Objective-C 快速语法指南：从入门到高级

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Objective-C Quick Syntax Reference.pdf 是一本针对苹果开发者的专业指南，它提供了一种简洁明了的方式，帮助读者快速掌握Objective-C（Objective-C）语言的关键语法和概念。Objective-C作为Apple平台的主要编程语言之一，尽管可能被认为是较为传统的语言，但其在iOS、macOS等应用开发中的地位依然稳固。本书的结构分为多个章节，覆盖了从基础入门到进阶特性的全面内容。第1章介绍了经典的"Hello, World!"程序，让初学者对编程环境和基本流程有初步了解。随后的章节分别讲解了变量（Chapter 3）、运算符（Chapter 4）、对象（Chapter 5）以及字符串（Chapter 6）、数字（Chapter 7）、数组（Chapter 8）和字典（Chapter 9）等数据结构的使用。对于循环控制，书中详细解释了for循环（Chapter 10）、while循环（Chapter 11）、do-while循环（Chapter 12）以及foreach循环（Chapter 13），这些是程序逻辑组织的基础。条件语句（Chapter 14）如if语句和switch语句（Chapter 15）展示了如何根据条件执行不同代码路径。面向对象编程的核心概念在第16章"定义类"中展开，涵盖了类方法（Chapter 17）、继承（Chapter 18）、分类（Category）以及Block（Chapter 20），这些都是构建复杂程序架构的重要组成部分。此外，关键值编码（Key-Value Coding, Chapter 21）和观察（Key-Value Observation, Chapter 22）提供了与对象交互的强大工具。面向协议（Chapter 23）和委托（Chapter 24）则展示了如何通过接口和代理实现多对象间的协作。单例模式（Chapter 25）是软件设计中常见的模式，用于确保只有一个实例存在。错误处理（Chapter 26）是任何程序员都必须重视的，它关乎程序的健壮性。最后，本书还讨论了如何在后台处理任务（Chapter 27），这对于创建响应式的用户界面和高效应用程序至关重要。 Objective-C Quick Syntax Reference.pdf是一本实用的参考手册，无论是初次接触Objective-C的开发者，还是希望巩固现有技能的程序员，都能从中受益匪浅。通过这本书，读者可以迅速掌握Objective-C的关键语法和核心编程技巧，进一步提升在苹果平台上的开发能力。

CHAPTER 3 ■ VARIABLES

Declaring Variables

Variables are declared in Objective-C with their data type first, followed by a variable

name. You must declare a variable before using it. Variable names should be meaningful,

but you can name a variable anything that you want.

Here is how you would declare an integer in Objective-C:

NSUInteger numberOfPeople;

Assigning Values

You can use the assignment operator (=) to assign a value to a variable, like so:

numberOfPeople = 100;

Once you have assigned a value, you can retrieve and use that value by referencing

the variable name.

NSLog(@"The number of people is %lu", numberOfPeople);

Note ■ You may have noticed that the NSLog statement required the %lu symbol. This

symbol is called a format specifier and NSLog will use it as a placeholder to insert values in

the comma-separated list that appears right after the string. See Table 3-1 for a list of the

format specifiers that you must use with Objective-C data types.

You can also declare variables and assign values on the same line if you like.

NSUInteger numberOfGroups = 20;

Integer Types

Integers are whole numbers, so any number that doesn’t need a decimal point is an

integer. In Objective-C, integers are expressed with the data types NSInteger and

NSUInteger.

NSUIntegers are unsigned integers, which means that they can only be positive

numbers. The maximum value that an NSUInteger can take depends on the system for

which the Objective-C code is compiled. If you compile for a 64-bit Mac, the maximum

value will be 18,446,744,073,709,551,615.

For 32-bit platforms like the iPhone 5 and below, the maximum value is

4,294,967,295. You can check these numbers yourself using the NSUIntegerMax constant.

NSLog(@"NSUIntegerMax is %lu", NSUIntegerMax);

CHAPTER 3 ■ VARIABLES

NSIntegers are signed integers, which means that they can be either positive or

negative. The maximum value of an NSInteger is half of the NSUInteger value because

NSInteger must support both positive and negative numbers.

So, if you need huge numbers, you may need to stick to NSUInteger, but if you need

to handle both positive and negative numbers, you will need NSInteger. You can check

the minimum and maximum value of NSInteger on your system with the NSIntegerMin

and NSIntegerMax constants.

NSLog(@"NSIntegerMin is %li", NSIntegerMin);

NSLog(@"NSIntegerMax is %li", NSIntegerMax);

Boolean Types

Boolean date types are used when values can either be true or false. In Objective-C, this

data type is declared as a BOOL type. BOOL types have values that are either YES or NO.

BOOL success = YES;

Since Objective-C stores BOOL values as 1 for YES and 0 for NO, you must use the %i

format specifier print out a BOOL value. %i is another format specifier for integers.

NSLog(@"success is %i", success);

The NSLog statement above will print out 1 for YES and 0 for NO, but some people

prefer to see the YES or NO strings printed out to the log. You can do so using this alternate

statement:

NSLog(@"success: %@", success ? @"YES" : @"NO");

Here the variable success was replaced with a statement that has to be evaluated.

This statement will return either the string YES or the string NO depending on the value

of the variable success. If success is zero, then whatever is in the last position of the

statement is returned, and if success is any other value then whatever is in the first

position is returned. The ternary operator (?) tells the compiler to evaluate the statement.

Float Types

Float types are represented in Objective-C with the CGFloat data type. CGFLoat is what

you use when you want decimal places in your number. For example, if you want to

represent a percent, you may do something like this:

CGFloat percent = 33.34;

You can find the maximum value of CGFloat values for 32-bit systems using FLT_MAX.

For 64-bit systems you must use DBL_MAX.

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