Perform image decoding/optimization in the IO thread instead of main thread for better responsiveness (fix #18)

This new optimization is currently disabled by default so you can test it and give us feedback.
To enable it, add #define ENABLE_SDWEBIMAGE_DECODER and to not forget to add SDWebImageDecoder class
to your projet.

Thanks to Adam Jernst (https://github.com/adamjernst) and James Tang (https://github.com/mystcolor)
for this great optimization. See https://github.com/rs/SDWebImage/pull/18 for more info.

SDImageCache.m

@@ -7,8 +7,13 @@
  */
 
 #import "SDImageCache.h"
+#import "SDWebImageDecoder.h"
 #import <CommonCrypto/CommonDigest.h>
 
+#ifdef ENABLE_SDWEBIMAGE_DECODER
+#import "SDWebImageDecoder.h"
+#endif
+
 static NSInteger cacheMaxCacheAge = 60*60*24*7; // 1 week
 
 static SDImageCache *instance;
@@ -173,6 +178,13 @@ static SDImageCache *instance;
     UIImage *image = [[[UIImage alloc] initWithContentsOfFile:[self cachePathForKey:key]] autorelease];
     if (image)
     {
+#ifdef ENABLE_SDWEBIMAGE_DECODER
+        UIImage *decodedImage = [UIImage decodedImageWithImage:image];
+        if (decodedImage)
+        {
+            image = decodedImage;
+        }
+#endif
         [mutableArguments setObject:image forKey:@"image"];
     }
 

SDWebImageDecoder.h

@@ -0,0 +1,35 @@
+/*
+ * This file is part of the SDWebImage package.
+ * (c) Olivier Poitrey <rs@dailymotion.com>
+ *
+ * Created by james <https://github.com/mystcolor> on 9/28/11.
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+#import <Foundation/Foundation.h>
+
+@protocol SDWebImageDecoderDelegate;
+
+@interface SDWebImageDecoder : NSObject
+{
+    NSOperationQueue *imageDecodingQueue;
+}
+
++ (SDWebImageDecoder *)sharedImageDecoder;
+- (void)decodeImage:(UIImage *)image withDelegate:(id <SDWebImageDecoderDelegate>)delegate userInfo:(NSDictionary *)info;
+
+@end
+
+@protocol SDWebImageDecoderDelegate <NSObject>
+
+- (void)imageDecoder:(SDWebImageDecoder *)decoder didFinishDecodingImage:(UIImage *)image userInfo:(NSDictionary *)userInfo;
+
+@end
+
+@interface UIImage (ForceDecode)
+
++ (UIImage *)decodedImageWithImage:(UIImage *)image;
+
+@end

SDWebImageDecoder.m

@@ -0,0 +1,124 @@
+/*
+ * This file is part of the SDWebImage package.
+ * (c) Olivier Poitrey <rs@dailymotion.com>
+ *
+ * Created by james <https://github.com/mystcolor> on 9/28/11.
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+#import "SDWebImageDecoder.h"
+
+#define DECOMPRESSED_IMAGE_KEY @"decompressedImage"
+#define DECODE_INFO_KEY @"decodeInfo"
+
+#define IMAGE_KEY @"image"
+#define DELEGATE_KEY @"delegate"
+#define USER_INFO_KEY @"userInfo"
+
+@implementation SDWebImageDecoder
+static SDWebImageDecoder *sharedInstance;
+
+- (void)notifyDelegateOnMainThreadWithInfo:(NSDictionary *)dict
+{
+    [dict retain];
+    NSDictionary *decodeInfo = [dict objectForKey:DECODE_INFO_KEY];
+    UIImage *decodedImage = [dict objectForKey:DECOMPRESSED_IMAGE_KEY];
+
+    id <SDWebImageDecoderDelegate> delegate = [decodeInfo objectForKey:DELEGATE_KEY];
+    NSDictionary *userInfo = [decodeInfo objectForKey:USER_INFO_KEY];
+
+    [delegate imageDecoder:self didFinishDecodingImage:decodedImage userInfo:userInfo];
+    [dict release];
+}
+
+- (void)decodeImageWithInfo:(NSDictionary *)decodeInfo
+{
+    UIImage *image = [decodeInfo objectForKey:IMAGE_KEY];
+
+    UIImage *decompressedImage = [UIImage decodedImageWithImage:image];
+    if (!decompressedImage)
+    {
+        // If really have any error occurs, we use the original image at this moment
+        decompressedImage = image;
+    }
+
+    NSDictionary *dict = [NSDictionary dictionaryWithObjectsAndKeys:
+                          decompressedImage, DECOMPRESSED_IMAGE_KEY,
+                          decodeInfo, DECODE_INFO_KEY, nil];
+
+    [self performSelectorOnMainThread:@selector(notifyDelegateOnMainThreadWithInfo:) withObject:dict waitUntilDone:NO];
+}
+
+- (id)init
+{
+    if ((self = [super init]))
+    {
+        // Initialization code here.
+        imageDecodingQueue = [[NSOperationQueue alloc] init];
+    }
+
+    return self;
+}
+
+- (void)decodeImage:(UIImage *)image withDelegate:(id<SDWebImageDecoderDelegate>)delegate userInfo:(NSDictionary *)info
+{
+    NSDictionary *decodeInfo = [NSDictionary dictionaryWithObjectsAndKeys:
+                                image, IMAGE_KEY,
+                                delegate, DELEGATE_KEY,
+                                info, USER_INFO_KEY, nil];
+
+    NSOperation *operation = [[NSInvocationOperation alloc] initWithTarget:self selector:@selector(decodeImageWithInfo:) object:decodeInfo];
+    [imageDecodingQueue addOperation:operation];
+    [operation release];
+}
+
+- (void)dealloc
+{
+    [imageDecodingQueue release], imageDecodingQueue = nil;
+    [super dealloc];
+}
+
++ (SDWebImageDecoder *)sharedImageDecoder
+{
+    if (!sharedInstance)
+    {
+        sharedInstance = [[SDWebImageDecoder alloc] init];
+    }
+    return sharedInstance;
+}
+
+@end
+
+
+@implementation UIImage (ForceDecode)
+
++ (UIImage *)decodedImageWithImage:(UIImage *)image
+{
+    CGImageRef imageRef = image.CGImage;
+    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
+    CGContextRef context = CGBitmapContextCreate(NULL,
+                                                 CGImageGetWidth(imageRef),
+                                                 CGImageGetHeight(imageRef),
+                                                 8,
+                                                 // Just always return width * 4 will be enough
+                                                 CGImageGetWidth(imageRef) * 4,
+                                                 // System only supports RGB, set explicitly
+                                                 colorSpace,
+                                                 // Makes system don't need to do extra conversion when displayed.
+                                                 kCGImageAlphaPremultipliedFirst | kCGBitmapByteOrder32Little); 
+    CGColorSpaceRelease(colorSpace);
+    if (!context) return nil;
+
+    CGRect rect = (CGRect){CGPointZero, CGImageGetWidth(imageRef), CGImageGetHeight(imageRef)};
+    CGContextDrawImage(context, rect, imageRef);
+    CGImageRef decompressedImageRef = CGBitmapContextCreateImage(context);
+    CGContextRelease(context);
+
+    UIImage *decompressedImage = [[UIImage alloc] initWithCGImage:decompressedImageRef];
+    CGImageRelease(decompressedImageRef);
+    return [decompressedImage autorelease];
+}
+
+@end

SDWebImageDownloader.m

@@ -8,6 +8,12 @@
 
 #import "SDWebImageDownloader.h"
 
+#ifdef ENABLE_SDWEBIMAGE_DECODER
+#import "SDWebImageDecoder.h"
+@interface SDWebImageDownloader (ImageDecoder) <SDWebImageDecoderDelegate>
+@end
+#endif
+
 NSString *const SDWebImageDownloadStartNotification = @"SDWebImageDownloadStartNotification";
 NSString *const SDWebImageDownloadStopNotification = @"SDWebImageDownloadStopNotification";
 
@@ -120,7 +126,12 @@ NSString *const SDWebImageDownloadStopNotification = @"SDWebImageDownloadStopNot
     if ([delegate respondsToSelector:@selector(imageDownloader:didFinishWithImage:)])
     {
         UIImage *image = [[UIImage alloc] initWithData:imageData];
+
+#ifdef ENABLE_SDWEBIMAGE_DECODER
+        [[SDWebImageDecoder sharedImageDecoder] decodeImage:image withDelegate:self userInfo:nil];
+#else
         [delegate performSelector:@selector(imageDownloader:didFinishWithImage:) withObject:self withObject:image];
+#endif
         [image release];
     }
 }
@@ -138,6 +149,15 @@ NSString *const SDWebImageDownloadStopNotification = @"SDWebImageDownloadStopNot
     self.imageData = nil;
 }
 
+#pragma mark SDWebImageDecoderDelegate
+
+#ifdef ENABLE_SDWEBIMAGE_DECODER
+- (void)imageDecoder:(SDWebImageDecoder *)decoder didFinishDecodingImage:(UIImage *)image userInfo:(NSDictionary *)userInfo
+{
+    [delegate performSelector:@selector(imageDownloader:didFinishWithImage:) withObject:self withObject:image];
+}
+#endif
+
 #pragma mark NSObject
 
 - (void)dealloc