Volley源码学习（三）：线程处理request,类CacheDispatcher，NetworkDispatcher,ExecutorDelivery

一.源码解析

volley处理request的流程：当调用RequestQueue的add(request)方法时，会进行一系列逻辑判断：如先从cache获取，没有或者过期，再从网络获取。这是通过将request添加到CacheQueue和NetworkQueue里，最终是由线程类CacheDispatcher，NetworkDispatcher进行相应处理的。

1.类CacheDispather，缓存请求处理线程类，在RequestQueue的start方法里启动
线程类，一直循环读取request,如果cache里没有或过期，然后添加request到networkQueue里从网络获取→有，将cacheentry变成networkresponse，并用request解析成response→不需更新就直接用mDelivery.postResponse(request, response);，需更新就之后再从网络获取

public CacheDispatcher(
BlockingQueue<Request> cacheQueue, BlockingQueue<Request> networkQueue,
Cache cache, ResponseDelivery delivery) {
mCacheQueue = cacheQueue;      //读取request,进行cache的操作
mNetworkQueue = networkQueue;    //将request add到networkqueue里进行网络请求
mCache = cache;            //cache缓存，这里使用DiskBasedCache(cacheDir)，将文件缓存到本地
mDelivery = delivery;      //用于将respone发送给request里的listener的方法
}

@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();    //缓存初始化

while (true) {
try {
// Get a request from the cache triage queue, blocking until
// at least one is available.
final Request request = mCacheQueue.take();       //从CacheQueue里获得request,没有就阻塞
request.addMarker("cache-queue-take");

// If the request has been canceled, don't bother dispatching it.
if (request.isCanceled()) {           //如果request被取消，就finish
request.finish("cache-discard-canceled");
continue;
}

// Attempt to retrieve this item from cache.
//没有缓存
Cache.Entry entry = mCache.get(request.getCacheKey());    //去缓存里获取
if (entry == null) {
request.addMarker("cache-miss");
// Cache miss; send off to the network dispatcher.
mNetworkQueue.put(request);             //没有就去网络获取
continue;
}

// If it is completely expired, just send it to the network.
//缓存过期了
if (entry.isExpired()) {           //cache过期，去网络请求
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(       //调用request方法解析networkResponse为response
new NetworkResponse(entry.data, entry.responseHeaders)); <span style="font-family: Arial, Helvetica, sans-serif;">//将cacheentry变成networkResponse</span>
request.addMarker("cache-hit-parsed");

if (!entry.refreshNeeded()) {                 //不需要更新就自己delivery response到对应request的listener
// 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.
mDelivery.postResponse(request, response, new Runnable() {     //postResponse后再从网络更新Cache
@Override
public void run() {
try {
mNetworkQueue.put(request);
} catch (InterruptedException e) {
// Not much we can do about this.
}
}
});
}

} catch (InterruptedException e) {
// We may have been interrupted because it was time to quit.
if (mQuit) {
return;
}
continue;
}
}
}

2.类NetworkDispatcher，线程处理网络请求
从queue里获取request→先判定是否取消，然后finish(request)→网络请求response = request.parseNetworkResponse(networkResponse);→
response = request.parseNetworkResponse(networkResponse);解析成response→需要缓存就调用mCache的put方法→mDelivery.postResponse(request, response);发送结果→异常时发送错误

public NetworkDispatcher(BlockingQueue<Request> queue,
Network network, Cache cache,
ResponseDelivery delivery) {
mQueue = queue;
mNetwork = network;
mCache = cache;
mDelivery = delivery;
}<pre name="code" class="html">    @Override
public void run() {
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
Request request;
while (true) {
try {
// Take a request from the queue.
request = mQueue.take();     //获取request,没有就阻塞
} catch (InterruptedException e) {
// We may have been interrupted because it was time to quit.
if (mQuit) {     //结束线程
return;
}
continue;
}

try {
request.addMarker("network-queue-take");

// If the request was cancelled already, do not perform the
// network request.
if (request.isCanceled()) {                  //取消request
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);       //获得networkResponse
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()) {           //如果没修改且已经发送repsonse，结束request
request.finish("not-modified");
continue;
}

// Parse the response here on the worker thread.
Response<?> response = request.parseNetworkResponse(networkResponse);    //解析成response
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) {              //保存到缓存里
mCache.put(request.getCacheKey(), response.cacheEntry);
request.addMarker("network-cache-written");
}

// Post the response back.
request.markDelivered();
mDelivery.postResponse(request, response);                   //将response结果post到requst的listener里，实际转接
} catch (VolleyError volleyError) {
parseAndDeliverNetworkError(request, volleyError);       //异常时发送错误
} catch (Exception e) {
VolleyLog.e(e, "Unhandled exception %s", e.toString());
mDelivery.postError(request, new VolleyError(e));
}
}
}

3.类ExecutorDelivery，用于发送response或error到request的listener

使用主线程的looper来处理线程。调用postResponse方法时处理一次线程，将response和error传给requset

public ExecutorDelivery(final Handler handler) {   //handler使用主线程Looper.getMainLooper()
// Make an Executor that just wraps the handler.
mResponsePoster = new Executor() {
@Override
public void execute(Runnable command) {
handler.post(command);
}
};
}    @Override
    public void postResponse(Request<?> request, Response<?> response, Runnable runnable) {
        request.markDelivered();
        request.addMarker("post-response");
        mResponsePoster.execute(new ResponseDeliveryRunnable(request, response, runnable));
    }<pre name="code" class="html">    @Override
public void postError(Request<?> request, VolleyError error) {
request.addMarker("post-error");
Response<?> response = Response.error(error);
mResponsePoster.execute(new ResponseDeliveryRunnable(request, response, null));
}

@SuppressWarnings("rawtypes")
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()) {      //取消就结束
mRequest.finish("canceled-at-delivery");
return;
}

// Deliver a normal response or error, depending.
if (mResponse.isSuccess()) {             //成功就调用request的方法，获得reponse.data
mRequest.deliverResponse(mResponse.result);
} else {
mRequest.deliverError(mResponse.error);       //失败，就调用request的方法获得response.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();
}
}
}