Volley拓展框架——Netroid，以及與Volley的差異

Netroid是一個基于Volley實現的Android Http庫。提供執行網絡請求、緩存返回結果、批量圖片加載、大文件斷點下載的常見Http交互功能。致力于避免每個項目重復開發基礎Http功能，實現顯著地縮短開發周期的愿景。

功能上的區別：

作為Volley的拓展框架，netroid增加了大文件斷點下載，并且netroid的可定制性更強。

實現上的區別：

1. 緩存的處理；

在volley中，緩存的過期時間是通過 ttl 和 softTtl 控制

        /** True if the entry is expired. */
        public boolean isExpired() {
            return this.ttl < System.currentTimeMillis();
        }

        /** True if a refresh is needed from the original data source. */
        public boolean refreshNeeded() {
            return this.softTtl < System.currentTimeMillis();
        }

而這兩個值的來源是HttpHeaderParser.parseCacheHeaders

public static Cache.Entry parseCacheHeaders(NetworkResponse response) {
    long now = System.currentTimeMillis();    
    long serverDate = 0;
    long lastModified = 0;
    long serverExpires = 0;
    long softExpire = 0;
    long finalExpire = 0;
    long maxAge = 0;
    long staleWhileRevalidate = 0;
    boolean hasCacheControl = false;
    boolean mustRevalidate = false;    
    String serverEtag = null;
    headerValue = headers.get("Date");
    if (headerValue != null) {
        serverDate = parseDateAsEpoch(headerValue);
    }    headerValue = headers.get("Cache-Control");
    if (headerValue != null) {
        hasCacheControl = true;
        String[] tokens = headerValue.split(",");
        for (int i = 0; i < tokens.length; i++) {
            String token = tokens[i].trim();
            if (token.equals("no-cache") || token.equals("no-store")) {
                return null;
            } else if (token.startsWith("max-age=")) {
                try {
                    maxAge = Long.parseLong(token.substring(8));
                } catch (Exception e) {
                }
            } else if (token.startsWith("stale-while-revalidate=")) {
                try {
                    staleWhileRevalidate = Long.parseLong(token.substring(23));
                } catch (Exception e) {
                }
            } else if (token.equals("must-revalidate") || token.equals("proxy-revalidate")) {
                mustRevalidate = true;
            }
        }
    }    
    headerValue = headers.get("Expires");
    if (headerValue != null) {
        serverExpires = parseDateAsEpoch(headerValue);
    }    
    headerValue = headers.get("Last-Modified");
    if (headerValue != null) {
        lastModified = parseDateAsEpoch(headerValue);
    // Cache-Control takes precedence over an Expires header, even if both exist and Expires is more restrictive.
    // 如果服務器返回的header中有Cache-Control字段，則可以按照制定的規則進行設定
    if (hasCacheControl) {
        softExpire = now + maxAge * 1000;
        finalExpire = mustRevalidate
                ? softExpire
                : softExpire + staleWhileRevalidate * 1000;
    } 
    // 若服務器返回的header中包含Expires和Date字段
    else if (serverDate > 0 && serverExpires >= serverDate) {
        // Default semantic for Expire header in HTTP specification is softExpire.
        softExpire = now + (serverExpires - serverDate);
        finalExpire = softExpire;
    }    
    Cache.Entry entry = new Cache.Entry();
    entry.data = response.data;
    entry.etag = serverEtag;
    entry.softTtl = softExpire;
    entry.ttl = finalExpire;
    entry.serverDate = serverDate;
    entry.lastModified = lastModified;
    return entry;
}

根據上述代碼中的中文注釋看，若服務器返回的header中沒有Cache-Control，Expires，Date等字段，則 ttl 和 softExpire 的值均為默認的0，從而使得緩存永遠是過期的，其影響是緩存不僅不能起效，反而每次網絡請求都需要更新緩存，最后就是拖累整體性能。

為此，netroid采用expireTime字段替代了 ttl 和 softExpire ，每次發起請求時，需指定過期時間

    // com.duowan.mobile.netroid.Request.java
    public void setCacheExpireTime(TimeUnit timeUnit, int amount) {
        this.mCacheExpireTime = System.currentTimeMillis() + timeUnit.toMillis(amount);
    }

    public final boolean shouldCache() {
        return mCacheExpireTime > 0;
    }

從上述代碼看出，若沒有設置過期時間時，不會產生緩存

        /** True if the entry is expired. */
        public boolean isExpired() {
            return expireTime < System.currentTimeMillis();
        }

        /** True if a refresh is needed from the original data source. */
        public boolean refreshNeeded() {
            // still unimplemented, might be use a constant like 'refreshTime'?
            return this.expireTime < System.currentTimeMillis();
        }

2. 網絡數據處理；

首先貼一段帶注釋的代碼：

    public NetworkResponse performRequest(Request<?> request) throws VolleyError {
        // Determine if request had non-http perform.
        // 若該請求不需要訪問網絡，則直接復寫perform方法。使用場景如，加載數據庫的數據，或者加載本地圖片，使用此框架可以統一處理此類耗時操作
        NetworkResponse networkResponse = request.perform();
        if (networkResponse != null) 
            return networkResponse;

        long requestStart = SystemClock.elapsedRealtime();
        while (true) {
            // If the request was cancelled already,
            // do not perform the network request.
            if (request.isCanceled()) {
                request.finish("perform-discard-cancelled");
                mDelivery.postCancel(request);
                throw new NetworkError(networkResponse);
            }

            HttpResponse httpResponse = null;
            byte[] responseContents = null;
            try {
                // prepare to perform this request, normally is reset the request headers.
                // 此方法默認實現為空，若請求有需要預處理的話，該設計也是極好的。使用場景如，在進行大文件斷點下載時，需要設置Range頭字段，但是網絡異常進行retry時就不太好處理range了，但是有這個方法就很簡單了
                request.prepare();

                httpResponse = mHttpStack.performRequest(request);

                StatusLine statusLine = httpResponse.getStatusLine();
                int statusCode = statusLine.getStatusCode();
                if (statusCode < 200 || statusCode > 299) throw new IOException();

                // 此方法的默認實現為volley的實現方法，但是可以復寫該方法。volley的實現方式是直接把請求到的數據轉為byte[]，此方式會限制請求的數據量不能太大，否則會OOM。
                // 若下載大文件時，就得復寫這個方法，將網絡請求的數據流讀寫到文件，而不是內存
                responseContents = request.handleResponse(httpResponse, mDelivery);

                // if the request is slow, log it.
                long requestLifetime = SystemClock.elapsedRealtime() - requestStart;
                logSlowRequests(requestLifetime, request, responseContents, statusLine);

                return new NetworkResponse(statusCode, responseContents, parseCharset(httpResponse));
            } 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) {
                。。。
            }
        }
    }

3. 數據請求過程回調；

在volley的實現中，是通過ExecutorDelivery將數據請求的結果回調給調用者。

public interface ResponseDelivery {
    /**
     * Parses a response from the network or cache and delivers it.
     */
    public 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.
     */
    public void postResponse(Request<?> request, Response<?> response, Runnable runnable);

    /**
     * Posts an error for the given request.
     */
    public void postError(Request<?> request, VolleyError error);
}

netroid在此基礎上增加了一些回調：

public interface Delivery {

    /** Posts request finished callback for the given request. */
    void postFinish(Request<?> request);

    /** Parses a response from the network or cache and delivers it. */
    public 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.
     */
    public void postResponse(Request<?> request, Response<?> response, Runnable runnable);

    /** Posts an error for the given request. */
    public void postError(Request<?> request, VolleyError error);

    /** Posts a cancel callback for the given request. */
    void postCancel(Request<?> request);

    /** Posts starting execute callback for the given request. */
    void postPreExecute(Request<?> request);

    /** Posts cache used callback for the given request. */
    void postUsedCache(Request<?> request);

    /** Posts networking callback for the given request. */
    void postNetworking(Request<?> request);

    /** Posts request retry callback for the given request. */
    void postRetry(Request<?> request);

    /** Posts file download progress stat. */
    void postDownloadProgress(Request<?> request, long fileSize, long downloadedSize);
}

可以看出，這些回調基本覆蓋了請求過程中的關鍵點，主要是有postDownloadProgress方法，進行回調文件下載進度。

                // com.duowan.mobile.netroid.NetworkDispatcher.java
                request.addMarker("network-queue-take");
                mDelivery.postPreExecute(request);

                // If the request was cancelled already,
                // do not perform the network request.
                if (request.isCanceled()) {
                    request.finish("network-discard-cancelled");
                    mDelivery.postCancel(request);
                    mDelivery.postFinish(request);
                    continue;
                }

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

                // Parse the response here on the worker thread.
                Response<?> response = request.parseNetworkResponse(networkResponse);
                request.addMarker("network-parse-complete");

                // Write to cache if applicable.
                if (mCache != null && request.shouldCache() && response.cacheEntry != null) {
                    response.cacheEntry.expireTime = request.getCacheExpireTime();
                    mCache.putEntry(request.getCacheKey(), response.cacheEntry);
                    request.addMarker("network-cache-written");
                }

                // Post the response back.
                request.markDelivered();
                mDelivery.postResponse(request, response);

從上述代碼看出，回調確實很多，若在這些回調中添加太多操作的話，肯定會影響數據請求的速度。

總的來說，netroid相對volley的改進還是不錯的，這也是這兩天看代碼的總結，如有遺漏，后面再補充！