通过Objective-C中的类别有条件地覆盖系统方法？ [英] Conditionally overriding a system method via categories in Objective-C?

问题描述

有没有办法提供一个方法实现（承载与框架定义的方法完全相同的名称）只有当该方法还没有在系统中定义？例如方法 [NSSomeClass someMethod：] 仅存在于Mac OS X 10.6中，如果我的应用程序在10.5中运行，我将在类别中提供该方法的定义。但是，当应用程序在10.6中运行时，我希望操作系统提供的方法运行。

Is there a way to provide a method implementation (that bears the exact same name of a method defined by the framework) only when the method isn't already defined in the system? For example method [NSSomeClass someMethod:] exists only in Mac OS X 10.6 and if my app runs in 10.5, I will provide that method's definition in a category. But when the app runs in 10.6, I want the OS-provided method to run.

背景： 10.5和10.6。问题是，我最近意识到方法 + [NSSortDescriptor sortDescriptorWithKey：ascending：] 仅存在于10.6中，我的代码已经被该方法调用。我可以为它提供一个默认实现（因为这一次它不是太难以自己实现），但我想要的原生的一个，当我的应用程序运行在10.6。此外，如果我在将来遇到类似的问题（使用更难以实现的方法），我可能无法避免提供一个单独的替换。

Background: I'm creating an app targeted for both 10.5 and 10.6. The problem is that I recently realized that method +[NSSortDescriptor sortDescriptorWithKey:ascending:] only exists in 10.6 and my code is already littered by that method call. I could provide a default implementation for it (since this time it's not too difficult to implement it myself), but I want the "native" one to be called whenever my app runs on 10.6. Furthermore if I encounter similar problems in the future (with more difficult-to-implement-myself methods), I might not be able to get away with providing a one-liner replacement.

这个问题类似于覆盖方法通过ObjC类和调用默认实现？，但是区别是我想提供实现只有当系统没有一个。

This question vaguely similar to Override a method via ObjC Category and call the default implementation? but the difference is that I want to provide implementations only when the system doesn't already have one.

谢谢。

推荐答案

是的，这是可能的。因为你的目标是10.5+我假设你使用ObjC2运行时，这使得它很简单。

Yes, this is possible. Since you are targetting 10.5+ I'm assuming you are using the ObjC2 runtime, which makes it fairly straightforward.

Objective-C运行时参考具有您需要的所有方法。具体来说，您可以使用class_getClassMethod或class_getInstanceMethod来查看方法是否已存在，然后使用class_addMethod将您的实现绑定到该选择器（如果该类还没有该选择器）。

The Objective-C Runtime Reference has all the methods you will need. Specifically, you can use class_getClassMethod or class_getInstanceMethod to see if the method already exists, and then class_addMethod to bind your implementation to that selector if the class doesn't already have it.

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