理解Objective-C：苹果平台的核心编程语言

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“Sams Teach Yourself iOS 6 Application Development in 24 Hours Chapter 03 - Chpater 3.pdf 第3章 - 最新的iOS6教程，包含Xcode详解和教程” 在这一章节中，我们将深入探讨Objective-C，这是苹果平台（包括iOS 6）上的主要开发语言。本小时的学习目标主要包括： 1. **Objective-C在项目中的应用**：Objective-C是编写iOS应用的基础，它提供了构建应用程序所需的各种语法和结构。不论是在OS X还是iOS上，Objective-C都是开发者的主要工具。 2. **面向对象编程基础**：了解面向对象编程（OOP）的概念至关重要。OOP是一种编程范式，通过类和对象来组织代码，使其更易于理解和维护。Objective-C基于C语言，并在其之上添加了面向对象的特性。 3. **Objective-C的简单语法**：Objective-C的语法与C语言类似，但增加了消息传递机制，使得对象之间的通信更加直观。例如，方法调用采用`[receiver message]`的格式，这在C++和Java中是看不到的。 4. **常用数据类型**：Objective-C支持基本的C语言数据类型，如int、float、char等，同时也引入了对象类型，如NSString、NSArray等，这些对象类型提供了丰富的功能和便利性。 5. **ARC（Automatic Reference Counting）与内存管理**：在iOS 6中，苹果引入了ARC技术，它自动处理对象的内存管理，大大简化了开发者的工作。理解ARC如何工作以及何时使用`strong`、`weak`、`autoreleasing`等引用计数属性对于避免内存泄漏至关重要。 6. **学习Objective-C的挑战与乐趣**：尽管Objective-C可能对初学者来说有些复杂，但一旦掌握，它的灵活性和表达力往往会让开发者对其爱不释手。本小时的教程将帮助你逐步熟悉Objective-C，开始在苹果平台上编写应用的旅程。通过本章的学习，你将能够开始理解Objective-C的基本概念，并能够使用它来构建简单的iOS应用。随着对语言的深入理解，你将能够利用其强大的特性，如类别（categories）、协议（protocols）和动态性（dynamic nature），来创建复杂且高效的应用程序。同时，结合Xcode的使用，你可以构建用户界面，调试代码，进行性能优化，从而全面掌握iOS应用开发。

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HOUR 3: Discovering Objective-C: The Language of Apple Platforms

CAUTION

Case Counts

Objective-C is case sensitive. If a program is failing, make sure you aren’t mixing case some-

where in the code.

Now that you have an idea of what OOP and Objective-C are, let’s take a look at how you’ll be

using them over the course of this book.

Exploring the Objective-C File Structure

In the preceding hour, you learned how to use Xcode to create projects and navigate their files.

As mentioned then, the vast majority of your time will be spent in the project group of Xcode,

which is shown for the MyNewApp project in Figure 3.1 . You’ll be adding methods to class files

that Xcode creates for you when you start a project or, occasionally, creating your own class files

to implement entirely new functionality in your application.

FIGURE 3.1

Most of your coding will occur within the files in your project group.

Okay, sounds simple enough, but where will the coding take place? If you create a new project

look, you’ll see quite a few different files staring back at you.

Header/Interface Files

Creating a class creates two different files: an interface (or header) file (.h extension) and an

implementation file (.m extension). The interface file is used to define a list of all of the meth-

ods and properties that your class will be using. This is useful for other pieces of code, including

Exploring the Objective-C File Structure

Interface Builder (IB) (which you learn about in Hour 5 , “Exploring Interface Builder”), to deter-

mine how to access information and features in your class.

The implementation file, however, is where you go to write the code that makes everything

defined in the header file work. Let’s review the structure of the very short, and entirely made-up,

interface file in Listing 3.1 .

LISTING 3.1 A Sample Interface File

1 : #import <UIKit/UIKit.h>

3: @interface myClass : myParent <myProtocol> {

4: NSString *_myString;

5: IBOutlet UILabel *_myLabel;

6: }

8: @property ( weak , nonatomic ) NSString *myString;

9: @property ( weak , nonatomic ) NSString *myOtherString;

10: @property ( weak , nonatomic ) IBOutlet UILabel *myOtherLabel;

11:

12: +( NSString )myClassMethod:( NSString * )aString;

13:

14: -( NSDate )myInstanceMethod:( NSString * )aString anotherParam:( NSURL * )aURL;

15:

16:

@end

CAUTION

Line Numbers Are for Reference Only!

Throughout the book, I present code samples like this one. Often, they include line numbers so

that I can easily reference the code and explain how it works. Objective-C does not require line

numbers, nor will the code work if you leave them in your application. If you see a line prefixed

with a number and a colon (#:), don’t type the line number prefix!

The #import Directive

1: #import <UIKit/UIKit.h>

First, in line 1, the interface file uses the #import directive to include any other interface files

that our application will need to access. The string

<UIKit/UIKit.h> designates the specific file

(in this case,

UIKit , which gives us access to a vast majority of the classes). If we need to import

a file, we explain how and why in the text. The UIKit example is included by default when

Xcode sets up your classes and covers most of what you need for this book’s examples.

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