IOS_压缩图片UIImage

常见压缩：

//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(void *info, void *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(void *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((void *)CFBridgingRetain(rep), [rep size], &callbacks);    CGImageSourceRef source = CGImageSourceCreateWithDataProvider(provider, NULL);    /*    压缩    */    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);//释放Image    return toReturn;}