Prevent NSKeyedArchiver throwing exception without Objective-C - swift

Question

On iOS version lower than 11 the throwing archivedData(withRootObject:requiringSecureCoding:) is unavailable, so I have tried to do the equivalent on versions less than iOS 11:
let archiveData: NSData
if #available(iOS 11.0, *) {
archiveData = try NSKeyedArchiver.archivedData(
withRootObject: rootObject,
requiringSecureCoding: true
) as NSData
} else {
NSKeyedArchiver.archivedData(withRootObject: userActivity)
let mutableData = NSMutableData()
let archiver = NSKeyedArchiver(forWritingWith: mutableData)
archiver.requiresSecureCoding = true
archiver.encode(rootObject, forKey: NSKeyedArchiveRootObjectKey)
if let error = archiver.error {
throw error
}
archiver.finishEncoding()
archiveData = mutableData
}
However, when the rootObject calls NSCoder.failWithError(_:) in the encode(with:) function an NSInvalidUnarchiveOperationException exception is raised.
If I subclass NSKeyedArchiver as such:
final class KeyedArchiver: NSKeyedArchiver {
override var decodingFailurePolicy: NSCoder.DecodingFailurePolicy {
return .setErrorAndReturn
}
}
It instead raises an NSInternalInconsistencyException exception with the message Attempting to set decode error on throwing NSCoder.
Is there a way to do this kind of archiving without throwing an exception, short of writing an Objective-C function to catch the exception and throwing it as an error?

Answer 1

The reason you're still getting the exception at encode time is that the .decodingFailurePolicy is effective only when decoding (i.e., unarchiving via NSKeyedUnarchiver), not encoding. Any object that calls .failWithError(_:) on encode will still produce the exception.
Calling .failWithError(_:) at encode-time is relatively rare: usually, once you have a fully constructed object at runtime, it's not terribly likely that it should be in a state that's not encodable. There are of course cases where this is possible, so you really have two options:
If you're working with objects you know and can check ahead of time whether they're in a valid state to encode, you should do that and avoid encoding invalid objects altogether
If you're working with arbitrary objects which you can't validate up-front, you're going to have to wrap your callout to NSKeyedArchiver via an Objective-C function which can catch the exception (and ideally throw an Error containing that exception, like the newer NSKeyedArchiver API does on your behalf)
Based on your comment above, option 2 is your best bet.
As an aside, you can shorten up your fallback code to avoid having to construct an intermediate NSMutableData instance:
let archiver = NSKeyedArchiver()
archiver.encode(rootObject, forKey: NSKeyedArchiveRootObjectKey)
archiveData = archiver.encodedData
The default initializer on NSKeyedArchiver constructs a new archiver with an internal mutable data instance to use, and NSKeyedArchiver.encodedData property automatically calls -finishEncoding on your behalf.