What is Category
The Category pattern is used to add methods to existing classes to achieve the purpose of extending existing classes. In many cases, Category is also a better choice than creating subclasses. The newly added method will also be automatically inherited by all subclasses of the extended class. When we know that a method in an existing class has a bug, but this class exists in the form of a library and we cannot directly modify the source code, Category can also be used to replace the entity of a method in this existing class, so as to achieve the purpose of fixing the bug. However, there is no convenient way to call the replaced method entity in the existing class. It should be noted that when you are preparing to replace a certain method, you must ensure that all functions of the original method are implemented, otherwise this replacement will be meaningless and will cause new bugs. Unlike subclasses, Category cannot be used to add instance variables to extended classes. Category is usually used as a tool for organizing framework code.
The purpose of Category
1. Implement extensions to existing classes without creating inherited classes.
2. Simplify the development of classes (when a class requires multiple programmers to develop together, Category can place the same class in different source files according to its purpose, so as to facilitate programmers to independently develop corresponding method collections).
3. Group common related methods.
4. It can be used to fix bugs without source code.
How to use Category
In Obj-C, the method of declaring a Category extension of an existing class is as follows:
@interface ClassName (CategoryName) -methodName1 -methodName2 @end
The above declaration is usually in the h file, and then we implement these methods in the m file:
@implementation ClassName (CategoryName) -methodName1 -methodName2 @end
We create an iOS Single View Appliance named CategoryExample. Then create a category extension of the NSString class. File->New->File and select Cocoa Touch Objective-C category. Name it ReverseNSString. The system will automatically generate a .h and .m file with a fixed format ClassName+CategoryName.
Declare Category
Open the NSString+ file and add the following code to it:
#import <Foundation/> @interface NSString (ReverseNSString) + (NSString*) reverseString:(NSString*)strSrc; @end
Implement Category
Implement reverseString method in NSString+ file:
#import"NSString+"
@implementationNSString (ReverseNSString)
+ (NSString*)reverseString:(NSString*)strSrc;
{
NSMutableString *reversedString =[[NSMutableString alloc]init];
NSInteger charIndex = [strSrc length];
while (charIndex > 0) {
charIndex--;
NSRange subStrRange =NSMakeRange(charIndex, 1);
[reversedString appendString:[strSrcsubstringWithRange:subStrRange]];
}
return reversedString;
}
@end
The rest of the work is to verify our Category, add a button ReverseString to the view, and set the corresponding action method to reverseString. Add a label on the view, named myString, and the default value is "HelloCategory Design Pattern!". Click the button to invert this string. The main code is as follows:
-(IBAction)reverseString:(id)sender {
NSString *test = [NSStringreverseString:_myString.text];
_myString.text = test;
}
Code Organization
Category is used for efficient decomposition of large classes. Usually, a method of a large class can be decomposed into different groups according to some logic or correlation. The larger the amount of code in a class, the more useful it is to decompose this class into different files. Each file is a collection of certain related methods of this class.
When multiple developers work together to complete a project, each person is responsible for the development and maintenance of a separate cagegory. This makes version control easier because there are fewer work conflicts between developers.
Category VS Add Subclasses
There is no clear boundary judgment standard to guide when to use Category and when to use the method of adding subclasses. But there are several guiding suggestions:
1. If you need to add a new variable, you need to add a subclass.
2. If you just add a new method, using Category is a better choice.
The above is all the content of this article. I hope it will be helpful to everyone's study and I hope everyone will support me more.