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

[Tony Parker]

FYI, here is the GitHub commit that addressed this issue:

https://github.com/apple/swift/commit/1f11ef2fd29f214bd9d9a5fa85246d36c04448f0 <https://github.com/apple/swift/commit/1f11ef2fd29f214bd9d9a5fa85246d36c04448f0>

It was also reported as SE-0140.

Basically, optional values were getting bridged to an opaque-to-Foundation “_SwiftValue” class, which means NSJSONSerialization couldn’t figure out what to do with it.

- Tony

> On Nov 18, 2016, at 11:46 AM, Dan Loewenherz via swift-users <swift-users at swift.org> wrote:
> 
> I've encountered a similar bug and filed a radar a few months ago. My report was marked as a duplicate but besides that I haven't received any follow-up. It does appear to have been fixed in Xcode 8.1 / Swift 3.0.1. http://www.openradar.me/28365419 <http://www.openradar.me/28365419>
> 
> Essentially, NSJSONSerialization would (does?) crash if you ask it to encode an array/dictionary of Anys containing an optional value containing a bridged Foundation type. The below snippet was the easiest demonstration of the runtime exception.
> 
>     let name: String? = "Spike"
>     let dogNames: [Any] = [name]
> 
>     do {
>         try JSONSerialization.data(withJSONObject: dogNames, options: .prettyPrinted)
>     } catch {
>         print("HI")
>     }
> 
> What version of Swift is the user of your framework using?
> 
> Dan
> 
> On Fri, Nov 18, 2016 at 1:37 PM, Jens Alfke via swift-users <swift-users at swift.org <mailto:swift-users at swift.org>> wrote:
> 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
> 
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