深入剖析Android的Volley库中的图片加载功能

public void displayImg(View view){ 
 ImageView imageView = (ImageView)this.findViewById(R.id.image_view); 
 RequestQueue mQueue = Volley.newRequestQueue(getApplicationContext()); 
   
 ImageLoader imageLoader = new ImageLoader(mQueue, new BitmapCache()); 
 
 ImageListener listener = ImageLoader.getImageListener(imageView,R.drawable.default_image, R.drawable.default_image); 
 imageLoader.get("http://developer.android.com/images/home/aw_dac.png", listener); 
 //指定图片允许的最大宽度和高度 
 //imageLoader.get("http://developer.android.com/images/home/aw_dac.png",listener, 200, 200); 
} 

public class BitmapCache implements ImageCache { 
 
 private LruCache<String, Bitmap> cache; 
 
 public BitmapCache() { 
  cache = new LruCache<String, Bitmap>(8 * 1024 * 1024) { 
   @Override 
   protected int sizeOf(String key, Bitmap bitmap) { 
    return bitmap.getRowBytes() * bitmap.getHeight(); 
   } 
  }; 
 } 
 
 @Override 
 public Bitmap getBitmap(String url) { 
  return cache.get(url); 
 } 
 
 @Override 
 public void putBitmap(String url, Bitmap bitmap) { 
  cache.put(url, bitmap); 
 } 
} 

<uses-permission android:name="android.permission.INTERNET"/> 

mReqQueue = Volley.newRequestQueue(mCtx);

#Volley

public static RequestQueue newRequestQueue(Context context) {
  return newRequestQueue(context, null);
 }

public static RequestQueue newRequestQueue(Context context, HttpStack stack)
 {
  return newRequestQueue(context, stack, -1);
 }
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;
 }

public RequestQueue(Cache cache, Network network) {
  this(cache, network, DEFAULT_NETWORK_THREAD_POOL_SIZE);
 }
public RequestQueue(Cache cache, Network network, int threadPoolSize) {
  this(cache, network, threadPoolSize,
    new ExecutorDelivery(new Handler(Looper.getMainLooper())));
 }
public RequestQueue(Cache cache, Network network, int threadPoolSize,
   ResponseDelivery delivery) {
  mCache = cache;
  mNetwork = network;
  mDispatchers = new NetworkDispatcher[threadPoolSize];
  mDelivery = delivery;
 }

#RequestQueue
 /**
  * Starts the dispatchers in this queue.
  */
 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();
  }
 }

#VolleyHelper
mImageLoader = new ImageLoader(mReqQueue, new ImageCache()
  {
   private final LruCache<String, Bitmap> mLruCache = new LruCache<String, Bitmap>(
     (int) (Runtime.getRuntime().maxMemory() / 10))
   {
    @Override
    protected int sizeOf(String key, Bitmap value)
    {
     return value.getRowBytes() * value.getHeight();
    }
   };

   @Override
   public void putBitmap(String url, Bitmap bitmap)
   {
    mLruCache.put(url, bitmap);
   }

   @Override
   public Bitmap getBitmap(String url)
   {
    return mLruCache.get(url);
   }
  });

#ImageLoader

public ImageLoader(RequestQueue queue, ImageCache imageCache) {
  mRequestQueue = queue;
  mCache = imageCache;
 }

 # VolleyHelper
 getInstance().getImageLoader().get(url, new ImageLoader.ImageListener());

#ImageLoader
 public ImageContainer get(String requestUrl, final ImageListener listener) {
  return get(requestUrl, listener, 0, 0);
 }
public ImageContainer get(String requestUrl, ImageListener imageListener,
   int maxWidth, int maxHeight) {
  return get(requestUrl, imageListener, maxWidth, maxHeight, ScaleType.CENTER_INSIDE);
 }
public ImageContainer get(String requestUrl, ImageListener imageListener,
   int maxWidth, int maxHeight, ScaleType scaleType) {

  // only fulfill requests that were initiated from the main thread.
  throwIfNotOnMainThread();

  final String cacheKey = getCacheKey(requestUrl, maxWidth, maxHeight, scaleType);

  // Try to look up the request in the cache of remote images.
  Bitmap cachedBitmap = mCache.getBitmap(cacheKey);
  if (cachedBitmap != null) {
   // Return the cached bitmap.
   ImageContainer container = new ImageContainer(cachedBitmap, requestUrl, null, null);
   imageListener.onResponse(container, true);
   return container;
  }

  // The bitmap did not exist in the cache, fetch it!
  ImageContainer imageContainer =
    new ImageContainer(null, requestUrl, cacheKey, imageListener);

  // Update the caller to let them know that they should use the default bitmap.
  imageListener.onResponse(imageContainer, true);

  // Check to see if a request is already in-flight.
  BatchedImageRequest request = mInFlightRequests.get(cacheKey);
  if (request != null) {
   // If it is, add this request to the list of listeners.
   request.addContainer(imageContainer);
   return imageContainer;
  }

  // The request is not already in flight. Send the new request to the network and
  // track it.
  Request<Bitmap> newRequest = makeImageRequest(requestUrl, maxWidth, maxHeight, scaleType,
    cacheKey);

  mRequestQueue.add(newRequest);
  mInFlightRequests.put(cacheKey,
    new BatchedImageRequest(newRequest, imageContainer));
  return imageContainer;
 }

#RequestQueue
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;
  }
 }

#CacheDispatcher
 public CacheDispatcher(
   BlockingQueue<Request<?>> cacheQueue, BlockingQueue<Request<?>> networkQueue,
   Cache cache, ResponseDelivery delivery) {
  mCacheQueue = cacheQueue;
  mNetworkQueue = networkQueue;
  mCache = cache;
  mDelivery = delivery;
 }

#CacheDispatcher

 @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();

  while (true) {
   try {
    // Get a request from the cache triage queue, blocking until
    // at least one is available.
    final Request<?> request = mCacheQueue.take();
    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.
     mDelivery.postResponse(request, response, new Runnable() {
      @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;
   }
  }
 }

# NetworkDispatcher
//new NetworkDispatcher(mNetworkQueue, mNetwork,mCache, mDelivery)

public NetworkDispatcher(BlockingQueue<Request<?>> queue,
   Network network, Cache cache,
   ResponseDelivery delivery) {
  mQueue = queue;
  mNetwork = network;
  mCache = cache;
  mDelivery = delivery;
 }
@Override
 public void run() {
  Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
  while (true) {
   long startTimeMs = SystemClock.elapsedRealtime();
   Request<?> request;
   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);
   }
  }
 }