IOS 图片压缩

 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：

[objc] view plaincopy

    //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：

[objc] view plaincopy

    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！