Objective-C编程语言详解：2009年Apple官方指南

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Objective-C编程语言是Apple Inc.于2009年10月19日发布的官方文档，该语言是专为Apple的开发环境和产品设计的，如iOS和macOS平台。Objective-C是一种面向对象的编程语言，它在C语言的基础上扩展了Smalltalk的概念，旨在提供更强大的灵活性和可扩展性，尤其是在iOS应用开发中扮演了核心角色。在《Objective-C Programming Language》中，文档详细介绍了工具和语言特性，包括类和对象的创建、消息传递机制（Message Passing）、继承与多态、Category（类别）以及基础数据类型和内存管理。此外，它还强调了版权政策，指出所有内容未经Apple书面许可不得复制、存储或通过任何形式的机械、电子方式传播，包括打印文档，除非用于个人学习用途且保留Apple的版权声明。值得注意的是，文档中的“keyboard” Apple logo（Option-Shift-K）是苹果商标，商业用途需获得事先书面同意，否则可能构成商标侵权和不公平竞争，这体现了对知识产权的尊重和保护。同时，文档明确指出，本资料不授予任何技术使用许可，苹果保留所有与所描述技术相关的知识产权，并强调其目的是为了辅助应用程序开发，而非商业目的的授权使用。在学习和使用Objective-C时，开发者需要遵守文档中的相关规定，确保合法合规地运用该语言进行项目开发。随着iOS和macOS生态系统的不断发展，Objective-C虽然在Swift等新语言的崛起中逐渐让位，但它在历史和遗产方面仍具有重要地位，对理解和维护旧有代码库或进行系统级编程仍然是不可或缺的技能。

[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

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