[Android]Volley源碼分析(三)Network

如果各位看官仔細看過我之前的文章,實際上Network這塊的只是點小功能的補充。我們來看下NetworkDispatcher的核心處理邏輯:

while (true) {
            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) {
                parseAndDeliverNetworkError(request, volleyError);
            } catch (Exception e) {
                VolleyLog.e(e, "Unhandled exception %s", e.toString());
                mDelivery.postError(request, new VolleyError(e));
            }
        }

我們看到NetworkDispatcher和CacheDispatcher的區別在於

                addTrafficStatsTag(request);

                // Perform the network request.
                NetworkResponse networkResponse = mNetwork.performRequest(request);
                request.addMarker("network-http-complete");

addTrafficStatsTag(request);方法用來優化網絡通道,並不屬於我們聊的重點。我們知道mNetwork通過HttpStack來實現網絡請求，我們先來看一下HurlStack。

@Override
    public HttpResponse performRequest(Request request, Map additionalHeaders)
            throws IOException, AuthFailureError {
        String url = request.getUrl();
        HashMap map = new HashMap();
        map.putAll(request.getHeaders());
        map.putAll(additionalHeaders);
        if (mUrlRewriter != null) {
            String rewritten = mUrlRewriter.rewriteUrl(url);
            if (rewritten == null) {
                throw new IOException("URL blocked by rewriter: " + url);
            }
            url = rewritten;
        }
        URL parsedUrl = new URL(url);
        HttpURLConnection connection = openConnection(parsedUrl, request);
        for (String headerName : map.keySet()) {
            connection.addRequestProperty(headerName, map.get(headerName));
        }
        setConnectionParametersForRequest(connection, request);
        // Initialize HttpResponse with data from the HttpURLConnection.
        ProtocolVersion protocolVersion = new ProtocolVersion("HTTP", 1, 1);
        int responseCode = connection.getResponseCode();
        if (responseCode == -1) {
            // -1 is returned by getResponseCode() if the response code could not be retrieved.
            // Signal to the caller that something was wrong with the connection.
            throw new IOException("Could not retrieve response code from HttpUrlConnection.");
        }
        StatusLine responseStatus = new BasicStatusLine(protocolVersion,
                connection.getResponseCode(), connection.getResponseMessage());
        BasicHttpResponse response = new BasicHttpResponse(responseStatus);
        response.setEntity(entityFromConnection(connection));
        for (Entry> header : connection.getHeaderFields().entrySet()) {
            if (header.getKey() != null) {
                Header h = new BasicHeader(header.getKey(), header.getValue().get(0));
                response.addHeader(h);
            }
        }
        return response;
    }

HurlStack是通過HttpUrlConnection的方式來請求數據。同時,它也聽過了接口 UrlRewriter mUrlRewriter;來實現url的重定向。HurlStack返回了HttpResponse給上層NetworkDispatcher.HttpClientStack大同小異我們就不看了。當然我們也看出了CacheDispatcher和NetworkDispatcher代碼上的不足,但是都是些小瑕疵。因為對於Request.isCancel這些方面的判斷完全可以采用模板方法的方式來統一。

NetworkDispatcher通過Network取得HttpResponse之後依舊跟CacheDispacher一樣用Request進行包裝:

Response response = request.parseNetworkResponse(networkResponse);
                request.addMarker("network-parse-complete");

最後判斷Request是否需要Cache如果需要就放到Cache緩沖池裡面:

if (request.shouldCache() && response.cacheEntry != null) {
                    mCache.put(request.getCacheKey(), response.cacheEntry);
                    request.addMarker("network-cache-written");
                }

那麼NetworkDispatcher的基本流程就走完了。下一章,我們將針對Volley提供給我們方便api結合源碼來看待Volley.