Watchkit new session does not work

Question

I have two view controllers in my watch extension.Whenever I call

[[WCSession defaultSession] sendMessage:applicationData replyHandler:^(NSDictionary *reply)  {}

I get response only for the first view controller, and error in my second viewcontroller

Error Domain=WCErrorDomain Code=7011 "Message reply failed." 
UserInfo={NSUnderlyingError=0x79f1f100 {Error Domain=WCErrorDomain Code=7010 "Payload contains unsupported type."
UserInfo={NSLocalizedRecoverySuggestion=Only pass valid types., NSLocalizedDescription=Payload contains unsupported type.}}, NSLocalizedDescription=Message reply failed.}

WCSession is initiated in both app and watch extension.Any suggestions?

Answer 1

In WCSessionDelegate's - session:didReceiveMessage:replyHandler: method, the replyHandler argument is defined as [String : AnyObject]. The AnyObject portion is misleading. It can only contain a property list data type: NSData, NSString, NSArray, NSDictionary, NSDate, and NSNumber. (Under these circumstances, it makes sense why AnyObject was chosen, since those 6 data types do not inherit from a common subclass besides NSObject.)

Usually people mention that NSCoding and NSKeyedArchiver can resolve the issue, but I haven't seen more examples/explanations beyond that.

The thing to note is that the replyHandler dictionary doesn't care about serialization. You could use NSKeyedArchiver, JSON, your own custom encoding, etc. So long as the dictionary only contains those 6 data types replyHandler will be happy. Otherwise you'll see the Payload contains unsupported type. error.

For this reason, you can never call the reply handler like so: replyHandler(["response": myCustomObject), even if myCustomObject implements the NSCoding protocol perfectly.

Summary of encoding choices:

• NSCoding: The main advantage is that when you unarchive, it'll automatically find the correct class and instantiate it for you including any objects in its subgraph.
• JSON
• Custom encoding: One advantage is that your object is not forced to inherit from NSObject, which is sometimes helpful in Swift.

---

If you do use NSCoding, this is what it'll look like:

iPhone App:

func session(session: WCSession, didReceiveMessage message: [String : AnyObject], replyHandler: ([String : AnyObject]) -> Void) {
    let data = NSKeyedArchiver.archivedDataWithRootObject(myCustomObject)
    replyHandler(["response": data])
}

Watch App:

WCSession.defaultSession().sendMessage([],
    replyHandler: {
        response -> Void in
        let myCustomObject = NSKeyedUnarchiver.unarchiveObjectWithData(response["response"])
    }, errorHandler: nil
)

Note that if you want to recover from a crash when unarchiving objects, you'll need to use the new iOS 9 API, unarchiveTopLevelObjectWithData, which throws an error if there was a problem.

Note: Your custom object must inherit from NSObject, otherwise you'll get the following error when archiving:

*** NSForwarding: warning: object ... of class 'Foo' does not implement methodSignatureForSelector: -- trouble ahead Unrecognized selector -[Foo replacementObjectForKeyedArchiver:]

Answer 2

"Payload contains unsupported type" likely means you are sending a custom object in your message dictionary. You will need to serialize this data to contain only the supported types (NSNumber, NSDate, NSString, NSData, NSArray, and NSDictionary).

I have a github project that automatically serializes your custom objects into safe ones for watchkit transfer. You can check it out here.