[swift-users] NSJSONSerialization barfs writing a Dictionary parsed from JSON: "Invalid type in JSON write (_SwiftValue)"

[Jens Alfke]

A developer using the framework I work on just reported <https://forums.couchbase.com/t/exception-caught-in-map-block-of-view-invalid-type-in-json-write--swiftvalue/10808> an exception which is triggered by NSJSONSerialization (see backtrace below.) Basically an NSDictionary gets parsed from JSON data (in Obj-C), passed to a Swift function, which then calls another function (written in Obj-C) that serializes it back to JSON data. That last step is crashing due to some problem with bridging.

The events leading up to this are:

1. The framework (written in Obj-C) parses some JSON data using NSJSONSerialization, resulting in an NSDictionary.
2. The NSDictionary is passed as a parameter to a block; the parameter is typed as `NSDictionary*`.
3. The block is part of the application, which is written in Swift.
4. The block in turn passes that NSDictionary to a function in the framework (through a parameter is typed as `id`)
5. The framework uses NSJSONSerialization to convert that id to data.

Step 5 is triggering an assertion failure in Foundation. Apparently it failed to un-bridge a Swift value in an array back into an NSObject. (From the contents of the dictionary I know that the single-object array in question contains a string.)

Any idea what’s going on here? I’ve never seen this happen before, and this part of our framework has been around for 2 ½ years. People have successfully built Swift apps using the framework. (This app was built with Xcode 8.0; don’t know what the OS is, though.)

	Invalid type in JSON write (_SwiftValue)
	3   Foundation                          0x0000000111263284 _writeJSONValue + 668
	4   Foundation                          0x00000001112636d9 ___writeJSONArray_block_invoke + 132
	5   CoreFoundation                      0x0000000118e1ce2a -[__NSSingleObjectArrayI enumerateObjectsWithOptions:usingBlock:] + 58
	6   Foundation                          0x00000001112633df _writeJSONArray + 330
	7   Foundation                          0x0000000111263204 _writeJSONValue + 540
	8   Foundation                          0x0000000111262f94 -[_NSJSONWriter dataWithRootObject:options:error:] + 124
	9   Foundation                          0x0000000111262e74 +[NSJSONSerialization dataWithJSONObject:options:error:] + 333
	...

—Jens
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.swift.org/pipermail/swift-users/attachments/20161118/7fc1c9fc/attachment.html>