Objective-C:@interface之前的@class指令? [英] Objective-C: @class Directive before @interface?

问题描述

这两个类声明之间有什么区别?我不明白为什么在这里使用@class.谢谢.

What is the difference between these two class declarations? I don't understand why @class is utilized here. Thanks.

@class TestClass;

@interface TestClass: UIView {
    UIImage *image1;
    UIImage *image2;
}

和

@interface TestClass: UIView {
    UIImage *image1;
    UIImage *image2;
}

推荐答案

@class存在以打破循环依赖关系.假设您有A类和B类.

@class exists to break circular dependencies. Say you have classes A and B.

@interface A:NSObject
- (B*)calculateMyBNess;
@end

@interface B:NSObject
- (A*)calculateMyANess;
@end

鸡肉；见蛋.因为A的接口依赖于B的定义，所以永远无法编译，反之亦然.

Chicken; meet Egg. This can never compile because A's interface depends on B being defined and vice-versa.

因此，可以使用@class对其进行修复:

Thus, it can be fixed by using @class:

@class B;
@interface A:NSObject
- (B*)calculateMyBNess;
@end

@interface B:NSObject
- (A*)calculateMyANess;
@end

@class有效地告诉编译器这样的类存在于某个地方，因此，声明为指向该类实例的指针是完全有效的.但是，您无法在类型仅定义为@class的实例引用上调用方法，因为编译器没有可用的其他元数据(我不记得它是否将调用站点还原为被评估为通过id致电).

@class effectively tells the compiler that such a class exists somewhere and, thus, pointers declared to point to instances of said class are perfectly valid. However, you couldn't call a method on an instance reference whose type is only defined as an @class because there is no additional metadata available to the compiler (I can't remember if it reverts the call site to being evaluated as a call through id or not).

在您的示例中，@class是无害的，但完全没有必要.

In your example, the @class is harmless, but entirely unnecessary.

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