Added PNG detection to be able to determine at runtime if a downloaded image should be saved as PNG or as JPEG

Author: bpoplauschi

SDWebImage/SDImageCache.h

@@ -90,13 +90,14 @@ typedef enum SDImageCacheType SDImageCacheType;
  * Store an image into memory and optionally disk cache at the given key.
  *
  * @param image The image to store
+ * @param recalculate BOOL indicates if imageData can be used or a new data should be constructed from the UIImage
  * @param data The image data as returned by the server, this representation will be used for disk storage
  *             instead of converting the given image object into a storable/compressed image format in order
  *             to save quality and CPU
  * @param key The unique image cache key, usually it's image absolute URL
  * @param toDisk Store the image to disk cache if YES
  */
-- (void)storeImage:(UIImage *)image imageData:(NSData *)data forKey:(NSString *)key toDisk:(BOOL)toDisk;
+- (void)storeImage:(UIImage *)image recalculateFromImage:(BOOL)recalculate imageData:(NSData *)imageData forKey:(NSString *)key toDisk:(BOOL)toDisk;
 
 /**
  * Query the disk cache asynchronously.

SDWebImage/SDImageCache.m

@@ -14,6 +14,24 @@
 #import <mach/mach_host.h>
 
 static const NSInteger kDefaultCacheMaxCacheAge = 60 * 60 * 24 * 7; // 1 week
+// PNG signature bytes and data (below)
+static unsigned char kPNGSignatureBytes[8] = {0x89, 0x50, 0x4E, 0x47, 0x0D, 0x0A, 0x1A, 0x0A};
+static NSData *kPNGSignatureData = nil;
+
+BOOL ImageDataHasPNGPreffix(NSData *data);
+BOOL ImageDataHasPNGPreffix(NSData *data)
+{
+    NSUInteger pngSignatureLength = [kPNGSignatureData length];
+    if ([data length] >= pngSignatureLength)
+    {
+        if ([[data subdataWithRange:NSMakeRange(0, pngSignatureLength)] isEqualToData:kPNGSignatureData])
+        {
+            return YES;
+        }
+    }
+
+    return NO;
+}
 
 @interface SDImageCache ()
 
@@ -25,15 +43,20 @@ @interface SDImageCache ()
 @end
 
 
-@implementation SDImageCache {
+@implementation SDImageCache
+{
     NSFileManager *_fileManager;
 }
 
 + (SDImageCache *)sharedImageCache
 {
     static dispatch_once_t once;
     static id instance;
-    dispatch_once(&once, ^{instance = self.new;});
+    dispatch_once(&once, ^
+    {
+        instance = self.new;
+        kPNGSignatureData = [NSData dataWithBytes:kPNGSignatureBytes length:8];
+    });
     return instance;
 }
 
@@ -66,7 +89,7 @@ - (id)initWithNamespace:(NSString *)ns
         {
             _fileManager = NSFileManager.new;
         });
-        
+
 #if TARGET_OS_IPHONE
         // Subscribe to app events
         [[NSNotificationCenter defaultCenter] addObserver:self
@@ -78,7 +101,7 @@ - (id)initWithNamespace:(NSString *)ns
                                                  selector:@selector(cleanDisk)
                                                      name:UIApplicationWillTerminateNotification
                                                    object:nil];
-        
+
         [[NSNotificationCenter defaultCenter] addObserver:self
                                                  selector:@selector(backgroundCleanDisk)
                                                      name:UIApplicationDidEnterBackgroundNotification
@@ -138,7 +161,7 @@ - (NSString *)cachedFileNameForKey:(NSString *)key
 
 #pragma mark ImageCache
 
-- (void)storeImage:(UIImage *)image imageData:(NSData *)imageData forKey:(NSString *)key toDisk:(BOOL)toDisk
+- (void)storeImage:(UIImage *)image recalculateFromImage:(BOOL)recalculate imageData:(NSData *)imageData forKey:(NSString *)key toDisk:(BOOL)toDisk
 {
     if (!image || !key)
     {
@@ -153,16 +176,35 @@ - (void)storeImage:(UIImage *)image imageData:(NSData *)imageData forKey:(NSStri
         {
             NSData *data = imageData;
 
-            if (!data)
+            if (image && (recalculate || !data))
             {
-                if (image)
-                {
 #if TARGET_OS_IPHONE
+                // We need to determine if the image is a PNG or a JPEG
+                // PNGs are easier to detect because they have a unique signature (http://www.w3.org/TR/PNG-Structure.html)
+                // The first eight bytes of a PNG file always contain the following (decimal) values:
+                // 137 80 78 71 13 10 26 10
+
+                // We assume the image is PNG, in case the imageData is nil (i.e. if trying to save a UIImage directly),
+                // we will consider it PNG to avoid loosing the transparency
+                BOOL imageIsPng = YES;
+
+                // But if we have an image data, we will look at the preffix
+                if ([imageData length] >= [kPNGSignatureData length])
+                {
+                    imageIsPng = ImageDataHasPNGPreffix(imageData);
+                }
+
+                if (imageIsPng)
+                {
                     data = UIImagePNGRepresentation(image);
+                }
+                else
+                {
+                    data = UIImageJPEGRepresentation(image, (CGFloat)1.0);
+                }
 #else
-                    data = [NSBitmapImageRep representationOfImageRepsInArray:image.representations usingType: NSJPEGFileType properties:nil];
+                data = [NSBitmapImageRep representationOfImageRepsInArray:image.representations usingType: NSJPEGFileType properties:nil];
 #endif
-                }
             }
 
             if (data)
@@ -183,12 +225,12 @@ - (void)storeImage:(UIImage *)image imageData:(NSData *)imageData forKey:(NSStri
 
 - (void)storeImage:(UIImage *)image forKey:(NSString *)key
 {
-    [self storeImage:image imageData:nil forKey:key toDisk:YES];
+    [self storeImage:image recalculateFromImage:YES imageData:nil forKey:key toDisk:YES];
 }
 
 - (void)storeImage:(UIImage *)image forKey:(NSString *)key toDisk:(BOOL)toDisk
 {
-    [self storeImage:image imageData:nil forKey:key toDisk:toDisk];
+    [self storeImage:image recalculateFromImage:YES imageData:nil forKey:key toDisk:toDisk];
 }
 
 - (BOOL)diskImageExistsWithKey:(NSString *)key
@@ -198,7 +240,7 @@ - (BOOL)diskImageExistsWithKey:(NSString *)key
     {
         exists = [_fileManager fileExistsAtPath:[self defaultCachePathForKey:key]];
     });
-    
+
     return exists;
 }
 
@@ -215,15 +257,15 @@ - (UIImage *)imageFromDiskCacheForKey:(NSString *)key
     {
         return image;
     }
-    
+
     // Second check the disk cache...
     UIImage *diskImage = [self diskImageForKey:key];
     if (diskImage)
     {
         CGFloat cost = diskImage.size.height * diskImage.size.width * diskImage.scale;
         [self.memCache setObject:diskImage forKey:key cost:cost];
     }
-    
+
     return diskImage;
 }
 
@@ -272,7 +314,7 @@ - (UIImage *)scaledImageForKey:(NSString *)key image:(UIImage *)image
 - (NSOperation *)queryDiskCacheForKey:(NSString *)key done:(void (^)(UIImage *image, SDImageCacheType cacheType))doneBlock
 {
     NSOperation *operation = NSOperation.new;
-    
+
     if (!doneBlock) return nil;
 
     if (!key)
@@ -295,7 +337,7 @@ - (NSOperation *)queryDiskCacheForKey:(NSString *)key done:(void (^)(UIImage *im
         {
             return;
         }
-        
+
         @autoreleasepool
         {
             UIImage *diskImage = [self diskImageForKey:key];
@@ -311,7 +353,7 @@ - (NSOperation *)queryDiskCacheForKey:(NSString *)key done:(void (^)(UIImage *im
             });
         }
     });
-    
+
     return operation;
 }
 
@@ -454,13 +496,13 @@ - (void)backgroundCleanDisk
         [application endBackgroundTask:bgTask];
         bgTask = UIBackgroundTaskInvalid;
     }];
-    
+
     // Start the long-running task and return immediately.
     dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^
     {
         // Do the work associated with the task, preferably in chunks.
         [self cleanDisk];
-        
+
         [application endBackgroundTask:bgTask];
         bgTask = UIBackgroundTaskInvalid;
     });
@@ -487,7 +529,7 @@ - (int)getDiskCount
     {
         count += 1;
     }
-    
+
     return count;
 }
 

SDWebImage/SDWebImageManager.m

@@ -199,8 +199,8 @@ - (BOOL)diskImageExistsForURL:(NSURL *)url
 
                             if (transformedImage && finished)
                             {
-                                NSData *dataToStore = [transformedImage isEqual:downloadedImage] ? data : nil;
-                                [self.imageCache storeImage:transformedImage imageData:dataToStore forKey:key toDisk:cacheOnDisk];
+                                BOOL imageWasTransformed = ![transformedImage isEqual:downloadedImage];
+                                [self.imageCache storeImage:transformedImage recalculateFromImage:imageWasTransformed imageData:data forKey:key toDisk:cacheOnDisk];
                             }
                         });
                     }
@@ -213,7 +213,7 @@ - (BOOL)diskImageExistsForURL:(NSURL *)url
 
                         if (downloadedImage && finished)
                         {
-                            [self.imageCache storeImage:downloadedImage imageData:data forKey:key toDisk:cacheOnDisk];
+                            [self.imageCache storeImage:downloadedImage recalculateFromImage:NO imageData:data forKey:key toDisk:cacheOnDisk];
                         }
                     }
                 }