类方法（class method）

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http://blog.youkuaiyun.com/sdnxr/article/details/43155705

A class method is a method that operates on class objects rather than instances of the class. In Objective-C, a class method is denoted by a plus (+) sign at the beginning of the method declaration and implementation:

类方法操作的是类对象而不是类的实例。在OC中，使用一个放置在方法声明和实现之前的“＋”号来标记类方法：

+ (void)classMethod;

To send a message to a class, you put the name of the class as the receiver in the message expression:

向一个类传递消息，你需要将这个类作为方法的接收者：

[MyClass classMethod];

Subclasses（子类）

You can send class messages to subclasses of the class that declared the method. For example,  NSArray  declares the class method  array  that returns a new instance of an array object. You can also use the method with  NSMutableArray , which is a subclass of  NSArray :

在子类中，你可以使用父类声明的方法。举个例子，NSArray类声明了类方法array，该方法返回一个新的数组实例对象。你可以在NSArray的子类NSMutableArray中使用该方法：

NSMutableArray *aMutableArray = [NSMutableArray array];

In this case, the new object is an instance of  NSMutableArray , not  NSArray .

上例中，新生成的对象是NSMutableArray的实例，而不是NSArray类。

Instance Variables（实例变量）/*注意*/

Class methods can’t refer directly to instance variables. For example, given the following class declaration:

类方法不能直接引用实例变量。举个例子，如下类的声明：

@interface MyClass : NSObject {

     NSString *title;

}

+ (void)classMethod;

@end

you cannot refer to  title  within the implementation of  classMethod .

在classMethod的实现中，你不可以引用title。

self（self指针）/*注意*/

Within the body of a class method,  self  refers to the class object itself. You might implement a factory method like this:

在类方法中，self指向类对象本身。你可以如下实现一个工厂方法：

+ (id)myClass {

      return [[[self alloc] init] autorelease];

}

In this method,  self refers to the class to which the message was sent . If you created a subclass of  MyClass :

在这个方法中，self指向消息的接受类。如果你创建了如下MyClass的子类：

@interface MySubClass : MyClass {

}

@end

and then sent a  myClass  message to the subclass:

然后向MySubClass发（diao）送（yong）了消（fang）息（fa）myClass：

id instance = [MySubClass myClass];

at runtime, within the body of the  myClass  method,  self  would refer to the  MySubClass  class (and so the method would return an instance of the subclass).

运行时，在myClass方法中，self指向MySubClass类（该方法返回一个MySubClass的实例）。

转载请注明出处

http://blog.youkuaiyun.com/sdnxr/article/details/43155705