Objective-C 2.0编程语言详解

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"The Objective-C 2.0 Programming Language" Objective-C是苹果公司开发的一种面向对象的编程语言，它是C语言的扩展，增加了Smalltalk式的消息传递机制。Objective-C 2.0是在原版Objective-C的基础上进行的一次重大更新，旨在提高开发效率和代码的可读性。本编程语言主要用于开发苹果的iOS和macOS平台上的应用程序，是Cocoa和Cocoa Touch框架的基础。在Objective-C 2.0中，主要改进包括： 1. **属性（Properties）的增强**：在Objective-C 2.0中，属性语法得到了显著增强，开发者可以更方便地定义和使用对象的属性。例如，自动合成、内存管理策略（如`nonatomic`和`atomic`）、属性的读写权限（`readonly`, `readwrite`）以及默认的getter和setter方法。 2. **分类（Categories）的改进**：分类允许开发者向已有的类添加新的方法，而无需继承。Objective-C 2.0增强了分类的功能，允许在分类中声明实例变量，尽管不能直接初始化这些变量，但可以通过关联对象（associated objects）来实现存储。 3. **垃圾回收（Garbage Collection）**：虽然iOS后来转向了引用计数的内存管理，但在Objective-C 2.0初期，引入了垃圾回收机制，自动管理内存，减少了程序员手动管理内存的工作。 4. **块（Blocks）**：块是Objective-C 2.0引入的一项重要特性，它是一种内联函数或者匿名函数，可以作为参数传递，也可以作为返回值。这使得编写异步操作、迭代和处理集合等任务变得更加简单。 5. **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 = @[obj1, obj2];`和`NSDictionary *dict = @{@"key": @"value"};`，这大大提高了代码的可读性和编写速度。 6. **新式初始化方法（Modern Initializers）**：通过`instancetype`关键字，Objective-C 2.0提供了更灵活的初始化方式，允许子类正确地返回自身的类型，而不是父类的类型。 7. **Key-Value Coding（KVC）和Key-Value Observing（KVO）**： KVC和KVO是Objective-C中的数据绑定技术，它们允许对象通过键值来访问和修改其他对象的属性，而无需知道具体实现。在Objective-C 2.0中，这些技术得到了进一步加强。 8. **NSManagedObject的改进**：对于Core Data框架，Objective-C 2.0改进了`NSManagedObject`的性能，简化了与数据库的交互。 9. **编译器优化**：编译器的改进带来了更快的编译速度和更高效的代码生成，有助于提高应用程序的运行效率。总体来说，Objective-C 2.0在保持与Objective-C兼容的同时，引入了许多现代编程语言的特性，使得开发过程更为高效和愉快。然而，随着Swift的出现，Objective-C的使用逐渐减少，但在许多现有的项目和库中，Objective-C仍然是重要的组成部分。

[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 (seeMessaging 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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