Very slow communication between apple watch and iPhone - swift

Question

I have wrote a very basic Xcode project that contains 3 targets:
- iOS target
- WatchKit app
- WatchKit extension
First of all, i do not understand why Xcode creates a second target (extension) for WatchKit app ? It seems that WatchKit app contains storyboard, and WatchKit extension contains swift code (controllers). Is there a particular reason for Xcode to design and split 2 targets instead of one single ?
Look at this very basic piece of code:
iOS controller:
override func viewDidLoad()
{
super.viewDidLoad()
if WCSession.isSupported()
{
let session = WCSession.default()
session.delegate = self
session.activate()
}
}
#IBAction func on_btn_tap(_ sender: Any)
{
if WCSession.isSupported()
{
let session = WCSession.default()
session.sendMessage(["mykey": "myvalue"], replyHandler: { (response) -> Void in
NSLog("OK")
}, errorHandler: { (error) -> Void in
NSLog("Error)
})
}
}
On watch extension (InterfaceController.swift):
override func awake(withContext context: Any?)
{
super.awake(withContext: context)
if WCSession.isSupported()
{
let session = WCSession.default()
session.delegate = self
session.activate()
}
}
extension InterfaceController: WCSessionDelegate
{
func session(_ session: WCSession,
activationDidCompleteWith activationState: WCSessionActivationState,
error: Error?)
{
}
func session(_ session: WCSession, didReceiveMessage message: [String : Any], replyHandler: #escaping ([String : Any]) -> Void)
{
self.btn.setBackgroundColor(UIColor.yellow)
}
}
As you certainly understand, i have a button on my iOS App. When i tap on this button, i send a message to Watch App and this app will change a button color.
There is a delay of about 5-6 seconds between the button tap on the iPhone and the color change. Do you know why ?
In the other communication side (watch to iPhone), it is worst (10-15 seconds)
Thanks

Answer 1

Since you are updating your UI you need to wrap it in a DispatchQueue, like this:
DispatchQueue.main.async {
self.btn.setBackgroundColor(UIColor.yellow)
}
These delegate callbacks are not on the main thread and you should never update your UI from any other thread than the main thread. Wrapping it like this results in much faster updating of your UI and safer code.