Catch Objective-C exception in Swift

Question

I am trying to set the value of an @objc object in my Swift code using obj.setValue(value, forKey: key).

It works fine when the object has the property being set. But if it doesn't, my app crashes hard with an uncaught NSException ("class is not key value coding-compliant…").

How can I catch and absorb this exception like I can in Objective-C so as to not crash my app? I tried wrapping it in a Swift try-catch, but it complains that none of the instructions throws and does nothing.

Do you really need KVC? Objects in dictionaries are supposed to be set with `setObject:forKey:` or key subscripting especially in Swift. — vadian, Jan 31 '16 at 21:20
Yes, I'm writing an XML parser and need to be able to set arbitrary properties on objects by their names. — devios1, Jan 31 '16 at 21:21
Then rather than catch an exception check if the key exists. — vadian, Jan 31 '16 at 21:22
That would totally work. I didn't think there was a way to do that though! — devios1, Jan 31 '16 at 21:23
Possible duplicate of [Catching NSException in Swift](https://stackoverflow.com/questions/32758811/catching-nsexception-in-swift) — kelin, Mar 23 '19 at 13:22

score 24 · Accepted Answer · edited May 23 '17 at 11:54

24

see this answer:

//
//  ExceptionCatcher.h
//

#import <Foundation/Foundation.h>    

NS_INLINE NSException * _Nullable tryBlock(void(^_Nonnull tryBlock)(void)) {
    @try {
        tryBlock();
    }
    @catch (NSException *exception) {
        return exception;
    }
    return nil;
}

edited May 23 '17 at 11:54

Community

1
1

answered Feb 01 '16 at 22:53

Casey

6,531
24
43

That did the trick! Not pretty, but I'm glad it's at least possible! – devios1 Feb 01 '16 at 23:00

score 15 · Answer 2 · answered May 15 '20 at 18:06

Swift 5 version with utility error type:

NS_INLINE NSException * _Nullable ExecuteWithObjCExceptionHandling(void(NS_NOESCAPE^_Nonnull tryBlock)(void)) {
    @try {
        tryBlock();
    }
    @catch (NSException *exception) {
        return exception;
    }
    return nil;
}

public struct NSExceptionError: Swift.Error {

   public let exception: NSException

   public init(exception: NSException) {
      self.exception = exception
   }
}

public struct ObjC {

   public static func perform(workItem: () -> Void) throws {
      let exception = ExecuteWithObjCExceptionHandling {
         workItem()
      }
      if let exception = exception {
         throw NSExceptionError(exception: exception)
      }
   }
}

score 13 · Answer 3 · edited Jun 20 '20 at 09:12

Not the answer I was hoping for, unfortunately:

Although Swift error handling resembles exception handling in Objective-C, it is entirely separate functionality. If an Objective-C method throws an exception during runtime, Swift triggers a runtime error. There is no way to recover from Objective-C exceptions directly in Swift. Any exception handling behavior must be implemented in Objective-C code used by Swift.

Excerpt From: Apple Inc. “Using Swift with Cocoa and Objective-C (Swift 2.1).” iBooks. https://itun.es/ca/1u3-0.l

My next plan of attack is to add an Objective-C function I can call out to that will wrap the attempt in @try/@catch. This really sucks, Apple.

score 1 · Answer 4 · answered Jul 18 '17 at 22:34

1

I wanted to call out the answer given in this question:

Catching NSException in Swift

as you can still use the Swift do/catch with the call in a natural fashion, the next best thing to Swift actually catching ObjC exceptions as it should.

answered Jul 18 '17 at 22:34

Kendall Helmstetter Gelner

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Catch Objective-C exception in Swift

4 Answers4

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