Chapter 1. Cocoa Touch: The Core iPhone

Cocoa is a collection of tools—libraries, frameworks, and APIs—used to build applications for the Mac OS. Most of the core functionality you would need to develop a rich Mac application is included in Cocoa. There are mechanisms for drawing to display, working with text, saving and opening data files, talking to the operating system, and even talking to other computers across a network. The look and feel of Mac applications is recognizable and relatively consistent in large part because of the breadth and quality of the Cocoa user interface framework.

The Cocoa frameworks include two areas of focus: classes that represent user interface objects and collect user input, and classes that simplify challenges like memory management, networking, filesystem operations, and time management.

Developing applications for the iPhone and iPod Touch is similar in many ways to building applications for Mac OS X. The same tools are used for writing and debugging code, laying out visual interfaces, and profiling performance, but mobile application development requires a supplemental set of software libraries and tools, called the iPhone SDK (software development kit).

Cocoa Touch is a modified version of Cocoa with device-specific libraries for the iPhone and iPod Touch. Cocoa Touch works in conjunction with other layers in the iPhone and iPod Touch operating systems and is the primary focus of this book.

Mac Frameworks

Mac OS X programmers use a framework called AppKit that supplies all the windows, buttons, menus, graphics contexts, and event handling mechanisms that have come to define the OS X experience. The Cocoa Touch equivalent is called UIKit. In addition to user interface elements, UIKit provides event handling mechanisms and handles drawing to the screen. UIKit is a very rich framework and is a major focus of user experience programmers. Nearly all user interface needs are accounted for in UIKit, and developers can create custom UI elements very easily. Many of the user experience problems and patterns addressed in this book will focus on UIKit programming with an emphasis on standard solutions.

The second Cocoa Touch framework is the Foundation framework. You can think of Foundation as the layer that abstracts many of the underlying operating system elements such as primitive types, bundle management, file operations, and networking from the user interface objects in UIKit. In other words, Foundation is the gateway to everything not explicitly part of the user interface. As you’ll see in this book, user experience programming goes deeper than the user interface controls and includes things such as latency management, error handling, data caching, and data persistence.

UIKit Overview

The user interface comprises the elements of a device or application that users see, click, and—in the case of Cocoa Touch—tilt, shake, or tap. User interfaces are a big part of user experience. They provide the face of your product, and often a bit more.

For the most part, UIKit is just a limited subset of the AppKit framework for Mac OS X. If you have experience developing Cocoa apps for the Mac, you will get your head around UIKit fairly quickly. The main differences are that UIKit is tuned for specific hardware interfaces and that it provides less functionality than AppKit. The reduced scope of UIKit is primarily due to the differences in robustness between typical computers and the iPhone or iPod Touch. Despite the omission of a few familiar elements, UIKit is a very capable toolset.

The best way to understand the breadth of UIKit is with a visual topology of the framework. Figures 1-1 and 1-2 show the layout of UIKit.

The core class from which all Cocoa objects inherit basic behavior is NSObject. The NS prefix has roots in the non-Apple origins of Cocoa at NeXT. The early versions of what is now Cocoa were called NextStep. Most Cocoa classes in Cocoa are subclasses of NSObject, and many classes assume that NSObject is the foundation of objects being passed around. For example, the class NSArray, which represents a collection of pointers, requires that any pointer it stores points to an NSObject subclass. In most cases, any custom class you create should inherit from NSObject.

The names of classes in Cocoa are often very descriptive. The following illustrations give an overview of the classes in UIKit. The purpose is to provide a view of the entire collection of classes so that developers of all experience levels can see the breadth of the frameworks.

In UIKit, all classes that respond to user input inherit from UIResponder, which is an NSObject subclass that provides functionality around handling user input. Figure 1-1 focuses on the subclasses of UIResponder.

UIKit classes: UIResponder tree
Figure 1-1. UIKit classes: UIResponder tree

In addition to the UIResponder class hierarchy, UIKit includes a set of classes acting as value objects, logical controllers, and abstractions of hardware features. Figure 1-2 shows these classes.

The documentation sets that accompany the iPhone SDK, in tandem with the Apple Developer Connection website, cover all the details of the Cocoa and Cocoa Touch framework classes. This book will further elaborate on key classes in the context of interaction design patterns and overall user experience, but the official documentation should remain the primary reference for developers, as each update to the iPhone SDK includes amended or edited documentation packages.

UIKit classes: controllers, value objectslogical controllershardware feature abstractionsvalue objects, device classes
Figure 1-2. UIKit classes: controllers, value objects, device classes

Foundation Overview

The Foundation layer of Cocoa Touch (and Cocoa on the Mac) provides an object-oriented abstraction to the core elements of the operating system. Foundation handles core features of Cocoa Touch, including:

  • Essential object behavior, such as memory management mechanisms

  • Inter-object notification mechanisms, such as event dispatching

  • Access to resource bundles (files bundled with your application)

  • Internationalization and localization of resources, such as text strings and images

  • Data management tools (SQLite, filesystem access)

  • Object wrappers of primitive types, such as NSInteger, NSFloat, and NSString

All Cocoa Touch applications must link against Foundation because Foundation contains the classes that make a Cocoa application work—including many classes that are integral in the functioning of the user interface framework. For example, many UIKit methods use NSString objects as arguments or return values from methods.

The Foundation class tree as supported by Cocoa Touch is illustrated in Figures 1-3 to 1-9. The class hierarchy diagrams are logically grouped according to coarse functionality. This conceptual grouping mirrors the organization used by Apple in the Cocoa Fundamentals Guide included in the developer documentation. You should consult the Cocoa Fundamentals Guide and the class documentation provided by Apple as part of the iPhone SDK install for updated, in-depth information about the framework classes.

Figure 1-3 shows a subset of NSObject subclasses that represent value objects. Value objects are used to represent non-functional values—primitives, dates, generic binary data—and to provide utility methods for those values.

Value objects
Figure 1-3. Value objects

Figure 1-4 maps the NSObject subclasses that focus on XML and string management. The XML classes are particularly useful when working with web services. Strings are used constantly, and Cocoa developers spend a lot of time working with NSString instances.

Foundation provides powerful classes for collection management. These classes are shown in Figure 1-5. Standard collection types such as arrays, sets, and hash tables are included, along with classes used for enumerating through collections.

XML and strings
Figure 1-4. XML and strings
Figure 1-5. Collections

Figure 1-6 illustrates classes that focus on operating system services, file operations, and inter-process communication (IPC).

Operating system services: operations, file operations, interprocess communication
Figure 1-6. Operating system services: operations, file operations, interprocess communication

Figure 1-7 illustrates the portion of Foundation that provides for URL handling functionality. Hybrid web/Cocoa applications use URL handling classes heavily.

Figure 1-8 shows the classes used to manage threading in Cocoa applications. Careful thread management can be an important part of optimizing the perception of responsiveness in an application.

Operating system services: URL handling
Figure 1-7. Operating system services: URL handling
Operating system services: locking and thread management
Figure 1-8. Operating system services: locking and thread management

Finally, Figure 1-9 shows classes providing notifications, archiving, and core language features such as exceptions.

Notifications, archiving and serialization, Objective-C language services
Figure 1-9. Notifications, archiving and serialization, Objective-C language services

Garbage Collection

One notable difference between the iPhone OS and Mac OS X is that the implementation of Foundation for Cocoa Touch does not automatically recover memory when objects are destroyed. This means developers must keep track of the objects they create and follow certain idioms in order to keep memory usage as low as possible.

The newest version of the Objective-C programming language, version 2.0, added support for automatic resource management, or garbage collection. Developers who have grown accustomed to using garbage collection in Cocoa applications on the Mac may find its omission in the iPhone SDK inconvenient. The performance implications of most garbage collection implementations are important considerations on mobile devices, and this is a key reason it was excluded from the iPhone. Battery life and processing speed are important elements of an elegant user experience, and often act as differentiating factors among competing devices in the consumer marketplace.

The Devices

The screen on both the iPhone and iPod Touch is an LCD-lit 3.5-inch (diagonal) widescreen Multi-Touch display. The screen resolution is 480×320 pixels at a resolution of 163 pixels per inch. The devices include the following sensors: accelerometer, proximity sensor, and ambient light sensor.

  • When activated, the accelerometer is used to detect device movement in space, providing information about movement along three axes.

  • The proximity sensor recognizes the proximity of the handset to another object, most commonly a human ear.

  • The ambient light sensor detects the level of ambient light hitting the device.

Both the iPhone and iPod Touch devices provide rocker switches for controlling volume, a hardware power button, and a depressible “home” button. These concrete interfaces are outside the scope of Cocoa Touch programming, but are notable in the overall UX (user experience) of the devices.

From the point of view of user experience programmers, the hardware interface elements are separate from the touch interface. Apple doesn’t provide any means of accessing the home button, lock button, volume controls, or the navigation controls included on headsets. This simplifies the domain of UX programming, but comes at a cost: there are certainly cases in which access to the home button or volume rocker could provide enhanced functionality to users.

The iPhone provides a few distinct advantageous features over the iPod Touch, aside from the telephony. For example, the iPhone includes GPS support and a hardware ringer/silence switch. For networking, in addition to 802.11g support, the iPhone 3G includes support for 3G wireless networking, and support for High-Speed Downlink Packet Access (HSDPA). The operating system will, unless told otherwise, automatically switch from 3G to wireless networks with known names. This feature is part of a larger UX pattern that is core to the iPhone: attempting to deliver the best experience possible given the environment, without interruption. We will cover this pattern in Chapter 8.

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