类别而不是继承的实际用途是什么? [英] What is the actual use of categories instead of inheritance?

问题描述

我试图了解类别的实际用法，而不是Objective-C中的继承.我什么时候应该使用类别?一个带有代码的真实示例将很有帮助.

I'm trying to understand the actual use of categories as opposed to inheritance in Objective-C. When should I prefer to use a category? A real-life example with code would be helpful.

推荐答案

当我需要将一些方便的函数添加到现有类中而又不需要子类来覆盖该类的某些现有函数时，我使用了类别班级.

I used categories when I need to add some convenient functions that I will use repeatedly to the existing class without having a need to subclass to overwrite some existing functions of that class.

例如，当我想检查一个空字符串，或删除一个字符串的所有前导和尾随空格时:

For example, when I want to check for an empty string, or remove all leading and trailing spaces of a string:

.h文件:

@interface NSString  (Extension)

-(BOOL)isEmptyString;
-(NSString *)trimLeadingAndTrailingWhiteSpaces;

@end

.m文件:

@implementation NSString  (Extension)

-(BOOL)isEmptyString
{
    NSString *myString = [self stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceCharacterSet]];
    if (myString.length == 0)
        return TRUE;
    else
       return FALSE;
}

-(NSString *)trimLeadingAndTrailingWhiteSpaces
{
    NSString *myString = [self stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceAndNewlineCharacterSet]];
    return myString;
}
@end

要使用它:

 someString = [someString trimLeadingAndTrailingWhiteSpaces];

 if ([someString isEmptyString])
       {
         //someString is empty, do whatever!
       }

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