Applying Core Animation Effects IPHONE APPS

Core Animation is an Objective-C framework that provides infrastructure for creating fluid, real time animations quickly and easily. Core Animation is not a drawing technology itself, in the sense that it does not provide primitive routines for creating shapes, images, or other types of content. Instead, it is a technology for manipulating and displaying content that you created using other technologies.

Most applications can benefit from using Core Animation in some form in iPhone OS. Animations provide feedback to the user about what is happening. For example, when the user navigates through the Settings application, screens slide in and out of view based on whether the user is navigating further down the preferences hierarchy or back up to the root node. This kind of feedback is important and provides contextual information for the user. It also enhances the visual style of an application.

In most cases, you may be able to reap the benefits of Core Animation with very little effort. For example, several properties of the UIView class (including the view’s frame,center, color, and opacity— among others) can be configured to trigger animations when their values change. You have to do some work to let UIKit know that you want these animations performed, but the animations themselves are created and run automatically for you. For information about how to trigger the built-in view animations, see “Animating Views”.

When you go beyond the basic animations, you must interact more directly with Core Animation classes and methods. The following sections provide information about Core Animation and show you how to work with its classes and methods to create typical animations in iPhone OS. For additional information about Core Animation and how to use it, see Core Animation Programming Guide.

About Layers

The key technology in Core Animation is the layer object. Layers are lightweight objects that are similar in nature to views, but that are actually model objects that encapsulate geometry, timing, and visual properties for the content you want to display. The content itself is provided in one of three ways:

  • You can assign a CGImageRef to the contents property of the layer object.
  • You can assign a delegate to the layer and let the delegate handle the drawing.
  • You can subclass CALayer and override one of the display methods.

When you manipulate a layer object’s properties, what you are actually manipulating is the model-level data that determines how the associated content should be displayed. The actual rendering of that content is handled separately from your code and is heavily optimized to ensure it is fast. All you must do is set the layer content, configure the animation properties, and then let Core Animation take over.

About Animations

When it comes to animating layers, Core Animation uses separate animation objects to control the timing and behavior of the animation. The CAAnimation class and its subclasses provide different types of animation behaviors that you can use in your code. You can create simple animations that migrate a property from one value to another, or you can create complex keyframe animations that track the animation through the set of values and timing functions you provide.

Core Animation also lets you group multiple animations together into a single unit, called a transaction. The CA Transaction object manages the group of animations as a unit. You can also use the methods of this class to set the duration of the animation.

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