iOS的图片压缩，目前我在用的有两种：

    一、在内存中压缩

    需要消耗内存，如果图片较多，可能会造成crash

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


    二、利用ALAsset压缩

#import 
#import 


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.
    NSLog(@"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);
}

- (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);
  
  CGImageRef p_w_picpathRef = CGImageSourceCreateThumbnailAtIndex(source, 0, (__bridge CFDictionaryRef) @{
                                                                                                    (NSString *)kCGImageSourceCreateThumbnailFromImageAlways : @YES,
                                                                                                    (NSString *)kCGImageSourceThumbnailMaxPixelSize : @(size),
                                                                                                    (NSString *)kCGImageSourceCreateThumbnailWithTransform : @YES,
                                                                                                    });
  CFRelease(source);
  CFRelease(provider);
  
  if (!p_w_picpathRef) {
    return nil;
  }
  
  UIImage *toReturn = [UIImage p_w_picpathWithCGImage:p_w_picpathRef];
  
  CFRelease(p_w_picpathRef);
  
  return toReturn;
}
#pragma mark - UIImagePickerControllerDelegate
- (void)p_w_picpathPickerController:(UIImagePickerController *)picker didFinishPickingMediaWithInfo:(NSDictionary *)info {
  NSLog(@"info:\n%@", info);
  UIImage *p_w_picpath = info[UIImagePickerControllerOriginalImage];
  NSData *imgData = UIImagePNGRepresentation(p_w_picpath);
  NSLog(@"length1: %lu", (unsigned long)imgData.length);
  
  NSURL *p_w_picpathURL = info[UIImagePickerControllerReferenceURL];
  ALAssetsLibrary *assetsLibrary = [[ALAssetsLibrary alloc] init];
  [assetsLibrary assetForURL:p_w_picpathURL resultBlock:^(ALAsset *asset) {
    UIImage *p_w_picpath = [self thumbnailForAsset:asset maxPixelSize:1];
    NSData *imgData = UIImagePNGRepresentation(p_w_picpath);
    NSLog(@"length2: %lu", (unsigned long)imgData.length);
  } failureBlock:nil];
  
  [picker dismissViewControllerAnimated:YES completion:^{
  }];
}

    使用第二种方法内存占用率很低，很适合做多张图片的压缩处理。