Swift: `NS_CLOSED_ENUM` And Preventing Invalid Raw Values

Hey guys! Let's dive into a peculiar issue that pops up when dealing with imported enums from Objective-C into Swift, specifically those declared using NS_CLOSED_ENUM. This can lead to unexpected behavior and even crashes, which, as developers, we definitely want to avoid! We'll explore why this happens and what we can do to mitigate these problems.

The Core of the Problem: Invalid Raw Values

When you work with enums imported from Objective-C, you might encounter situations where you initialize an enum with a raw value that doesn't actually correspond to a defined case. The root of this issue often lies in how Swift handles these imported enums and how they interact with Objective-C's more permissive approach to enum values. Let's break it down.

As the original post points out, the behavior of enums declared with NS_ENUM and NS_CLOSED_ENUM differs. With NS_ENUM, Swift's behavior is designed to be compatible with C. C allows any value to be stored in an enumeration, even values that aren’t explicitly defined in the enum's header. This means that when you initialize a Swift enum imported as NS_ENUM with an invalid raw value, it won’t necessarily fail. You might get a valid enum instance, which you can then try to use. This design choice aims to provide flexibility and compatibility with existing C codebases.

However, the problem arises when we consider NS_CLOSED_ENUM. The intent behind NS_CLOSED_ENUM is to create a more strict, frozen enum. When you use a raw value that isn’t defined within an NS_CLOSED_ENUM, the expected behavior is that it should return nil. This behavior is safer and prevents unexpected crashes. The current implementation, however, doesn't always adhere to this expectation, leading to potential issues.

The NS_ENUM Case

Let’s look at an example using NS_ENUM to illustrate the problem. If we have the following Objective-C code:

typedef NS_ENUM(NSInteger, IntEnum) {
 IntEnumZero,
 IntEnumOne,
};

In Swift, this is imported, and you might write:

if let ie = IntEnum(rawValue: 100) {
 switch ie {
 case .zero:
 print("zero")
 case .one:
 print("one")
 @unknown default:
 print("unknown")
 }
}

Since this is a non-frozen enum, you have to handle the @unknown default case. This is a safety measure to ensure that your switch statement correctly handles any future cases added to the enum in Objective-C, even if you haven't updated your Swift code. This approach allows your Swift code to gracefully handle new cases without crashing.

The NS_CLOSED_ENUM Case and the Crash

Now, let's look at NS_CLOSED_ENUM. Here's the Objective-C:

typedef NS_CLOSED_ENUM(NSInteger, IntEnum) {
 IntEnumZero,
 IntEnumOne,
};

Ideally, when we try to initialize with rawValue: 100, it should return nil. But, if we write the following Swift code:

if let ie = IntEnum(rawValue: 100) {
 switch ie {
 case .zero:
 print("zero")
 case .one: // Thread 1: Fatal error: unexpected enum case 'IntEnum(rawValue: 100)'
 print("one")
 }
}

You'll get a crash. The Swift compiler, in this case, isn't behaving as expected. The raw value of 100 isn't a valid case in the enum, so when Swift tries to evaluate the switch statement, it doesn't know what to do, resulting in an unexpected enum case and a crash.

Reproduction and Expected Behavior

Reproducing the issue is pretty straightforward. You define an NS_CLOSED_ENUM in Objective-C. You then try to initialize it in Swift with a raw value that isn't defined in the enum. When you use this value in a switch statement without a default case, the crash occurs.

The expected behavior is that IntEnum(rawValue: 100) should return nil, as it isn't a valid value for the enum. The inability of NS_CLOSED_ENUM to correctly handle invalid raw values leads to runtime errors, making the code less safe and potentially causing unforeseen problems.

Understanding the Technical Details

Let's delve a bit deeper into what's happening under the hood. The stack dump provided in the original post gives us some clues:

#0 0x0000000193dab0dc in _swift_runtime_on_report ()
#1 0x0000000193e91d08 in _swift_stdlib_reportFatalError ()
#2 0x0000000193ec398c in closure #1 (Swift.UnsafeBufferPointer<Swift.UInt8>) -> () in Swift._assertionFailure(_: Swift.StaticString, _: Swift.String, flags: Swift.UInt32) -> Swift.Never ()
#3 0x0000000193e9f378 in _assertionFailure ()
#4 0x0000000193ec4734 in _diagnoseUnexpectedEnumCaseValue ()

This stack trace indicates that the crash originates from an assertion failure (_assertionFailure) within the Swift runtime, specifically when dealing with an unexpected enum case. The _diagnoseUnexpectedEnumCaseValue function is triggered because the rawValue doesn't match any of the defined cases, and the code isn't prepared to handle such a situation.

Workarounds and Best Practices

Given this issue, what can we do to make our code more robust and avoid these crashes? Here are a few workarounds and best practices:

Using if let and Early Exit

Always use if let when dealing with raw value initializations. This prevents you from entering the potentially problematic switch statement if the raw value isn't valid. Example:

if let ie = IntEnum(rawValue: 100) {
 // Proceed only if the raw value is valid
 switch ie {
 case .zero:
 print("zero")
 case .one:
 print("one")
 }
} else {
 // Handle the case where the raw value is invalid
 print("Invalid raw value")
}

Utilizing the @unknown default Case (Even with NS_CLOSED_ENUM)

Although NS_CLOSED_ENUM is intended to be frozen, it is still a good idea to include an @unknown default case in your switch statements. This is particularly useful if the enum definition might change in future versions, or if there's a possibility of unexpected behavior from the Objective-C side. This approach adds a layer of safety, making your code more resilient to unexpected situations.

if let ie = IntEnum(rawValue: 100) {
 switch ie {
 case .zero:
 print("zero")
 case .one:
 print("one")
 @unknown default:
 print("Unknown case")
 }
}

Thorough Testing

Testing is crucial! Make sure you write unit tests that explicitly test your enums with invalid raw values. This proactive approach helps to catch these issues early and ensures your code behaves as expected under various conditions.

Swift Bridging and Code Review

Be mindful of how your Objective-C enums are bridged into Swift. Review the generated Swift code to understand how your enums are being imported. Code reviews are also very important; having a second pair of eyes can often catch potential problems.

Conclusion: Navigating the NS_CLOSED_ENUM Challenge

Dealing with enums imported from Objective-C, especially NS_CLOSED_ENUM, can be tricky, but by understanding the underlying behavior, and adopting some best practices, we can write safer and more reliable Swift code.

Remember to always be cautious about raw value initializations, and validate your code by testing with both valid and invalid values. As Swift continues to evolve, we can hope for improvements in how these Objective-C enums are handled, reducing the chances of unexpected crashes. Until then, these strategies will help you write code that is more resilient and more friendly to handle any potentially unforeseen situations, providing a smoother experience for you and your users!

I hope this helps you guys avoid those pesky crashes and write more stable Swift code! Keep coding and keep learning!