如果需要，在头文件中使用前向声明，并为在实现中使用的任何类导入头文件。换句话说，您总是#import您在实现中使用的文件，如果您需要在头文件中引用一个类，也可以使用forward声明。

例外的是，你应该在头文件中导入一个类或正式协议(在这种情况下，你不需要在实现中导入它)。

向前声明只是为了防止编译器显示错误。

编译器会知道你在头文件中声明的类的名称。

三个简单的规则:

只在头文件(.h文件)中#导入超类和采用的协议。 #导入所有类和协议，你发送消息到实现(。m文件)。 其他所有东西的前向声明。

如果在实现文件中进行前向声明，则可能会出错。

查看关于ADC的Objective-C编程语言文档

在定义类|类接口一节中，它描述了为什么这样做:

@class指令最大限度地减少了编译器和链接器看到的代码量，因此是给出类名正向声明的最简单方法。由于简单，它避免了导入仍然导入其他文件的文件时可能出现的潜在问题。例如，如果一个类声明了另一个类的静态类型实例变量，并且它们的两个接口文件相互导入，那么两个类都不能正确编译。

有关文件依赖关系& #import & @class的更多信息，请查看以下内容:

http://qualitycoding.org/file-dependencies/ 这是篇好文章

文章摘要

imports in header files: #import the superclass you’re inheriting, and the protocols you’re implementing. Forward-declare everything else (unless it comes from a framework with a master header). Try to eliminate all other #imports. Declare protocols in their own headers to reduce dependencies. Too many forward declarations? You have a Large Class. imports in implementation files: Eliminate cruft #imports that aren’t used. If a method delegates to another object and returns what it gets back, try to forward-declare that object instead of #importing it. If including a module forces you to include level after level of successive dependencies, you may have a set of classes that wants to become a library. Build it as a separate library with a master header, so everything can be brought in as a single prebuilt chunk. Too many #imports? You have a Large Class.

我的问题是这样的。什么时候使用#import，什么时候使用@class?

简单的回答:当存在物理依赖时，使用#import或#include。否则，使用正向声明(@class MONClass, struct MONStruct， @protocol MONProtocol)。

以下是一些常见的身体依赖的例子:

任何C或c++值(指针或引用不是物理依赖项)。如果您将CGPoint作为ivar或属性，编译器将需要看到CGPoint的声明。 你的超类。 你使用的方法。

有时如果我使用@class声明，我看到一个常见的编译器警告，如下所示: “警告:接收端‘FooController’是转发类，对应的@interface可能不存在。”

The compiler's actually very lenient in this regard. It will drop hints (such as the one above), but you can trash your stack easily if you ignore them and don't #import properly. Although it should (IMO), the compiler does not enforce this. In ARC, the compiler is more strict because it is responsible for reference counting. What happens is the compiler falls back on a default when it encounters an unknown method which you call. Every return value and parameter is assumed to be id. Thus, you ought to eradicate every warning from your codebases because this should be considered physical dependence. This is analogous to calling a C function which is not declared. With C, parameters are assumed to be int.

The reason you would favor forward declarations is that you can reduce your build times by factors because there is minimal dependence. With forward declarations, the compiler sees there is a name, and can correctly parse and compile the program without seeing the class declaration or all of its dependencies when there is no physical dependency. Clean builds take less time. Incremental builds take less time. Sure, you will end up spending a little more time making sure the all the headers you need are visible to every translation as a consequence, but this pays off in reduced build times quickly (assuming your project is not tiny).

如果您使用#import或#include来代替，则会向编译器抛出大量不必要的工作。您还引入了复杂的头依赖项。你可以把它比作一个蛮力算法。当您使用#import时，会拖入大量不必要的信息，这需要大量内存、磁盘I/O和CPU来解析和编译源代码。

ObjC is pretty close to ideal for a C based language with regards to dependency because NSObject types are never values -- NSObject types are always reference counted pointers. So you can get away with incredibly fast compile times if you structure your program's dependencies appropriately and forward where possible because there is very little physical dependence required. You can also declare properties in the class extensions to further minimize dependence. That's a huge bonus for large systems -- you would know the difference it makes if you have ever developed a large C++ codebase.

因此，我的建议是尽可能使用forward，然后在有物理依赖的地方使用#import。如果您看到警告或其他暗示身体依赖的警告，请全部修复。修复方法是在实现文件中使用#import。

在构建库时，您可能会将一些接口分类为一组，在这种情况下，您将#导入引入物理依赖的库(例如#import <AppKit/AppKit.h>)。这可能会引入依赖关系，但是库维护者通常可以根据需要为您处理物理依赖关系——如果他们引入了一个特性，他们可以将其对构建的影响最小化。