如何在不引用类的情况下在Objective C中调用+ class方法? [英] How do I call +class methods in Objective C without referencing the class?

问题描述

我有一系列策略"对象，我认为将这些对象方便地实现为一组策略类上的类方法.我为此指定了一个协议，并创建了要遵守的类(如下所示)

I have a series of "policy" objects which I thought would be convenient to implement as class methods on a set of policy classes. I have specified a protocol for this, and created classes to conform to (just one shown below)

@protocol Counter   
+(NSInteger) countFor: (Model *)model;
@end

@interface CurrentListCounter : NSObject <Counter> 
+(NSInteger) countFor: (Model *)model;
@end

然后我有一个符合该协议的类的数组(就像CurrentListCounter一样)

I then have an array of the classes that conform to this protocol (like CurrentListCounter does)

+(NSArray *) availableCounters {
return [[[NSArray alloc] initWithObjects: [CurrentListCounter class], [AllListsCounter class], nil] autorelease];
}

请注意我是如何使用类似对象的类的(这可能是我的问题-在Smalltalk类中，对象是与其他所有对象一样的-我不确定它们是否在Objective-C中吗?)

Notice how I am using the classes like objects (and this might be my problem - in Smalltalk classes are objects like everything else - I'm not sure if they are in Objective-C?)

我的确切问题是当我从数组中取出一个策略对象时要调用该方法:

My exact problem is when I want to call the method when I take one of the policy objects out of the array:

id<Counter> counter = [[MyModel availableCounters] objectAtIndex: self.index];
return [counter countFor: self];

我在return语句上收到警告-它说-countFor:在协议中找不到(因此假设它是我要调用类方法的实例方法).但是，由于数组中的对象是类的实例，所以它们现在就像实例方法(或者从概念上讲应该是实例方法).

I get a warning on the return statement - it says -countFor: not found in protocol (so its assuming its an instance method where I want to call a class method). However as the objects in my array are instances of class, they are now like instance methods (or conceptually they should be).

是否有一种神奇的方法来调用类方法?还是这只是个坏主意，我应该只创建策略对象的实例(而不使用类方法)?

Is there a magic way to call class methods? Or is this just a bad idea and I should just create instances of my policy objects (and not use class methods)?

推荐答案

此

id <Counter> counter = [[Model availableCounters] objectAtIndex:0];
return ( [counter countFor: nil] );

应该是

Class <Counter> counter = [[Model availableCounters] objectAtIndex:0];
return ( [counter countFor: nil] );

在第一个实例中，您有一个符合<Counter>的实例.在第二个中，您有一个符合<Counter>的类.编译器警告是正确的，因为符合<Counter>的实例不会响应countFor:，只有类会响应.

In the first you have an instance that conforms to <Counter>. In the second you have a class that conforms to <Counter>. The compiler warning is correct because instances that conform to <Counter> don't respond to countFor:, only classes do.

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