An Objective-C operation to resize your images correctly and quickly.

Features

• Thread safe: as a subclass of NSOperation, BOSImageResizeOperation can easily be used with an NSOperationQueue to resize images without blocking the current thread.
• Designed for the common use case: resizes images proportionally to fit within a given size.
• Can also crop to fit a given aspect ratio.
• Respects EXIF/UIImage orientation.

Requirements

BOSImageResizeOperation uses ARC and has been tested with iOS 4.3 and above.

Installation

Manual

Add BOSImageResizeOperation.h and BOSImageResizeOperation.m to your project. You'll need to enable ARC for your project, or just for BOSImageResizeOperation.m, which is left as an exercise for the reader.

Usage

1. Create an instance of BOSImageResizeOperation and initialize it with either a path to a file or a UIImage object.

   // We already have the image in memory, e.g. from UIImagePickerController
   UIImage* image = info[UIImagePickerControllerOriginalImage];
   BOSImageResizeOperation* op = [[BOSImageResizeOperation alloc] initWithImage:image];

   // We don't have the image in memory; it will be loaded on a background thread
   NSString* documentsPath = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES)[0];
   NSString* inputPath = [documentsPath stringByAppendingPathComponent:@"large_image.jpg"];
   BOSImageResizeOperation* op = [[BOSImageResizeOperation alloc] initWithPath:inputPath]; 

2. Specify how you'd like the image resized. Any of these can be combined.

   Resize the image proportionally so it fits within the given size:

   [op resizeToFitWithinSize:CGSizeMake(200.f, 200.f)];

   Crop the image to the given aspect ratio:

   [op cropToAspectRatioWidth:16 height:9];

   Write the image to disk when finished (optional). The format will be PNG by default and JPEG if the output path ends in ".jpg". For JPEG output, the JPEGcompressionQuality property can be used to specify compression quality, from 0.0 to 1.0. The default is 0.8.

   NSString* outputPath = [documentsPath stringByAppendingPathComponent:@"small_image.jpg"];
   op.JPEGcompressionQuality = 0.5;
   [op writeResultToPath:outputPath];

3. Start the operation.

   On the current thread (for example, if we're already on a background thread):

   [op start];

   In the background, using an NSOperationQueue:

   NSOperationQueue* queue = [[NSOperationQueue alloc] init];
   
   // Avoid a retain cycle (ARC & iOS 5+)
   __weak BOSImageResizeOperation* weakOp = op;
   
   [op setCompletionBlock:^{
       UIImage* smallerImage = weakOp.result;
   }];
   
   [queue addOperation:op];

   In the background, using Grand Central Dispatch:

   dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
       [op start];
       UIImage* smallerImage = op.result;
   });

4. Do something with the image. If an error ocurred while resizing the image, the result property will be nil.

TODO

• More image resizing options
• Better error handling
• Support for mirrored orientations
• More extensive tests

Brought to you by

BOSImageResizeOperation was written by Alex Michaud, based on techniques cobbled together from all over the Internet and Stack Overflow. Contributions are welcome and will help create a utopia where all iOS apps resize images the right way, no matter what edge cases are uncovered. Follow or say hi on Twitter: @bucketosoftware

License

BOSImageResizeOperation is available under the MIT license. See the LICENSE file for more information.