Introduction to IOS
iOS is the operating system that runs on iPad, iPhone, and iPod
touch devices. The operating system manages the device hardware and provides
the technologies required to implement native apps. The operating system also
ships with various system apps, such as Phone, Mail, and Safari, that provide
standard system services to the user.
The iOS
Software Development Kit (SDK) contains the tools and interfaces
needed to develop, install, run, and test native apps that appear on an iOS
device’s Home screen. Native apps are built using the iOS system frameworks and
Objective-C language and run directly on iOS. Unlike web apps, native apps are
installed physically on a device and are therefore always available to the
user, even when the device is in Airplane mode. They reside next to other
system apps, and both the app and any user data is synced to the user’s
computer through iTunes.
At the highest level,
iOS acts as an intermediary between the underlying hardware and the apps you
create. Apps do not talk to the underlying hardware directly. Instead, they
communicate with the hardware through a set of well-defined system interfaces.
These interfaces make it easy to write apps that work consistently on devices
having different hardware capabilities.
The implementation of
iOS technologies can be viewed as a set of layers, which are shown in Figure I-1. Lower layers contain
fundamental services and technologies. Higher-level layers build upon the lower
layers and provide more sophisticated services and technologies.
As you write
your code, it is recommended that you prefer the use of higher-level frameworks
over lower-level frameworks whenever possible. The higher-level frameworks are
there to provide object-oriented abstractions for lower-level constructs. These
abstractions generally make it much easier to write code because they reduce
the amount of code you have to write and encapsulate potentially complex
features, such as sockets and threads. You may use lower-level frameworks and
technologies, too, if they contain features not exposed by the higher-level
frameworks.
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