What does it do?

JLNRGravityImageView is a lightweight UIImageView subclass that pins image edges to view edges in UIViewContentModeAspectFill mode (not in AspectFit mode, though).

This is useful, for example, if you want to have a photo fill an area on the screen, but you do not want to clip off the head of a person or a logo that is placed near the top of the image. Simply set alignTop to YES and gravity will pull the image up as far as possible.

Its interface is compatible with UIImageViewAligned (four properties alignTop, alignBottom, alignLeft, alignRight), and can be configured in Interface Builder thanks to IBInspectable.

How is it implemented? How does it differ from UIImageViewAligned?

Just like UIImageViewAligned, this is a UIImageView subclass so that existing categories work. Apart from setting the Custom Class, no changes to XIBs and storyboards are required.

Unlike UIImageViewAligned, JLNRGravityImageView does not use nested views at all. This prevents issues where UIImageView keeps resetting the image property on the outer UIImageView, showing two images instead of one.

Instead, JLNRGravityImageView sets the layer.contentsRect property to a rectangle that is larger than 1x1. This lets us influence how UIImageView (actually CALayer) centres the image.

(One other approach that I have tried was to use a content mode like Top, Left etc. and scale the image using ìmageView.contentScaleFactor or imageView.transform, however the results were too inconsistent.)

Requirements

JLNRGravityImageView should work on all versions of iOS.

Installation

You can simply copy the two files in Classes into your project, but the easiest way is to use CocoaPods.

CocoaPods is a dependency manager for Objective-C, which automates and simplifies the process of using 3rd-party libraries like JLNRGravityImageView in your projects. See the "Getting Started" guide for more information.

Podfile

platform :ios, '7.0'
pod "JLNRGravityImageView", :git => "https://github.com/jlnr/JLNRGravityImageView"

License

JLNRGravityImageView is MIT-licensed.