Swift - AppToApp communication 

https://developer.apple.com/documentation/uikit/core_app/communicating_with_other_apps_using_custom_urls

communicate with using URL
---------
create url schemes
    go to prokject-> info -> urlTypes
        fill identifier - Actually there is no usage at all. you can use anything. only the sceme and role considered.
        URL schemes - unique app scheme which other apps use to open the app, cannot use existing ios system schemes in here (https://developer.apple.com/library/content/featuredarticles/iPhoneURLScheme_Reference/Introduction/Introduction.html#//apple_ref/doc/uid/TP40007899)
        Role -
            Editor - schemes if we are defines
            Viewer - existing schemes in iOS system (like photo viewer, email , sms apps)

    how it works
        if we want to open app2 from app1
            in app2
                identifier - dont know the correct usage. in apple Doc also not gives a full Description
                schecme - used iPay
                role - Editor (this is our own define scheme. so need to use as Editor. if systemdefine scheme should as view)

            in App1
                app1 we should also have to define url scheme for matching for appys2. scheme should be same and role should None.
                    identifier - can use anything
                    schecme - used iPay
                    role - None

                let kCustomURLScheme = "ipay://" // only the sceme used. not identifier
                let customURL = URL(string: customURLScheme)!
                if UIApplication.shared.canOpenURL(customURL) {
                    if #available(iOS 10.0, *) {
                        UIApplication.shared.open(customURL, completionHandler: { (success) in
                            print("Open url : \(success)")
                        })
                    } else {
                        UIApplication.shared.openURL(customURL)
                    }
                    return true
                }
    ** if we use same scheme for two application

        system starts first installed application. if we uninstall first app then it will open second app.