我如何子类化 NSDate? [英] How do I subclass NSDate?

问题描述

我第一次尝试对 NSDate 进行子类化以提供我需要的 2 个方法.编译正常，但在运行时我尝试访问它时出现错误.

I first time tried to subClassed an NSDate to give it 2 methods that I need. Compiles fine, but in runtime I try to access it I get an error.

假设我只想要子类中未修改的当前日期:

Lets say I just want the current date which is unmodified in the subClass:

[myNSDate date];

我收到错误

-[NSDate initWithTimeIntervalSinceReferenceDate:]: method only defined for 
 abstract class.  Define -[myNSDate initWithTimeIntervalSinceReferenceDate:]!

有什么不同?

推荐答案

NSD 的回答是正确的，我就尽量简单地重申一下.NSDate 不是一个可以轻松子类化的简单类.这是一个 类簇，简而言之，当你得到一个 NSDate 类型的值时，它实际上是一个不同的私有类的实例，与 NSDate.换句话说，<代码>的NSDate 不是你想的东西的子类.

NSD’s answer corect, I’ll just try to reiterate in simple terms. NSDate is not a simple class you could easily subclass. It’s a class cluster, which in short means that when you get a value of type NSDate, it’s actually instance of a different, private class that has the same interface as NSDate. In other words, NSDate is not something you would want to subclass.

如果您只想添加方法(而不是实例变量)，您可以使用类别轻松实现:

If you just want to add methods (not instance variables), you can easily do that using a category:

@interface NSDate (MyExtensions)
- (void) doFoo;
@end

@implementation NSDate (MyExtensions)
- (void) doFoo {
    NSLog(@"Foo");
}
@end

现在您可以调用[date doFoo].另请参阅类簇子类化教程 迈克·阿什.

Now you can call [date doFoo]. See also a class cluster subclassing tutorial by Mike Ash.

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