Use of Objective-C syntax Category

No matter how perfect a class design is, there will inevitably be unpredictable demands. How can we expand the existing class? Of course, inheritance is a good choice. However, Objective-C provides a special way to extend the class, called Catagory. It can dynamically add new behaviors for existing classes. This ensures that the required functions can be added with minor changes based on the original class. When you use Category to expand a class, you do not need to access its source code or create a subclass. In this way, you can extend the class provided by the system. Category uses a simple method to modularize the class-related methods and allocate different class methods to different classification files.

A simple example shows how to use Category.

Now we have a class named MyClass.

#import <Foundation/Foundation.h>@interface MyClass : NSObject-(void) myPrint;@end

# Import "MyClass. h" @ implementation MyClass-(void) myPrint {NSLog (@ "myPrint called");} @ end

It has an instance method: myPrint. We can call it after the extension.

Now, with the above MyClass, we need to add a HelloWorld method without adding a subclass or modifying the MyClass class. How can we add it? You only need to add two files: MyClass + HelloWorld. h and MyClass + HelloWorld. m.

In the declaration file and implementation file, use "()" to enclose the Category name. The original class name + Category "is the agreed file naming method.

See how these two files are implemented. On Xcoed, press Command + N to create a new file and select the Objective-C category method to create a class. In this way, Xcode will automatically create a file with the specified naming method.

The Category on class is MyClass. The correct one is selected.

In this way, Xcode will help you create the MyClass + HelloWorld. h and MyClass + HelloWorld. m files.

Now we add a HelloWorld method. The code after implementation is as follows:

#import "MyClass.h"@interface MyClass (HelloWorld)-(void)HelloWorld;@end

# Import "MyClass + HelloWorld. h" @ implementation MyClass (HelloWorld)-(void) HelloWorld {NSLog (@ "Hello London Olympics! ");} @ End

Call in main

        MyClass *myclass = [[[MyClass alloc]init]autorelease];        [myclass HelloWorld];        [myclass myPrint];

Run the print result:

11:24:16. 697 objectiveC [16053: 403] Hello, London Olympics! 11:24:16. 699 objectiveC [16053: 403] myPrint called

 

What are the application scenarios of Category:
1. The class contains many methods to implement, and these methods need to be implemented by members of different teams.
2. When you are using the classes in the base class library, you do not want to inherit these classes but just want to add some methods.
 
Category can meet the above requirements. Of course, you need to pay attention to the following issues when using Category:
1. Category can access the instance variables of the original class, but cannot add instance variables. If you want to add variables, You can inherit to create subclass.
2. Category can reload the methods of the original class. It is not recommended to do so. This will overwrite the methods of the original class. If you do need to overload it, you can create a subclass by inheriting it.
3. What is different from a common interface is that the instance method in the Category implementation file does not need to implement all declared methods as long as you do not call it.

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