Swift: Downcast causes application to crash, why? - ios

Question

I have following snippets of code where I have crash on some devices:
Crashed: com.apple.root.default-qos
EXC_BAD_ACCESS KERN_INVALID_ADDRESS at 0x00000000cbd
Code:
var obj:AnyObject = command.arguments[0] as AnyObject!
var theData:AnyObject = obj["getContactImagesByEmails"] as AnyObject!
if let contactImagesByEmails:AnyObject = obj["emails"]{
if contactImagesByEmails is Array<String>{
/*line 176*/ let array:Array<String> =
contactImagesByEmails as Array<String> // CRASH happens here
results = WmSqliteImagesModel.getInstance.getImagesByEmailAsWmContactImage(array) as Dictionary<String,AnyObject>
}
}
Full stack trace
Thread : Crashed: com.apple.root.default-qos
0 libswiftCore.dylib 0x0000000100559794 swift_unknownRetain + 32
1 MyApp 0x000000010017c8a0 MyApp.Plugin.(getContactImagesByEmails (MyApp.Plugin) -> (ObjectiveC.CDVInvokedUrlCommand) -> ()).(closure #1) (Plugin.swift:176)
2 MyApp 0x000000010017c8a0 MyApp.Plugin.(getContactImagesByEmails (MyApp.Plugin) -> (ObjectiveC.CDVInvokedUrlCommand) -> ()).(closure #1) (Plugin.swift:176)
3 MyApp 0x00000001001790b0 partial apply forwarder for reabstraction thunk helper from #callee_owned () -> (#unowned ()) to #callee_owned (#in ()) -> (#out ()) with unmangled suffix "125" (Plugin.swift:62)
4 MyApp 0x0000000100179120 partial apply forwarder for reabstraction thunk helper from #callee_owned (#in ()) -> (#out ()) to #callee_owned () -> (#unowned ()) with unmangled suffix "128" (Plugin.swift:62)
5 libdispatch.dylib 0x00000001937e13ac _dispatch_call_block_and_release + 24
6 libdispatch.dylib 0x00000001937e136c _dispatch_client_callout + 16
7 libdispatch.dylib 0x00000001937ed40c _dispatch_root_queue_drain + 1152
8 libdispatch.dylib 0x00000001937ee75c _dispatch_worker_thread3 + 108
9 libsystem_pthread.dylib 0x00000001939bd2e4 _pthread_wqthread + 816
The Plugin.swift:176 points to:
let array:Array<String> = contactImagesByEmails as Array<String>
Do I miss something? I think this code should be safe.
if contactImagesByEmails is Array<String> returns true, why contactImagesByEmails as Array<String> fails?
Please help,
[EDIT]
command has type of CDVInvokedUrlCommand
#interface CDVInvokedUrlCommand : NSObject {
NSString* _callbackId;
NSString* _className;
NSString* _methodName;
NSArray* _arguments;
}

Answer 1

I don't know if this can actually fixes the problem, but there's a bit of redundancy in the 2 ifs and the following let. It can simply be written as:
if let contactImagesByEmails = obj["emails"] as? Array<String> {
results = WmSqliteImagesModel.getInstance.getImagesByEmailAsWmContactImage(contactImagesByEmails) as Dictionary<String,AnyObject>
}
Moreover, it would be safer to combine optional binding and optional downcast when invoking getImagesByEmailAsWmContactImage
if let results = WmSqliteImagesModel.getInstance.getImagesByEmailAsWmContactImage(contactImagesByEmails) as? Dictionary<String,AnyObject> {
...
}
Last, are you sure getInstance is a property and not a method? Shouldn't it be:
if let results = WmSqliteImagesModel.getInstance().getImagesByEmailAsWmContactImage(contactImagesByEmails) as? Dictionary<String,AnyObject> {
...
}

Answer 2

Perhaps try an alternate approach to getting the casted value:
if let contactImagesByEmails = obj["emails"] as? [String] {
results = WmSqliteImagesModel.getInstance.getImagesByEmailAsWmContactImage(contactImagesByEmails) as Dictionary<String,AnyObject>
}
I also wonder if your crash is not from naming your variable "array" in your original code...

Answer 3

Simplify this step by step with fewer references to AnyObject (and certainly not AnyObject!). The compiler is probably letting through something that is illegal, because when you start pulling out AnyObject you're saying "I know exactly what I'm doing; don't check this."
You'd expect this code to be something like:
let obj = command.arguments[0]
if let contactImagesByEmails = obj["emails"] as? [String] {
results = WmSqliteImagesModel.getInstance.getImagesByEmailAsWmContactImage(array) as? [String: AnyObject]
}
In the end results would be a [String:AnyObject]?
The key point here is that you should get rid of as many references to AnyObject as you can, and you should use if-let-as? to determine if the types are what you expect.
The fact that command.arguments is filled with things of various types that you have to type-check is pretty dangerous and a sign of a serious design problem. If there really are several types in there, you should use an enum rather than type-checking against random things like [String:AnyObject]. That's what enums are explicitly for. (If this is bridging from ObjC; enums may not be possible here, but even in ObjC, the right answer is seldom to have an NSArray filled with heterogeneous types. You create a class there to hold them rather than an enum.)