Why is 'Simulate Background Fetch' leading to crash (libsystem_kernel.dylib`mach_msg_trap) since Xcode 8?

Question

I\'m on macOS Sierra, Xcode 8, and get crashes whenever I try to Simulate Background Fetch on an actual iOS 10 device. This does NOT occur when using the simulator. This o

Answer 1

I just had this issue, it had something to do with pulling data from my backend and a casting type.

If you aren't familiar with firebase just look at the last two lines of this. If you are, here is how I fixed it. My data coming in looked something like this

"Node" : {
      "SubNode" : {
        "-KoB8OMIO0PLiTs8fUkJ" : {
      "ImageName" : "DSC05833-2.jpg",
      "price" : 100,
    },
    "-KoB8Rh9PtSMaMUlaD91" : {
      "ImageName" : "DSC05780-2.jpg",
      "price" : 0,
    },

And my code to pull.

ref.child("Node").child("SubNode").child(uniqueidID).observeSingleEvent(of: DataEventType.value, with: { (snapshot) in
        if snapshot.childrenCount > 0
        {
            let jsonPhoto = snapshot.value as? [String: AnyObject]
            let mageName = jsonPhoto?["ImageName"] as! String            
            photoObj.imagePrice = jsonPhoto?["price"] as! Double //error, Swift didn't catch, just froze my app
        }
})

it worked if value was 100 -> Double, and then for some reason firebase was failing when I casted 0 -> Double

So I fixed how I casted it

I originallyy had this:

photoObj.imagePrice = jsonPhoto?["price"] as! Double // caused error

to..

 photoObj.imagePrice = jsonPhoto?["price"] as? Double ?? 0.00 //fixed error