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 的存在是为了打破循环依赖.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；遇见蛋蛋.这永远无法编译，因为 A 的接口取决于 B 的定义，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，它可以通过使用@class来修复:

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处，因此，声明指向所述类的实例的指针是完全有效的.但是，您不能在类型仅定义为 @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(我不记得它是否恢复调用网站是否通过 id 被评估为调用).

@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.

这篇关于Objective-C:@interface 之前的@class 指令?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！