifetime = SystemClock.elapsedRealtime() - requestStart;
logSlowRequests(requestLifetime, request, responseContents, statusLine);
if (statusCode < 200 || statusCode > 299) {
throw new IOException();
}
return new NetworkResponse(statusCode, responseContents, responseHeaders, false,
SystemClock.elapsedRealtime() - requestStart);
} catch (Exception e) {
···
}
}
}這段代碼中，先10和11行代碼將cache的屬性設置給header，接著第12行調用mHttpStack對象的performRequest方法並傳入請求對象和頭部來進行請求，得到一個HttpResponse對象。
接著將HttpResponse對象中的狀態碼取出，如果值為HttpStatus.SC_NOT_MODIFIED（也就是304），則表示請求得到的Response沒有變化，直接顯示緩存內容。
第45行表示請求成功並且獲取到請求內容，將內容取出並作為一個NetworkResponse對象的屬性並返回給NetworkDispatcher。
在NetworkDispatcher中收到了NetworkResponse這個返回值後又會調用Request的parseNetworkResponse()方法來解析NetworkResponse中的數據，以及將數據寫入到緩存，這個方法的實現是交給Request的子類來完成的，因為不同種類的Request解析的方式也肯定不同，這就是為什麼我們在自定義Request的時候必須要重寫parseNetworkResponse()這個方法的原因了。
在解析完了NetworkResponse中的數據之後，又會調用ExecutorDelivery的postResponse()方法來回調解析出的數據。
接著是postResponse()
- postResponse()
@Override
public void postResponse(Request<?> request, Response<?> response, Runnable runnable) {
request.markDelivered();
request.addMarker("post-response");
mResponsePoster.execute(new ResponseDeliveryRunnable(request, response, runnable));
}這裡看到第5行調用了mResponsePoster的execute方法並傳入了一個ResponseDeliveryRunnable對象，再看mResponsePoster的定義：
public ExecutorDelivery(final Handler handler) {
// Make an Executor that just wraps the handler.
mResponsePoster = new Executor() {
@Override
public void execute(Runnable command) {
handler.post(command);
}
};
}也就是我們在這裡把ResponseDeliveryRunnable對象通過Handler的post方法發送出去了。這裡為什麼要發送到MainLooper中？因為RequestQueue是在子線程中執行的，回調到的代碼也是在子線程中的，如果在回調中修改UI，就會報錯。再者，為什麼要使用post方法？原因也很簡單，因為我們在消息發出之後再進行回調，post方法允許我們傳入一個Runnable的實現類，post成功會自動執行它的run方法，這個時候在run方法中進行結果的判斷並且進行回調：
- run()
@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();
}
}可以看到，11行是調用Request的deleverResponse方法將結果回調給Request。舉例看一下StringRequest中該方法是如何實現的：
- deliverResponse()
@Override
protected void deliverResponse(String response) {
if (mListener != null) {
mListener.onResponse(response);
}
}直接通過我們構造StringRequest時傳進來的Listener的回調方法onResponse來將結果回調給Activity。deleverError也是同樣的做法。
看完網路線程NetworkDispatcher之後再來看一下緩存線程CacheDispatcher是如何工作的
最後來看CacheDispatcher的run()方法
- run()
@Override
public void run() {
if (DEBUG) VolleyLog.v("start new dispatcher");
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
// Make a blocking call to initialize the cache.
mCache.initialize();
Request<?> request;
while (true) {
// release previous request object to avoid leaking request object when mQueue is drained.
request = null;
try {
// Take a request from the queue.
request = mCacheQueue.take();
} catch (InterruptedException e) {
// We may have been interrupted because it was time to quit.
if (mQuit) {
return;
}
continue;
}
try {
request.addMarker("cache-queue-take");
// If the request has been canceled, don't bother dispatching it.
if (request.isCanceled()) {
request.finish("cache-discard-canceled");
continue;
}
// Attempt to retrieve this item from cache.
Cache.Entry entry = mCache.get(request.getCacheKey());
if (entry == null) {
request.addMarker("cache-miss");
// Cache miss; send off to the network dispatcher.
mNetworkQueue.put(request);
continue;
}
// If it is completely expired, just send it to the network.
if (entry.isExpired()) {
request.addMarker("cache-hit-expired");
request.setCacheEntry(entry);
mNetworkQueue.put(request);
continue;
}
// We have a cache hit; parse its data for delivery back to the request.
request.addMarker("cache-hit");
Response<?> response = request.parseNetworkResponse(
new NetworkResponse(entry.data, entry.responseHeaders));
request.addMarker("cache-hit-parsed");
if (!entry.refreshNeeded()) {
// Completely unexpired cache hit. Just deliver the response.
mDelivery.postResponse(request, response);
} else {
// Soft-expired cache hit. We can deliver the cached response,
// but we need to also send the request to the network for
// refreshing.
request.addMarker("cache-hit-refresh-needed");
request.setCacheEntry(entry);
// Mark the response as intermediate.
response.intermediate = true;
// Post the intermediate response back to the user and have
// the delivery then forward the request along to the network.
final Request<?> finalRequest = request;
mDelivery.postResponse(request, response, new Runnable() {
@Override
public void run() {
try {
mNetworkQueue.put(finalRequest);
} catch (InterruptedException e) {
// Not much we can do about this.
}
}
});
}
} catch (Exception e) {
VolleyLog.e(e, "Unhandled exception %s", e.toString());
}
}
}首先在10行可以看到一個while(true)迴圈，說明緩存線程始終是在運行的，
接著在第33行會嘗試從緩存當中取出響應結果，如何為空的話則把這條請求加入到網路請求隊列中，如果不為空的話再判斷該緩存是否已過期，如果已經過期了則同樣把這條請求加入到網路請求隊列中，否則就認為不需要重髮網絡請求，直接使用緩存中的數據即可。
之後會在第39行調用Request的parseNetworkResponse()方法來對數據進行解析，再往後就是將解析出來的數據進行回調了，跟上面的回掉思路是完全一樣的！
至此，我們可以通過通過Volley官方提供的流程圖重新回顧一下整個的流程
其中藍色部分代表主線程，綠色部分代表緩存線程，橙色部分代表網路線程。我們在主線程中調用RequestQueue的add()方法來添加一條網路請求，這條請求會先被加入到緩存隊列當中，如果發現可以找到相應的緩存結果就直接讀取緩存並解析，然後回調給主線程。如果在緩存中沒有找到結果，則將這條請求加入到網路請求隊列中，然後處理髮送HTTP請求，解析響應結果，寫入緩存，並回調主線程。
希望通過這個系列的文章你能夠清晰的掌握和理解Volley，儘管他現在已經不流行了，接下來我會持續為大家講解比較好的開源框架，TX
