Fixes Issue:538 - Memory problem/leak on iOS 7

This was happening as the decompressed images take up lot of memory on older iOS devices
such as iPad 2 with a collection view and crashes the app without a memory warning most of the times.
These options don't break existing features but adds:
    * Adds Queue options to SDWebImagePrefetcher.
    * Adds option to decompress images in cache and post download.

SDWebImage/SDImageCache.h

@@ -36,6 +36,12 @@ typedef void(^SDWebImageCalculateSizeBlock)(NSUInteger fileCount, NSUInteger tot
  */
 @interface SDImageCache : NSObject
 
+/**
+ * Decompressing images that are downloaded and cached can improve peformance but can consume lot of memory.
+ * Defaults to YES. Set this to NO if you are experiencing a crash due to excessive memory consumption.
+ */
+@property (assign, nonatomic) BOOL shouldDecompressImages;
+
 /**
  * The maximum "total cost" of the in-memory image cache. The cost function is the number of pixels held in memory.
  */

SDWebImage/SDImageCache.m

@@ -77,6 +77,9 @@ - (id)initWithNamespace:(NSString *)ns {
         NSArray *paths = NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES);
         _diskCachePath = [paths[0] stringByAppendingPathComponent:fullNamespace];
 
+        // Set decompression to YES
+        _shouldDecompressImages = YES;
+
         dispatch_sync(_ioQueue, ^{
             _fileManager = [NSFileManager new];
         });
@@ -266,7 +269,9 @@ - (UIImage *)diskImageForKey:(NSString *)key {
     if (data) {
         UIImage *image = [UIImage sd_imageWithData:data];
         image = [self scaledImageForKey:key image:image];
-        image = [UIImage decodedImageWithImage:image];
+        if (self.shouldDecompressImages) {
+            image = [UIImage decodedImageWithImage:image];
+        }
         return image;
     }
     else {

SDWebImage/SDWebImageDownloader.h

@@ -49,8 +49,6 @@ typedef NS_OPTIONS(NSUInteger, SDWebImageDownloaderOptions) {
      * Put the image in the high priority queue.
      */
     SDWebImageDownloaderHighPriority = 1 << 7,
-
-
 };
 
 typedef NS_ENUM(NSInteger, SDWebImageDownloaderExecutionOrder) {
@@ -79,12 +77,17 @@ typedef NSDictionary *(^SDWebImageDownloaderHeadersFilterBlock)(NSURL *url, NSDi
  */
 @interface SDWebImageDownloader : NSObject
 
+/**
+ * Decompressing images that are downloaded and cached can improve peformance but can consume lot of memory.
+ * Defaults to YES. Set this to NO if you are experiencing a crash due to excessive memory consumption.
+ */
+@property (assign, nonatomic) BOOL shouldDecompressImages;
+
 @property (assign, nonatomic) NSInteger maxConcurrentDownloads;
 
 /**
  * Shows the current amount of downloads that still need to be downloaded
  */
-
 @property (readonly, nonatomic) NSUInteger currentDownloadCount;
 
 

SDWebImage/SDWebImageDownloader.m

@@ -65,6 +65,7 @@ + (SDWebImageDownloader *)sharedDownloader {
 - (id)init {
     if ((self = [super init])) {
         _operationClass = [SDWebImageDownloaderOperation class];
+        _shouldDecompressImages = YES;
         _executionOrder = SDWebImageDownloaderFIFOExecutionOrder;
         _downloadQueue = [NSOperationQueue new];
         _downloadQueue.maxConcurrentOperationCount = 6;
@@ -158,6 +159,7 @@ - (void)setOperationClass:(Class)operationClass {
                                                             if (!sself) return;
                                                             [sself removeCallbacksForURL:url];
                                                         }];
+        operation.shouldDecompressImages = wself.shouldDecompressImages;
 
         if (wself.username && wself.password) {
             operation.credential = [NSURLCredential credentialWithUser:wself.username password:wself.password persistence:NSURLCredentialPersistenceForSession];

SDWebImage/SDWebImageDownloaderOperation.h

@@ -17,6 +17,9 @@
  */
 @property (strong, nonatomic, readonly) NSURLRequest *request;
 
+
+@property (assign, nonatomic) BOOL shouldDecompressImages;
+
 /**
  * Whether the URL connection should consult the credential storage for authenticating the connection. `YES` by default.
  *

SDWebImage/SDWebImageDownloaderOperation.m

@@ -47,6 +47,7 @@ - (id)initWithRequest:(NSURLRequest *)request
             cancelled:(SDWebImageNoParamsBlock)cancelBlock {
     if ((self = [super init])) {
         _request = request;
+        _shouldDecompressImages = YES;
         _shouldUseCredentialStorage = YES;
         _options = options;
         _progressBlock = [progressBlock copy];
@@ -289,7 +290,12 @@ - (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data {
                 UIImage *image = [UIImage imageWithCGImage:partialImageRef scale:1 orientation:orientation];
                 NSString *key = [[SDWebImageManager sharedManager] cacheKeyForURL:self.request.URL];
                 UIImage *scaledImage = [self scaledImageForKey:key image:image];
-                image = [UIImage decodedImageWithImage:scaledImage];
+                if (self.shouldDecompressImages) {
+                    image = [UIImage decodedImageWithImage:scaledImage];
+                }
+                else {
+                    image = scaledImage;
+                }
                 CGImageRelease(partialImageRef);
                 dispatch_main_sync_safe(^{
                     if (self.completedBlock) {
@@ -358,7 +364,9 @@ - (void)connectionDidFinishLoading:(NSURLConnection *)aConnection {
 
             // Do not force decoding animated GIFs
             if (!image.images) {
-                image = [UIImage decodedImageWithImage:image];
+                if (self.shouldDecompressImages) {
+                    image = [UIImage decodedImageWithImage:image];
+                }
             }
             if (CGSizeEqualToSize(image.size, CGSizeZero)) {
                 completionBlock(nil, nil, [NSError errorWithDomain:@"SDWebImageErrorDomain" code:0 userInfo:@{NSLocalizedDescriptionKey : @"Downloaded image has 0 pixels"}], YES);

SDWebImage/SDWebImagePrefetcher.h

@@ -58,6 +58,11 @@ typedef void(^SDWebImagePrefetcherCompletionBlock)(NSUInteger noOfFinishedUrls,
  */
 @property (nonatomic, assign) SDWebImageOptions options;
 
+/**
+ * Queue options for Prefetcher. Defaults to Main Queue.
+ */
+@property (nonatomic, assign) dispatch_queue_t prefetcherQueue;
+
 @property (weak, nonatomic) id <SDWebImagePrefetcherDelegate> delegate;
 
 /**

SDWebImage/SDWebImagePrefetcher.m

@@ -40,6 +40,7 @@ - (id)init {
     if ((self = [super init])) {
         _manager = [SDWebImageManager new];
         _options = SDWebImageLowPriority;
+        _prefetcherQueue = dispatch_get_main_queue();
         self.maxConcurrentDownloads = 3;
     }
     return self;
@@ -82,9 +83,8 @@ - (void)startPrefetchingAtIndex:(NSUInteger)index {
                                 totalCount:self.prefetchURLs.count
             ];
         }
-
         if (self.prefetchURLs.count > self.requestedCount) {
-            dispatch_async(dispatch_get_main_queue(), ^{
+            dispatch_async(self.prefetcherQueue, ^{
                 [self startPrefetchingAtIndex:self.requestedCount];
             });
         }