精通Objective-C：iOS开发进阶指南

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"这是一本关于OC（Objective-C）语言编程的书籍，主要针对iOS开发，旨在帮助读者快速提升技能，改正编码习惯，并深入理解语法。书中涵盖了Objective-C的基础到高级概念，包括C语言在Objective-C中的应用、类、字符串与容器类、分类与协议、继承、blocks、动态绑定、内存管理、键值编码、文件系统，以及Foundation框架、编译器、预处理器、单元测试、调试工具的使用，并指导如何在OSX和iOS平台上编写应用程序。" 这本书的内容结构分为四个部分： 1. **第一部分：语言** - 这部分从基础开始介绍Objective-C，包括C语言在Objective-C中的作用，讲解了类、字符串和容器类（如NSArray、NSDictionary等）的使用，接着深入到类别（Category）和协议（Protocol）的概念，以及继承机制。这些章节帮助读者构建扎实的语言基础。 2. **第二部分：参考** - 这部分重点介绍了Foundation框架，这是iOS和OSX开发中不可或缺的部分，提供了许多核心数据类型和系统服务。通过学习这一章，读者可以更好地理解和使用Foundation框架中的各种类和功能。 3. **第三部分：工具** - 这里探讨了编译器的工作原理、预处理器的应用，以及单元测试和调试技巧。这些章节有助于提高开发效率，确保代码质量和可靠性。 4. **第四部分：编写OSX和iOS应用程序** - 最后，书中的这两章分别讲解了如何在OSX桌面平台和iOS移动平台上构建图形用户界面应用程序，包括使用Cocoa或Cocoa Touch框架，以及针对不同平台的特定设计和实现细节。通过阅读这本书，开发者不仅能深入理解Objective-C语言，还能掌握Apple生态系统下的开发实践，从而提升自己的iOS开发能力。书中的每个章节都包含了丰富的示例代码和解释，便于读者理解和应用所学知识。无论是初学者还是有一定经验的开发者，都能从中受益匪浅。

8 CHAPTER 1: The C in Objective-C

declared as signed (when negative numbers are allowed) or unsigned (in case it is known that no

negative number will be stored).

Logical Values

A variable can also store a single logical value, true or false. Although the C language doesn’t have

a specialized Boolean value, in Objective-C you use the BOOL type to represent this kind of variable.

A variable of type BOOL can store the values YES or NO, which can also be spelled as TRUE or FALSE.

The language, however, allows the conversion of any integer type into a Boolean. In that case, the

value zero is converted to FALSE, while anything different from zero is considered to be TRUE. This

may be a source of mistakes for programmers that are not careful with the conversion rules between

these types.

Pointers

Objective-C supports variables that store memory addresses, and this is one of the most flexible

methods to refer to data stored by a program (the use of such variables will be covered later in this

chapter). Pointers are declared using the star notation, and here is an example of declaring pointers

to common types:

int *pointer_to_int;

double *double_pointer;

The first example is a variable that can store the address of an integer. The second is a pointer that

can be used to store the address where a value of type double is contained. The pointer notation is

frequently used when creating objects in Objective-C, as you will see in the next chapter.

The void Type

The keyword void is a special type name used in Objective-C. Unlike the other type names, void

doesn’t represent a real data type, but instead signifies the absence of data, or the fact that the type

is unknown. The type void can appear only in a few situations. One case happens when a function

has no return value. This is indicated by the use of the void keyword in place of the return type.

Another situation in which void is employed is to mark pointers to unknown data types. When a type

is unknown and void is used instead, the resulting pointer will have no information about the actual

data type stored in that memory location. Such a variable can be used in Objective-C as a generic

pointer, which can be applied to any data type. You will see examples of this use when learning

about pointers in a later section.

Note Since void is just a special keyword, it is not possible to declare a variable of void type. For example,

trying to declare a variable like void var; will result in a compilation error, since the compiler won’t be able

to deduce the real type of the variable.

9CHAPTER 1: The C in Objective-C

Comments

Software used in a large-scale project is not always easy to understand. Although every effort should

be made to make code easier to comprehend, complicated algorithms and other assumptions may

hinder the understanding of a particular area of the program. That is why there is always the need for

adding documentation to the program itself.

Objective-C uses two different styles of code comments. The first style, and the simplest one to use,

employs the sequence // as the beginning of a new comment. Any comment started this way will be

in force until the end of the line.

int integerVariable; // this variable holds an integer

float floatVariable; // a floating number variable

A second comment style is used for comments that span multiple lines. In that case, the comment is

started with the characters /*. All text from that position on is considered to be part of the comment,

until the pair of characters */ are reached.

/* this is an example of multiple line comments. The

following expressions are used to define variables of

the type integer and floating point, respectively */

int integerVariable;

float floatVariable;

Multi-line comments cannot be embedded. That is, if you already have a comment, you can’t just

use a second pair of comment characters around them. That will not work because the first time the

characters */ are seen, they will close any multi-line comment started previously, not just the latest.

/* a new comment

/* original comment, not embedded */

this is not a comment, since the first multi line comment

was already closed. */

Note One of the reasons why some programmers avoid the use of multi-line comments is that they can

lead to confusion, as noted above. To avoid this, use only single-line comments, and reserve multi-line

comments for specific situations, such as the general comment at the top of an Objective-C file.

Arithmetic Expressions

One of the simplest ways to use variables is as part of arithmetic expressions. These are used to

calculate numeric quantities, using one of the common operators such as addition, subtraction,

multiplication, and division.

10 CHAPTER 1: The C in Objective-C

Objective-C has a rich set of operators that can be used in numeric expressions. The most evident

way to use such operators is to express the four common arithmetic operations. A not-so-obvious

operator, however, is the remainder operator. It returns the integer remainder of the division of the

first number by the second.

int remainder = 10 % 4;

The result of this operation should be equal to two. A common use for this kind of expression is to

decide if a number is even.

if (number % 2 == 0)

{

/* this is an even number */

}

Expressions can be combined using parenthesis, so that programmers can control the order in

which the operations will be grouped by the compiler.

int value = 25;

int result = (10 + value) * (2 / value);

Function Fundamentals

Objective-C programs are composed of a mix of functions, declarations, and classes. Functions are

the basic concept used by C programs, while classes are the basic extension mechanism provided

by Objective-C. Accordingly, in this section we will only discuss functions; we’ll introduce classes in

the next chapter.

Every program starts in a function called main. The main function is not called from any part of

the program, but instead it is called by the programming environment whenever the application is

started. The program ends exactly when the main function returns. Here is a simple example of a

main function:

int main()

{

NSLog(@"hello to objective-C");

return 0;

}

The purpose of this program is to write a single logging string. The program achieves this using the

NSLog function, which is part of the Foundation framework provided by Objective-C, and therefore

is not available in standard C programs. The argument to NSLog is a string literal that starts with the

@ character; such string literals are also exclusive to Objective-C. Everything else in the program is

there just to comply with the requirements of the language.

The name of the function is main, and the pair of parenthesis after the name of the function is used to

determine any parameters that are required as arguments. In this particular case, the main function

has been defined with an empty set of parameters, meaning that there are no arguments. Some

applications, however, expect to receive arguments, which can be provided from the command line.

If that is the case, the main function can also be declared in a slightly different way.

11CHAPTER 1: The C in Objective-C

int main(int argc, char *argv[])

{

NSLog(@"hello from objective-C");

return 0;

}

In this example, the program is receiving two parameters: the first is an integer value argc, also

known as the argument counter, which specifies the number of arguments passed to the program.

The second parameter, argv, is an array of pointers to characters, which can be used to store each

of the arguments passed to the application. In future examples, you will learn how to use these

arguments in your programs.

Defining New Functions

A function is a simple way to package a set of instructions so that programmers don’t need to repeat

themselves. For example, here is a simple function to calculate the average of two numbers:

float avg(float a, float b)

{

return (a+b)/2;

}

There are several advantages in using functions whenever possible.

Simplifying a program: Whenever a common operation needs to be performed

repeatedly, you can instead create a function that encapsulates the operation.

In the example above, by calling the function avg you don’t need to repeat in

other parts of your code the addition and division operations used to calculate

the average. This may seem unnecessary here because the function is so

simple. However, when an algorithm is more complex, it becomes even more

important to streamline its use. For example, you could have a routine for

mathematical integration as part of a large-scale engineering package. Or you

could have a window-drawing routine in a graphical application. In each case,

using functions will result in a simplification of the resulting program.

Improving maintenance: Things change, and programmers have to adapt to

change on a daily basis. If the client decides to modify the way a calculation is

made and you are not using a function, then the required changes have to be

propagated to all places where the calculation is currently done. On the other

hand, if you were clever enough to use a function instead, there’s only one

place where the change needs to be applied. Every place in the code where

the function is called will use the updated version automatically. Such use of

functions can reduce the maintenance burden. In some cases, modifications can

be avoided in hundreds or thousands of lines of code—and remain contained to

just a single place.

12 CHAPTER 1: The C in Objective-C

Improving documentation: Creating a function is also a way to document a

procedure. Whenever you create a function, you need to give it a name that

describes what it does. If this is done well, callers will have at least a high-level idea

of what the function does. This technique can be seen as a simple but effective

form of self-documentation. Such a practice, when use regularly by everyone in

a programming project, can result in code that is much more maintainable and

future-proof.

Because of all these advantages, functions are widely used in Objective-C, and are in fact the basic

principle of code organization in the C language as well. You will see in the next chapter that classes

can be seen as a generalization of functions, so that you can achieve even more by using these

principles.

Function Syntax

The syntax of a function is determined by its input arguments and result type, along with the

braces and the set of statements contained within them. The argument list is the set of parameters

displayed inside parenthesis, after the name of the function. Even if no argument is needed by a

function, it will have a pair of empty parenthesis after its name. The name of the function is preceded

by the type of values that are returned as result. For example,

int add_values(int a, int b)

{

return a + b;

}

will always return an integer value that depends on the two arguments a and b.

The following examples in this chapter will use the syntax for functions, so that you can have an idea

of how functions may be used in programming projects.

Conditional Operations

A conditional operation is used to codify logical statements. In a programming language like

Objective-C, any expression that produces a non-zero value is considered to be true, while zero

(or NULL) values are considered to be false. Logical operations are used to manipulate such true or

false expressions, and form the basis for conditional execution using the if statement, for example.

Consider the following code fragment:

void test_function(int x)

{

if (x > 10)

NSLog(@"Number is greater than 10");

}

This function illustrates the use of the expression (x > 10) as part of an if statement. Logical

expressions can use one of the operators: >, <, >=, <=, ==, &&, ||, or !. The first five operators

provide a way to express the logical relations of greater, less, greater than, less than, or equal to.

They can be applied to numbers or characters, as well as to Boolean values.

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