Objective-C编程语言指南：iOS应用开发基石

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Objective-C编程语言是一种面向对象的编程语言，主要用于开发iOS和Mac OS X平台的应用程序。由Apple Inc.创建并维护，最初是为了支持其在iPhone和iPad等设备上的应用开发而设计的。该语言结合了C和Smalltalk的特性，提供了一个强大且灵活的开发环境。标题"TheObjective-CProgrammingLanguage"明确了主题，它详细介绍了Objective-C的核心概念、工具和语言特性。该文档于2011年发布，版权属于Apple Inc.，强调了任何复制或传播文档内容都必须得到书面许可，除非是出于个人学习目的，打印文档也需保留版权信息。 Objective-C的工具与语言环境包括了Apple提供的Cocoa框架，这是一组用于构建高性能图形用户界面（GUI）的类库，使开发者能够更高效地开发应用程序。Cocoa允许开发者利用Objective-C进行事件驱动编程，同时也支持MVC（Model-View-Controller）设计模式，这是一种组织代码结构的方法，有助于模块化和重用。文档中提到的"2011-10-12"可能是版本更新的时间，表明此文档反映了当时的最新进展。文档地址位于 InfiniteLoop，Cupertino, CA 95014，这是Apple的总部所在地。文档强调，所有关于技术的描述只适用于Apple品牌的计算机，这意味着开发者只能使用Objective-C来为苹果设备开发应用，并且苹果保留了所有与所描述技术相关的知识产权。此外，苹果明确指出，没有针对文档中技术的直接或隐含许可，任何未经授权的商业使用都是不允许的。 Objective-C编程语言的学习者可以通过这份文档深入了解其语法、类和对象的概念、以及如何在Cocoa平台上构建iOS和Mac应用。对于iOS开发者来说，掌握Objective-C是至关重要的，因为它不仅是iOS应用开发的基础，也是理解和构建现代苹果生态系统的关键组成部分。

Memory Management

In any program, it is important to ensure that objects are deallocated when they are no longer

needed—otherwise your application’s memory footprint becomes larger than necessary. It is also important

to ensure that you do not deallocate objects while they’re still being used.

Objective-C offers three mechanisms for memory management that allow you to meet these goals:

● Automatic Reference Counting (ARC), where the compiler reasons about the lifetimes of objects.

● Manual Reference Counting (MRC, sometimes referred to as MRR for “manual retain/release”), where

you are ultimately responsible for determining the lifetime of objects.

Manual reference counting is described in Advanced Memory Management Programming Guide.

● Garbage collection, where you pass responsibility for determining the lifetime of objects to an automatic

“collector.”

Garbage collection is described in Garbage Collection Programming Guide. (Not available for iOS—you

cannot access this document through the iOS Dev Center.)

Object Messaging

This section explains the syntax of sending messages, including how you can nest message expressions. It

also discusses the scope or “visibility” of an object’s instance variables, and the concepts of polymorphism

and dynamic binding.

Message Syntax

To get an object to do something, you send it a message telling it to apply a method. In Objective-C, message

expressions are enclosed in brackets:

[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 parameters 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 statement in C.

Because the method name in a message serves to “select” a method implementation, method names in

messages are often referred to as selectors.

Methods can also take parameters, sometimes called arguments. A message with a single parameter affixes

a colon (:) to the name and puts the parameter right after the colon:

Object Messaging 15

CHAPTER 1

Objects, Classes, and Messaging

[myRectangle setWidth:20.0];

For methods with multiple parameters, Objective-C's method names are interleaved with the parameters

such that the method’s name naturally describes the parameters expected by the method. The imaginary

message below tells the myRectangle object to set its origin to the coordinates (30.0, 50.0):

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

// multiple parameters

A selector name includes all the parts of the name, including the colons, so the selector in the preceding

example is named setOriginX:y:. It has two colons, because it takes two parameters. The selector name

does not, however, include anything else, such as return type or parameter types.

Important: The subparts of an Objective-C selector name are not optional, nor can their order be varied. In

some languages, the terms “named parameters” and “keyword parameters” carry the implications that the

parameters can vary at runtime, can have default values, can be in a different order, and can possibly have

additional named parameters. None of these characteristics about parameters are true for Objective-C.

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

additional parameters (see “Messaging” in the Objective-C Runtime Programming Guide). Thus, the structure

of an Objective-C method declaration differs from the structure of a method that uses named or keyword

parameters in a language like Python, as the following Python example illustrates:

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

pass

In this Python example, Thing and NeatMode might be omitted or might have different values when called.

In principle, a Rectangle class could instead implement a setOrigin:: method with no label for the

second parameter, which would be invoked as follows:

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

parameters

While syntactically legal, setOrigin:: does not interleave the method name with the parameters. Thus,

the second parameter is effectively unlabeled and it is difficult for a reader of this code to determine the kind

or purpose of the method’s parameters.

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

parameters are separated by commas after the end of the method name. (Unlike colons, the commas are

not considered part of the name.) In the following example, the imaginary makeGroup: method is passed

one required parameter (group) and three parameters 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]];

Object Messaging

CHAPTER 1

Objects, Classes, and Messaging

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

object’s accessor methods. The dot operator is often used in conjunction with the declared properties feature

(see “Declared Properties” (page 53)) and is described in “Dot Syntax” (page 19).

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 the spouse object here 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.

● 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 struct. 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 a valid use of sending a message to nil.

id anObjectMaybeNil = nil;

// this is valid

if ([anObjectMaybeNil methodThatReturnsADouble] == 0.0)

{

// implementation continues...

}

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 parameters. For example, the primaryColor method illustrated above takes no parameters,

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 parameters.

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 parameters 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 earlier are used for just this purpose.

Object Messaging 17

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Objects, Classes, and Messaging

See “Defining a Class” (page 31) for more information on referring to instance variables.

Polymorphism

As the earlier examples 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 don’t behave in the same way

that function calls do.

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. Therefore, two objects can respond differently to the same message. For example, each kind of object

that receives 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 parameters 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 invoked to respond to a message can be

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

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 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. Because each object can have its own version of a method,

an Objective-C statement can achieve a variety of results, not by varying the message but by varying the

object that receives the message. Receivers can be decided as the program runs; the choice of receiver can

be made dependent on factors such as user actions.

When executing code based upon the Application Kit (AppKit), for example, users determine which objects

receive messages from menu commands such as 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

to a copy message than a Rectangle would. Because messages do not select methods until runtime (from

another perspective, because binding of methods to messages does not occur until runtime), these differences

in behavior are isolated to the methods themselves. The code that sends the message doesn’t have to be

concerned with them; it doesn’t even have to enumerate the possibilities. An application’s objects can each

respond in its own way to copy messages.

Object Messaging

CHAPTER 1

Objects, Classes, and Messaging

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