Objective-C编程全面指南：从入门到实战

需积分: 9 192 浏览量更新于2024-07-28 收藏 1.15MB PDF 举报

Objective-C教程是一份全面介绍Objective-C编程语言的指南，它旨在帮助读者从基础知识入门并逐步深入实践。该教程由Apple Inc.于2009年发布，版权归属Apple，强调了未经授权复制或传播文档的必要限制。 Objective-C是一种面向对象的编程语言，是Apple开发的，主要用于iOS、macOS和其他Apple平台的应用程序开发。本教程首先介绍了Objective-C的核心概念，包括类和对象、消息传递机制、继承与多态性等，这些都是Objective-C程序设计的基础。通过这部分学习，开发者将理解如何创建和操作对象，以及如何利用Objective-C的动态类型系统进行高效编程。工具与语言部分，教程可能涵盖了Xcode（Apple的集成开发环境，IDE）的使用，这是编写Objective-C代码的主要平台，提供了丰富的编辑器、构建工具和调试功能。此外，还可能涉及Objective-C与C和Objective-C++之间的关系，因为Objective-C是基于C语言的，同时也支持C++的一些特性。 2009年的版本可能着重于当时的语言特性和框架，如Foundation框架和UIKit（用于iOS开发），这些是构建现代应用不可或缺的部分。然而，随着时间的推移，Objective-C逐渐被Swift取代，作为Apple的新一代编程语言，但Objective-C在遗留项目和库维护中仍然有其重要地位。文档中强调个人使用的版权规定，表明阅读者只能在个人电脑上存储文档，并且可以打印用于个人学习，但不能进行任何形式的商业复制或传播，除非获得Apple的书面许可。同时，未经允许使用带有苹果商标的键盘快捷键（如Option-Shift-K）用于商业目的可能会构成商标侵权和不公平竞争。最后，教程提醒读者，文档中所描述的技术并不授予任何许可，Apple保留所有与此技术相关的知识产权。该文档仅为提供应用开发的辅助，旨在帮助开发者更好地理解和运用Objective-C技术。对于那些希望在现代软件开发环境中继续使用Objective-C的人来说，理解这份教程仍然是宝贵的知识资源。

[receiver message]

The receiver is an object, and the message tells it what to do. In source code, the message is simply the name

of a method and any arguments that are passed to it. When a message is sent, the runtime system selects

the appropriate method from the receiver’s repertoire and invokes it.

For example, this message tells the myRectangle object to perform its display method, which causes the

rectangle to display itself:

[myRectangle display];

The message is followed by a “;” as is normal for any line of code in C.

The method name in a message serves to “select” a method implementation. For this reason, method names

in messages are often referred to as selectors.

Methods can also take parameters, or “arguments.” A message with a single argument affixes a colon (:) to

the selector name and puts the argument right after the colon. This construct is called a keyword; a keyword

ends with a colon, and an argument follows the colon, as shown in this example:

[myRectangle setWidth:20.0];

A selector name includes all keywords, including colons, but does not include anything else, such as return

type or parameter types. The imaginary message below tells the myRectangle object to set its origin to the

coordinates (30.0, 50.0):

[myRectangle setOrigin:30.0 :50.0]; // This is a bad example of multiple arguments

Since the colons are part of the method name, the method is named setOrigin::. It has two colons as it

takes two arguments. This particular method does not interleave the method name with the arguments and,

thus, the second argument is effectively unlabeled and it is difficult to determine the kind or purpose of the

method’s arguments.

Instead, method names should interleave the name with the arguments such that the method's name naturally

describes the arguments expected by the method. For example, the Rectangle class could instead implement

a setOriginX:y: method that makes the purpose of its two arguments clear:

[myRectangle setOriginX: 30.0 y: 50.0]; // This is a good example of multiple

arguments

Object Messaging

CHAPTER 1

Objects, Classes, and Messaging

Important: The sub-parts of the method name—of the selector—are not optional, nor can their order be

varied. “Named arguments” and “keyword arguments” often carry the implication that the arguments to a

method can vary at runtime, can have default values, can be in a different order, can possibly have additional

named arguments. This is not the case with Objective-C.

For all intents and purposes, an Objective-C method declaration is simply a C function that prepends two

additional arguments (see Messaging in the Objective-C Runtime Programming Guide). This is different from

the named or keyword arguments available in a language like Python:

def func(a, b, NeatMode=SuperNeat, Thing=DefaultThing):

pass

where Thing (and NeatMode) might be omitted or might have different values when called.

Methods that take a variable number of arguments are also possible, though they’re somewhat rare. Extra

arguments are separated by commas after the end of the method name. (Unlike colons, the commas aren’t

considered part of the name.) In the following example, the imaginary makeGroup: method is passed one

required argument (group) and three that are optional:

[receiver makeGroup:group, memberOne, memberTwo, memberThree];

Like standard C functions, methods can return values. The following example sets the variable isFilled to

YES if myRectangle is drawn as a solid rectangle, or NO if it’s drawn in outline form only.

BOOL isFilled;

isFilled = [myRectangle isFilled];

Note that a variable and a method can have the same name.

One message expression can be nested inside another. Here, the color of one rectangle is set to the color of

another:

[myRectangle setPrimaryColor:[otherRect primaryColor]];

Objective-C also provides a dot (.) operator that offers a compact and convenient syntax for invoking an

object’s accessor methods. This is typically used in conjunction with the declared properties feature (see

“Declared Properties” (page 67)), and is described in “Dot Syntax” (page 20).

Sending Messages to nil

In Objective-C, it is valid to send a message to nil—it simply has no effect at runtime. There are several

patterns in Cocoa that take advantage of this fact. The value returned from a message to nil may also be

valid:

■ If the method returns an object, then a message sent to nil returns 0 (nil), for example:

Person *motherInLaw = [[aPerson spouse] mother];

If aPerson’s spouse is nil, then mother is sent to nil and the method returns nil.

■ If the method returns any pointer type, any integer scalar of size less than or equal to sizeof(void*),

a float, a double, a long double, or a long long, then a message sent to nil returns 0.

Object Messaging 17

CHAPTER 1

Objects, Classes, and Messaging

■ If the method returns a struct, as defined by the Mac OS X ABI Function Call Guide to be returned in

registers, then a message sent to nil returns 0.0 for every field in the data structure. Other struct data

types will not be filled with zeros.

■ If the method returns anything other than the aforementioned value types the return value of a message

sent to nil is undefined.

The following code fragment illustrates valid use of sending a message to nil.

id anObjectMaybeNil = nil;

// this is valid

if ([anObjectMaybeNil methodThatReturnsADouble] == 0.0)

{

// implementation continues...

}

Note: The behavior of sending messages to nil changed slightly with Mac OS X v10.5.

On Mac OS X v10.4 and earlier, a message to nil also is valid, as long as the message returns an object, any

pointer type, void, or any integer scalar of size less than or equal to sizeof(void*); if it does, a message

sent to nil returns nil. If the message sent to nil returns anything other than the aforementioned value

types (for example, if it returns any struct type, any floating-point type, or any vector type) the return value

is undefined. You should therefore not rely on the return value of messages sent to nil unless the method’s

return type is an object, any pointer type, or any integer scalar of size less than or equal to sizeof(void*).

The Receiver’s Instance Variables

A method has automatic access to the receiving object’s instance variables. You don’t need to pass them to

the method as arguments. For example, the primaryColor method illustrated above takes no arguments,

yet it can find the primary color for otherRect and return it. Every method assumes the receiver and its

instance variables, without having to declare them as arguments.

This convention simplifies Objective-C source code. It also supports the way object-oriented programmers

think about objects and messages. Messages are sent to receivers much as letters are delivered to your home.

Message arguments bring information from the outside to the receiver; they don’t need to bring the receiver

to itself.

A method has automatic access only to the receiver’s instance variables. If it requires information about a

variable stored in another object, it must send a message to the object asking it to reveal the contents of

the variable. The primaryColor and isFilled methods shown above are used for just this purpose.

See “Defining a Class” (page 35) for more information on referring to instance variables.

Polymorphism

As the examples above illustrate, messages in Objective-C appear in the same syntactic positions as function

calls in standard C. But, because methods “belong to” an object, messages behave differently than function

calls.

Object Messaging

CHAPTER 1

Objects, Classes, and Messaging

In particular, an object can be operated on by only those methods that were defined for it. It can’t confuse

them with methods defined for other kinds of object, even if another object has a method with the same

name. This means that two objects can respond differently to the same message. For example, each kind of

object sent a display message could display itself in a unique way. A Circle and a Rectangle would respond

differently to identical instructions to track the cursor.

This feature, referred to as polymorphism, plays a significant role in the design of object-oriented programs.

Together with dynamic binding, it permits you to write code that might apply to any number of different

kinds of objects, without you having to choose at the time you write the code what kinds of objects they

might be. They might even be objects that will be developed later, by other programmers working on other

projects. If you write code that sends a display message to an id variable, any object that has a display

method is a potential receiver.

Dynamic Binding

A crucial difference between function calls and messages is that a function and its arguments are joined

together in the compiled code, but a message and a receiving object aren’t united until the program is

running and the message is sent. Therefore, the exact method that’s invoked to respond to a message can

only be determined at runtime, not when the code is compiled.

The precise method that a message invokes depends on the receiver. Different receivers may have different

method implementations for the same method name (polymorphism). For the compiler to find the right

method implementation for a message, it would have to know what kind of object the receiver is—what

class it belongs to. This is information the receiver is able to reveal at runtime when it receives a message

(dynamic typing), but it’s not available from the type declarations found in source code.

The selection of a method implementation happens at runtime. When a message is sent, a runtime messaging

routine looks at the receiver and at the method named in the message. It locates the receiver’s implementation

of a method matching the name, “calls” the method, and passes it a pointer to the receiver’s instance variables.

(For more on this routine, see Messaging in the Objective-C Runtime Programming Guide.)

This dynamic binding of methods to messages works hand-in-hand with polymorphism to give object-oriented

programming much of its flexibility and power. Since each object can have its own version of a method, a

program can achieve a variety of results, not by varying the message itself, but by varying just the object

that receives the message. This can be done as the program runs; receivers can be decided “on the fly” and

can be made dependent on external factors such as user actions.

When executing code based upon the Application Kit, for example, users determine which objects receive

messages from menu commands like Cut, Copy, and Paste. The message goes to whatever object controls

the current selection. An object that displays text would react to a copy message differently from an object

that displays scanned images. An object that represents a set of shapes would respond differently from a

Rectangle. Since messages don’t select methods (methods aren’t bound to messages) until runtime, these

differences are isolated in the methods that respond to the message. The code that sends the message

doesn’t have to be concerned with them; it doesn’t even have to enumerate the possibilities. Each application

can invent its own objects that respond in their own way to copy messages.

Objective-C takes dynamic binding one step further and allows even the message that’s sent (the method

selector) to be a variable that’s determined at runtime. This is discussed in the section Messaging in the

Objective-C Runtime Programming Guide.

Object Messaging 19

CHAPTER 1

Objects, Classes, and Messaging

剩余136页未读，继续阅读

每天进步一..

粉丝: 235
资源: 4

Objective-C编程全面指南：从入门到实战

Objective-C语言教程

Objective-C基础教程(入门教程).pdf

objective-c语言教程

objective-c教程

objective-c基础教程 第2版 pdf

objective-c入门教程 pdf

objectivec基础教程 第2版 pdf

swift unreal 混编

ios开发 java

class-dump教程

最新资源

objective-c基础教程第2版 pdf

objectivec基础教程第2版 pdf