//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;
  }