Objective-C 2.0编程语言详解

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"Objective-C 2.0 是一种面向对象的编程语言，主要应用于苹果的软件开发，如iOS和macOS。本文档是Apple Inc.于2009年发布，详细介绍了Objective-C 2.0的主要特性和使用方法。" Objective-C 2.0是在Objective-C的基础上进行的一次重大更新，引入了许多改进和新特性，使得编程更加高效和简洁。以下是一些Objective-C 2.0的关键知识点： 1. **块（Blocks）**: Objective-C 2.0引入了块的概念，这是一种内联函数或匿名函数，可以捕获并存储其定义时的上下文变量。块允许程序员创建可重用的代码段，特别适用于异步操作和算法实现。 2. **分类（Categories）**: 分类在Objective-C 2.0中得到了加强，允许开发者向已有的类添加方法，而无需继承。这增强了代码的模块化，使得为已有类添加功能变得更加灵活。 3. **属性（Properties）**: Objective-C 2.0对属性的支持更加完善，提供了更多的属性特性，如`nonatomic`、`atomic`、`strong`、`weak`、`assign`等，用于控制内存管理、线程安全以及对象引用计数。 4. **协议（Protocols）**: 协议在Objective-C中扮演着接口的角色，Objective-C 2.0强化了对协议的支持，允许一个类同时遵循多个协议，增加了代码的可扩展性。 5. **垃圾回收（Garbage Collection）**: 虽然在后来的版本中被自动引用计数（ARC）取代，但Objective-C 2.0引入了垃圾回收机制，自动管理内存，减少了程序员手动释放内存的负担。 6. **字面量语法（Literal Syntax）**: 对于数组和字典，Objective-C 2.0引入了更简洁的字面量语法，如`NSArray *array = @[@"item1", @"item2"]`和`NSDictionary *dict = @{@"key": @"value"}`，这大大提高了代码的可读性。 7. ** dot notation（点语法）**: Objective-C 2.0允许使用点语法访问和设置属性，如`object.property`，使得代码看起来更像其他现代面向对象语言。 8. **更好的编译器支持**: Objective-C 2.0的编译器提供了一些优化，比如类型推断，使得编写代码更加方便。 9. **类别中的实例变量**: 在Objective-C 2.0中，开发者可以在类别中声明私有的实例变量，增强了类别的功能和封装性。 10. **Runtime改进**: Objective-C的运行时系统在2.0版本中也有所增强，包括更快的消息传递和更多调试信息。 Objective-C 2.0的这些改进极大地提升了开发者的生产力，并且为构建复杂和高性能的应用程序提供了强大的支持。对于iOS和macOS开发者来说，深入理解Objective-C 2.0的这些特性至关重要。

[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 2.0 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 2.0 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 57)), 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 2.0 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 2.0 Runtime Programming Guide.

Object Messaging 19

CHAPTER 1

Objects, Classes, and Messaging

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