A sample application for cropping images, written in Swift.
The main class is the CropppableImageView class, which is a subclass of UIView.
To use a CroppableImageView in your project, drag a UIView into your XIB/storyboard. Then use the "Idenity Inspector" to change the type to CroppableImageView.
If you need to be notified if there is a valid crop area defined, set up a delegate object that conforms to the CroppableImageViewDelegateProtocol. That protocol only has 1 method, haveValidCropRect(). The CroppableImageView will call your haveValidCropRect() method when the user selects/deselects a crop rectangle. You can use the haveValidCropRect() method to enable/disable a crop button, for example.
The CropppableImageView has a method croppedImage() that returns a new image containing the portion of the source image the user has selected, or nil if the selection rectangle isn't valid.
The CropppableImageView class uses another class, CornerpointView, to draw the cornerpoints of the image view, and allow dragging of the cornerpoints. A CropppableImageView sets up 4 CornerpointView objects and adds them as subviews in it's init method.
The initalizers for CornerpointView create pan gesture recognziers and connect them to the view so CornerpointView objects are automatically draggable.
The CornerpointViews centerPoint property is optional and is initially nil. The centerPoint property has a didSet method that hides the CornerpointView if the centerPoint is nil and un-hides the corner point if the centerPoint is not nil.
The CornerpointView class has an optional cornerpointDelegate property. (If you set a conerpointDelegate, it must conform to the CornerpointClientProtocol.) The CropppableImageView sets itself up as the delegate of it's CornerpointViews.
The only method in the CornerpointClientProtocol is cornerHasChanged. It simply tells the delegate that the user has moved the corner point. It passes a pointer to itself so the delegate can tell which corner has changed.
The ViewController class coordinates between the CropppableImageView and the button that triggers image cropping.
The ViewController class also offers a button to load a new image into the image view.
Loading a new image is handled by the handleSelectImgButton IBAction method. This method uses the new UIAlertController class, added in iOS 8 instead of the now-deprecated UIAlertView. (Note that if you want your app to run under iOS 7 and 8, you will still have to use a UIAlertView, or write code that uses a UIAlertView on iOS 7 and a UIAlertController under iOS 8)
A UIAlertController uses a modern block-based design pattern, where you create one or more UIAlertAction objects and attach them to the UIAlertController. These UIAlertAction objects are usually drawn as buttons, and inlcude a block of code that's executed when the user chooses that option.
The "Take a New Picture" action and the "Select Picture from library" action both call the method pickImageFromSource. This method creates and displays a UIImagePickerController. The docs for UIImagePickerController say that you must use a popover to display the picker controller in a popover on iPad for anything but taking a picture with the camera. However, I've found that displaying a full-screen picker works on iPad, and it gives the user more room to navigate their photo library.
The crop button on the view controller's view is linked to the handleCropButton() IBAction method. The handleCropButton() method calls the CropppableImageViews croppedImage() mehod to create a croppped image. It then plays a shutter sound, displays a white view on top of the image to simulate a flash of light, then finally calls the Cocoa Touch method UIImageWriteToSavedPhotosAlbum to save the cropped image to the user's photo album.
There is code at the bottom of the handleCropButton() method that will save the cropped image to the user's documents directory instead, in case that's what you need to do in your app.