IOS 图片压缩

原文：http://blog.csdn.net/apple_app/article/details/38847357

最近做论坛功能，发帖的时候需要用到从相册中选取图片然后上传，由于每次上传图片的最大数量为9张，所以需要对图片进行压缩。开始时用了以前经常用的压缩的方法：

 //压缩图片质量
 +(UIImage *)reduceImage:(UIImage *)image percent:(float)percent
 {
     NSData *imageData = UIImageJPEGRepresentation(image, percent);
     UIImage *newImage = [UIImage imageWithData:imageData];
     return newImage;
 }
 //压缩图片尺寸
 + (UIImage*)imageWithImageSimple:(UIImage*)image scaledToSize:(CGSize)newSize
 {
     // Create a graphics image context
     UIGraphicsBeginImageContext(newSize);
     // new size
     [image drawInRect:CGRectMake(0,0,newSize.width,newSize.height)];
     // Get the new image from the context
     UIImage* newImage = UIGraphicsGetImageFromCurrentImageContext();

     // End the context
     UIGraphicsEndImageContext();
     // Return the new image.
     return newImage;
 }

上面的方法比较常见，可是需要加载到内存中来处理图片，当图片数量多了的时候就会收到内存警告，程序崩溃。研究半天终于在一篇博客中找到了解决方法：

 static size_t getAssetBytesCallback(voidvoid *info, voidvoid *buffer, off_t position, size_t count) {
     ALAssetRepresentation *rep = (__bridge id)info;

     NSError *error = nil;
     size_t countRead = [rep getBytes:(uint8_t *)buffer fromOffset:position length:count error:&error];

     if (countRead == 0 && error) {
         // We have no way of passing this info back to the caller, so we log it, at least.
         NDDebug(@"thumbnailForAsset:maxPixelSize: got an error reading an asset: %@", error);
     }

     return countRead;
 }

 static void releaseAssetCallback(voidvoid *info) {
     // The info here is an ALAssetRepresentation which we CFRetain in thumbnailForAsset:maxPixelSize:.
     // This release balances that retain.
     CFRelease(info);
 }

 // Returns a UIImage for the given asset, with size length at most the passed size.
 // The resulting UIImage will be already rotated to UIImageOrientationUp, so its CGImageRef
 // can be used directly without additional rotation handling.
 // This is done synchronously, so you should call this method on a background queue/thread.
 - (UIImage *)thumbnailForAsset:(ALAsset *)asset maxPixelSize:(NSUInteger)size {
     NSParameterAssert(asset != nil);
     NSParameterAssert(size > 0);

     ALAssetRepresentation *rep = [asset defaultRepresentation];

     CGDataProviderDirectCallbacks callbacks = {
         .version = 0,
         .getBytePointer = NULL,
         .releaseBytePointer = NULL,
         .getBytesAtPosition = getAssetBytesCallback,
         .releaseInfo = releaseAssetCallback,
     };

     CGDataProviderRef provider = CGDataProviderCreateDirect((voidvoid *)CFBridgingRetain(rep), [rep size], &callbacks);
     CGImageSourceRef source = CGImageSourceCreateWithDataProvider(provider, NULL);

     CGImageRef imageRef = CGImageSourceCreateThumbnailAtIndex(source, 0, (__bridge CFDictionaryRef) @{
                                                                                                       (NSString *)kCGImageSourceCreateThumbnailFromImageAlways : @YES,
                                                                                                       (NSString *)kCGImageSourceThumbnailMaxPixelSize : [NSNumber numberWithInt:size],
                                                                                                       (NSString *)kCGImageSourceCreateThumbnailWithTransform : @YES,
                                                                                                       });
     CFRelease(source);
     CFRelease(provider);

     if (!imageRef) {
         return nil;
     }

     UIImage *toReturn = [UIImage imageWithCGImage:imageRef];

     CFRelease(imageRef);

     return toReturn;
 }

采用上面的方法之后内存占用率很低！

原文地址：http://www.mindsea.com/2012/12/downscaling-huge-alassets-without-fear-of-sigkill/