swift4 - The use of Swift 3 @objc inference in Swift 4 mode is deprecated?

Question

Briefly, while using Xcode 9 Beta, I have run into the following warning:

The use of Swift 3 @objc inference in Swift 4 mode is deprecated. Please address deprecated @objc inference warnings, test your code with “Use of deprecated Swift 3 @objc inference” logging enabled, and disable Swift 3 @objc inference.**

After some research, I still have no idea how to fix the issue. I would greatly appreciate any tips on how to fix this issue as well as an explanation of what is going on.

My goal is to grasp a better understanding of what is happening with my code.

Answer 1

I got rid of this warning by changing the "Swift 3 @objc Inference" build setting of my targets to "Default".

From this article:

Before Swift 4, the compiler made some Swift declarations automatically available to Objective-C. For example, if one subclassed from NSObject, the compiler created Objective-C entry points for all methods in such classes. The mechanism is called @objc inference.

In Swift 4, such automatic @objc inference is deprecated because it is costly to generate all those Objective-C entry points. When "Swift 3 @objc Inference" setting is set to "On", it allows the old code to work. However, it will show deprecation warnings that need to be addressed. It is recommended to "fix" these warnings and switch the setting to "Default", which is the default for new Swift projects.

Please also refer to this Swift proposal for more information.