OS X Has a Layered Architecture with Key Technologies in Each Layer

It’s helpful to view the implementation of OS X as a set of layers. The lower layers of the system provide the fundamental services on which all software relies. Subsequent layers contain more sophisticated services and technologies that build on (or complement) the layers below.

The lower the layer a technology is in, the more specialized are the services it provides. Generally, technologies in higher layers incorporate lower-level technologies to provide common app behaviors. A good rule of thumb is to use the highest-level programming interface that meets the goals of your app. Here is a brief summary of the layers of OS X.

• The Cocoa (Application) layer includes technologies for building an app’s user interface, for responding to user events, and for managing app behavior.

• The Media layer encompasses specialized technologies for playing, recording, and editing audiovisual media and for rendering and animating 2D and 3D graphics.

• The Core Services layer contains many fundamental services and technologies that range from Automatic Reference Counting and low-level network communication to string manipulation and data formatting.

• The Core OS layer defines programming interfaces that are related to hardware and networking, including interfaces for running high-performance computation tasks on a computer’s CPU and GPU.

• The Kernel and Device Drivers layer consists of the Mach kernel environment, device drivers, BSD library functions (libSystem), and other low-level components. The layer includes support for file systems, networking, security, interprocess communication, programming languages, device drivers, and extensions to the kernel.

You Can Create Many Different Kinds of Software for Mac

Using the developer tools and system frameworks, you can develop a wide variety of software for Mac, including the following:

• Apps. Apps help users accomplish tasks that range from creating content and managing data to connecting with others and having fun. OS X provides a wealth of system technologies such as app extensions and handoff, that you use to extend the capabilities of your apps and enhance the experience of your users.

• Frameworks and libraries. Frameworks and libraries enable code sharing among apps.

• Command-line tools and daemons. Command-line tools allow sophisticated users to manipulate data in the command-line environment of the Terminal app. Daemons typically run continuously and act as servers for processing client requests.

• App plug-ins and loadable bundles. Plug-ins extend the capabilities of other apps; bundles contain code and resources that apps can dynamically load at runtime.

• System plug-ins. System plug-ins, such as audio units, kernel extensions, I/O Kit device drivers, preference panes, Spotlight importers, and screen savers, extend the capabilities of the system.

When Porting a Cocoa Touch App, Be Aware of API Similarities and Differences

The technology stacks on which Cocoa and Cocoa Touch apps are based have many similarities. Some system frameworks are identical (or nearly identical) in each platform, including Foundation, Core Data, and AV Foundation. This commonality of API makes some migration tasks—for example, porting the data model of your Cocoa Touch app—easy.

Other migration tasks are more challenging because they depend on frameworks that reflect the differences between the platforms. For example, porting controller objects and revising the user interface are more demanding tasks because they depend on AppKit and UIKit, which are the primary app frameworks in the Cocoa and CocoaTouch layers, respectively.