OGImage

Backwards compatibility

Note that 0.0.4 on the master branch breaks backwards compatibility with < 0.0.3. You'll need to change all your addObserver: calls to addObserver:context: and removeObserver: to removeObserver:context: See Pull Request 23 for more information.

Introduction

The idea behind OGImage is to encapsulate best practices for loading images over HTTP in a simple, extensible interface.

Philosophy

• The default use case should be ridiculously simple to execute. In OGImage, you can load, cache, and scale an image with the following call:

  static NSString *KVOContext = @"OGImage observation";
  
  ...
  
  OGScaledImage *ogImage = [[OGScaledImage alloc] initWithURL:imageURL size:renderSize key:nil];
  /*
   * This is shorthand for calling KVO methods for @"image", @"scaledImage", and @"error"
   */
  [ogImage addObserver:self context:&KVOContext];
  
  // check to see if the image loaded instantly (e.g., from cache)
  if (nil != ogImage.image) {
      // we already have an image, so do whatever we need with it, otherwise
      // we'll be notified in `observeValueForKeyPath:ofObject:change:context:` whenever the image changes
      [self displayImage:ogImage.image];
      // ooh, we also got all the image's metadata! Sweet!
      NSDictionary *exifData = [ogImage.originalFileProperties valueForKey:kCGImagePropertyExifDictionary];
  }

• Networking belongs in the Model. Views (and to some degree, Controllers) shouldn't know anything about where images come from: That's the model's job. OGImage is designed as a model object: Controllers and Views learn about changes to it (e.g., when an image has loaded) through KVO.

• Generally, more recently requested images should have higher loading priority. For example, if images are being loaded on demand as a user scrolls through a table view, the user experience is improved if they don't have to wait for all the images they've scrolled past to load before the images that are visible are loaded. At the same time, we don't believe in waiting until a table view has finished scrolling to start loading images. OGImage addresses these competing requirements by pushing image load requests into a LIFO queue.

• Each subsystem should have its own GCD queue.

Installation

To use OGImage in your projects, simply add the files in the OGImage subdirectory to your target.

You'll need to add AssetsLibrary.framework and ImageIO.framework to your target's "Link Binary With Libraries" build phase. If you're using OGScaledImage and/or OGImageProcessing, you'll additionally need to add Accelerate.framework and AssetsLibrary.framework to your target's "Link Binary With Libraries" build phase.

Usage

OGImage has a simple interface; just instantiate with the image's URL and listen for KVO notifications on the image property. You can also specify a placeholder image to use until loading is complete with initWithURL:placeholderImage

#import "OGImage.h"

static NSString *KVOContext = @"OGImage observation";

...

OGImage *image = [[OGImage alloc] initWithURL:[NSURL URLWithString:@"http://somedomain.com/someimage.jpg"]];
[image addObserver:self context:&KVOContext];

...

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context {
    if ((void *)&KVOContext == context) {
        if ([keyPath isEqualToString:@"image"]) {
            // image was loaded!
            ...
        } else if ([keyPath isEqualToString:@"error"]) {
            // error loading image
        }
    }
    else {
        [super observeValueForKeyPath:keyPath ofObject:object change:change context:context];
    }
}

There are also caching (OGCachedImage) and caching+scaling (OGScaledImage) subclasses that provide useful functionality and demonstrate how to go about bending OGImage to your needs via subclassing.

#import "OGScaledImage.h"

static NSString *KVOContext = @"OGImage observation";

...

/*
 * The image at `imageURL` will be loaded from one of in-memory cache, disk cache or
 * over the network and scaled to `scaledSize`. Note that if the image at `imageURL`
 * has already been scaled to `scaledSize` in the past, OGScaledImage will simply
 * load the scaled image from cache.
 */
OGScaledImage *image = [[OGScaledImage alloc] initWithURL:imageURL size:scaledSize key:nil];

/*
 * Note that here we're interested in the `scaledImage` property, not the full-size `image`
 * property.
 */
[image addObserver:self context:&KVOContext];

...

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context {
    if ((void *)&KVOContext == context) {
        if ([keyPath isEqualToString:@"scaledImage"]) {
            // image was loaded and scaled!
            ...
        } else if ([keyPath isEqualToString:@"error"]) {
            // error loading image
        }
    else {
        [super observeValueForKeyPath:keyPath ofObject:object change:change context:context];
    }
}

Demo and Tests

This repo contains a very simple project that demonstrates the use of OGScaledImage with a stock UITableView.

To run the demo and tests in the OGImageDemo project, you'll need CocoaPods If you already have cocoapods on your system, just run pod install from the terminal in the OGImageDemo directory.

Note that you do not need cocoapods to use OGImage in your projects--there are no external dependencies for the library itself, only for the tests (GHUnit) and demo (CocoaLumberjack)

Key-Value Observing

If you haven't used KVO before or if you're rusty, be sure check out Key-Value Observing Programming Guide and Dave Dribin's excellent 'Proper Key-Value Observer Usage'

TODO:

See issues