这篇文章主要分析网络请求和结果交付的过程。
NetWork工作原理
之前已经说到通过mNetWork.performRequest()方法来得到NetResponse，看一下该方法具体的执行流程,performRequest是一个接口方法，真正实现该方法以及被调用的是BasicNetWork,其具体的performRequest代码如下：
@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);
}
// 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 (SocketTimeoutException e) {
attemptRetryOnException("socket", request, new TimeoutError());
} catch (ConnectTimeoutException e) {
attemptRetryOnException("connection", request, new TimeoutError());
} catch (MalformedURLException e) {
throw new RuntimeException("Bad URL " + request.getUrl(), e);
} catch (IOException e) {
int statusCode;
if (httpResponse != null) {
statusCode = httpResponse.getStatusLine().getStatusCode();
} else {
throw new NoConnectionError(e);
}
VolleyLog.e("Unexpected response code %d for %s", statusCode, request.getUrl());
NetworkResponse networkResponse;
if (responseContents != null) {
networkResponse = new NetworkResponse(statusCode, responseContents,
responseHeaders, false, SystemClock.elapsedRealtime() - requestStart);
if (statusCode == HttpStatus.SC_UNAUTHORIZED ||
statusCode == HttpStatus.SC_FORBIDDEN) {
attemptRetryOnException("auth",
request, new AuthFailureError(networkResponse));
} else if (statusCode >= 400 && statusCode <= 499) {
// Don't retry other client errors.
throw new ClientError(networkResponse);
} else if (statusCode >= 500 && statusCode <= 599) {
if (request.shouldRetryServerErrors()) {
attemptRetryOnException("server",
request, new ServerError(networkResponse));
} else {
throw new ServerError(networkResponse);
}
} else {
// 3xx? No reason to retry.
throw new ServerError(networkResponse);
}
} else {
attemptRetryOnException("network", request, new NetworkError());
}
}
}
}
上述代码可以看出，真正进行网络请求还是HttpStack.performRequest，在请求结束以后，对返回的状态码进行封装，这里主要说一下304状态码，该状态码，由于只返回header,不返回body，因此body需要从以前缓存的entity中取出body。而HttpStack的两个实现类一个是HttpClientStack,另一个是HurlStack，他们也是直接调用Java自带的http请求方法来得到response。
下面要看的是NetWorkResponse的封装
public NetworkResponse(int statusCode, byte[] data, Map<String, String> headers,
boolean notModified, long networkTimeMs) {
this.statusCode = statusCode;
this.data = data;
this.headers = headers;
this.notModified = notModified;
this.networkTimeMs = networkTimeMs;
}
这就是全部的NetWorkResponse的封装，封装的目的是为了对response进行交付。
ResponseDelivery工作原理
ResponseDelivery是一个接口，其定义了3个方法，2个传递response的重载方法与一个传递error方法
/**
* Parses a response from the network or cache and delivers it.
*/
void postResponse(Request<?> request, Response<?> response);
/**
* Parses a response from the network or cache and delivers it. The provided
* Runnable will be executed after delivery.
*/
void postResponse(Request<?> request, Response<?> response, Runnable runnable);
/**
* Posts an error for the given request.
*/
void postError(Request<?> request, VolleyError error);
其实现类ExecutorDelivery是真正执行的功能类。其内部非常简单，代码如下：
public class ExecutorDelivery implements ResponseDelivery {
/** Used for posting responses, typically to the main thread. */
private final Executor mResponsePoster;
/**
* Creates a new response delivery interface.
* @param handler {@link Handler} to post responses on
*/
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);
}
};
}
/**
* Creates a new response delivery interface, mockable version
* for testing.
* @param executor For running delivery tasks
*/
public ExecutorDelivery(Executor executor) {
mResponsePoster = executor;
}
@Override
public void postResponse(Request<?> request, Response<?> response) {
postResponse(request, response, null);
}
@Override
public void postResponse(Request<?> request, Response<?> response, Runnable runnable) {
request.markDelivered();
request.addMarker("post-response");
mResponsePoster.execute(new ResponseDeliveryRunnable(request, response, runnable));
}
@Override
public void postError(Request<?> request, VolleyError error) {
request.addMarker("post-error");
Response<?> response = Response.error(error);
mResponsePoster.execute(new ResponseDeliveryRunnable(request, response, null));
}
/**
* A Runnable used for delivering network responses to a listener on the
* main thread.
*/
@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()) {
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();
}
}
}
}
其构造方法唯一目的就是实例化了Excutor类型的mResponsePoster，Volley默认调用的是第一个构造方法，也就是包装了一个Handler.该类最主要的方法是postResponse，其核心的实现
mResponsePoster.execute(new ResponseDeliveryRunnable(request, response, runnable));
执行的是ResponseDeliveryRunnable的run方法,而run方法中是通过
mRequest.deliverResponse(mResponse.result);
而Request的该方法则是由有其具体的实现类 StringRequest,JsonRequest,ImageRequest实现，其实现都一致，则是
@Override
protected void deliverResponse(T response) {
mListener.onResponse(response);
}
调用回调方法。
因此，整个分发过程就清晰了，通过Dispatcher进行处理的每个Request,对request进行处理完，则通过ResponseDelivery进行交付，其交付通过得到一个UI线程的handler，通过该Handler的post,调用Request类的delivery方法，在该方法中，又会调用我们构造request时，传入的回调方法。