The use of Swift 3 @objc inference in Swift 4 mode

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

You can simply pass to "default" instead of "ON". Seems more adherent to Apple logic.

(but all the other comments about the use of @obj remains valid.)

Answer 2

Migrator cannot identify all the functions that need @objc Inferred Objective-C thunks marked as deprecated to help you find them
• Build warnings about deprecated methods
• Console messages when running deprecated thunks

Answer 3

Indeed, you'll get rid of those warnings by disabling Swift 3 @objc Inference. However, subtle issues may pop up. For example, KVO will stop working. This code worked perfectly under Swift 3:

for (key, value) in jsonDict {
    if self.value(forKey: key) != nil {
        self.setValue(value, forKey: key)
    }
}

After migrating to Swift 4, and setting "Swift 3 @objc Inference" to default, certain features of my project stopped working. It took me some debugging and research to find a solution for this. According to my best knowledge, here are the options:

• Enable "Swift 3 @objc Inference" (only works if you migrated an existing project from Swift 3)
• Mark the affected methods and properties as @objc
• Re-enable ObjC inference for the entire class using @objcMembers

Re-enabling @objc inference leaves you with the warnings, but it's the quickest solution. Note that it's only available for projects migrated from an earlier Swift version. The other two options are more tedious and require some code-digging and extensive testing.

See also https://github.com/apple/swift-evolution/blob/master/proposals/0160-objc-inference.md

Answer 4

I had this warning with "Swift 3 @objc Inference" = "Default" setting. Then I realized that is was set for the Project - not for the target. So, make sure that you have "Default" setting in your target to get rid of the warning.

Answer 5

I'm an occasional iOS dev (soon to be more) but I still couldn't find the setting as guided by the other answer (since I did not have that Keychain item the answer shows), so now that I found it I thought I might just add this snapshot with the highlighted locations that you will need to click and find.

1. Start in the upper left
2. Choose the project folder icon
3. Choose your main project name below the project folder icon.
4. Choose Build Settings on the right side.
5. Choose your project under TARGETS.
6. Scroll very far down (or search for the word inference in search text box)

Answer 6

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.