NSArray element failed to match the Swift Array Element type

General
1

Hi,
I’m still new with backendless and I’ve been trying to get my head around this for a while now. I followed the tutorials yet I’m still getting an error while trying to cast from BackendlessCollection to Swift array.
func loadMessages() {
let query = BackendlessDataQuery()
let whereClause = “user = ‘((currentUser?.objectId)!)’”
let queryOptions = QueryOptions()
queryOptions.sortBy([“updated DESC”])
query.whereClause = whereClause
query.queryOptions = queryOptions

let dataStore = backendless.data.of(Message.ofClass())
dataStore.find(query,
response: { (collection: BackendlessCollection!) -> Void in
self.messages.removeAll()
self.messages += collection.getCurrentPage() as! [Message]
self.tableView.reloadData()
}) { (error) -> Void in
print(“Error: (error)”)
}
}
class Message: NSObject {
var ownerId: String?
var objectId: String?
var created: NSDate?
var updated: NSDate?

var lastUser: String?
var user: String?
var lastMessage: String?
var groupId: String?
var counter: Int = 0
var descriptionM: String?
}
On this line: self.messages += collection.getCurrentPage() as! [Message], I get the following error: “fatal error: NSArray element failed to match the Swift Array Element type”
Can someone please point me in a good direction? I tried a lot of different ways to do this and I get the same error each time. A similar problem was reported here: http://support.backendless.com/t/how-to-downcast-a-groupscollection-array-to-a-swift-array, but I’ve had no luck trying all of those solutions.
Thanks

2

Hi Florin,

The “getCurrentPage()” method returns a collection of Message objects, therefore casting the whole collection to just one Message will cause the error. Take a look at the following example which shows how to get the individual objects from the returned collection:
https://backendless.com/feature-16-data-retrieval-api-how-to-load-objects-from-an-mbaas-storage/

Regards,
Mark

3

Hi Mark,

Thanks for your incredibly quick answer. I’ve tried this, from the tutorial you gave me:

func loadMessages() {

    let query = BackendlessDataQuery()

    let whereClause = "user = '\((currentUser?.objectId)!)'"

    let queryOptions = QueryOptions()

    queryOptions.sortBy(["updated DESC"])

    query.whereClause = whereClause

    query.queryOptions = queryOptions

    

    backendless.persistenceService.of(Message.ofClass()).find(query,

        response: { (collection: BackendlessCollection!) -> Void in




            let currentPage = collection.getCurrentPage()

            for item in currentPage as! [Message]{

                print("item: \(item.groupId)")

            }




        }) { (error) -> Void in

            print("Error while retrieved messages: \(error)")

            self.refreshControl?.endRefreshing()

    }

}

I get the same error on line “for item in currentPage as! [Message]{”
“fatal error: NSArray element failed to match the Swift Array Element type”

What am I missing?

4

Hi Florin,

What is you set a breakpoint and check in the debugger what the “currentPage” variable contains?

Regards,
Mark

5

Btw, is the Message class defined in a separate .swift file? If not, please make sure it is not defined in the controller code.

6

Yes, the Message class is defined in a separate .swift file.

The currentPage variable contains one (Message *) object. I attached a screenshot if there is anything else you’d like to check.

Thanks!

Screen Shot 2016-02-22 at 11.21.40 PM.png
Screen Shot 2016-02-22 at 11.21.40 PM.png1376×756 138 KB

7

Florin,

The screenshot helped. I think what happened is the object in the collection was deserialized as our own Message class. I am not sure how to disambiguate between your class and the one included into the SDK (I’d need to defer to our iOS experts). A quick and dirty solution would be to rename the table and your class to something that does not cause the naming collision.

Regards,
Mark

8

Mark,

I guess I’ll take the dirty solution for now. Hopefully you’ll find a way to fix this so others won’t encounter the same issue.

Once again, thank you for answering me so quickly. I don’t always have the time to work on this so your quick response is really appreciated.

Florin

9

Hi Florin,

You should rename your Message class, because Backendless SDK already has public Message class - https://github.com/Backendless/ios-SDK/blob/master/SDK/backendlessAPI/Classes/Messaging/Message.h

On your screenshot you can see that Message objects, you’ve got by getCurrenPage, was casted as Backendless SDK class - not yours (take note on the names of the properties):

http://support.backendless.com/public/attachments/1f1465cf6a2d6f2abb27fc7f13e8fab2.png</img>

So, fix it and try again please.

Regards,
Slava

1f1465cf6a2d6f2abb27fc7f13e8fab2.png
1f1465cf6a2d6f2abb27fc7f13e8fab2.png936×542 216 KB

10

Backendless SDK Message class cannot be rename, because this will bring the problems for existing apps