ios: only detect the device in the backend once paired

The device first connects. Pairing happens later when a secured
characteristic is used. We assume the device is paired when we can
read the product string, at which point the backend register the
device. Otherwise, the app would launch into the device workflows in
the UI and get overlaid with the pairing code suddenly, which looks broken.

Author: benma

frontends/ios/BitBoxApp/BitBoxApp/Bluetooth.swift

@@ -49,6 +49,8 @@ class BluetoothManager: NSObject, ObservableObject, CBCentralManagerDelegate, CB
     var pWriter: CBCharacteristic?
     var pReader: CBCharacteristic?
     var pProduct: CBCharacteristic?
+    
+    private var isPaired: Bool = false
 
     // Peripherals in this set will not be auto-connected even if previously paired.
     // This is for failed connections to not enter an infinite connect loop.
@@ -66,7 +68,7 @@ class BluetoothManager: NSObject, ObservableObject, CBCentralManagerDelegate, CB
     }
 
     func isConnected() -> Bool {
-        return connectedPeripheral != nil && pReader != nil && pWriter != nil;
+        return isPaired && connectedPeripheral != nil && pReader != nil && pWriter != nil;
     }
 
     func connect(to peripheralID: UUID) {
@@ -100,13 +102,7 @@ class BluetoothManager: NSObject, ObservableObject, CBCentralManagerDelegate, CB
             restartScan()
         case .poweredOff, .unauthorized, .unsupported, .resetting, .unknown:
             print("BLE: unavailable or not supported")
-            connectedPeripheral = nil
-            pReader = nil
-            pWriter = nil
-            pProduct = nil
-            state.discoveredPeripherals.removeAll()
-            state.connecting = false
-            updateBackendState()
+            handleDisconnect()
         @unknown default:
             print("BLE: Unknown Bluetooth state")
         }
@@ -242,22 +238,36 @@ class BluetoothManager: NSObject, ObservableObject, CBCentralManagerDelegate, CB
         }
         if characteristic == pProduct {
             print("BLE: product changed: \(String(describing: parseProduct()))")
+            // We can only read the product characteristic when paired.
+            isPaired = true
             // Invoke device manager to scan now, which will make it detect the device being connected
             // (or disconnected, in case the product string indicates that) now instead of waiting for
             // the next scan.
             MobileserverUsbUpdate()
         }
     }
+    
+    func handleDisconnect() {
+        connectedPeripheral = nil
+        pReader = nil
+        pWriter = nil
+        pProduct = nil
+        state.discoveredPeripherals.removeAll()
+        state.connecting = false
+        isPaired = false
+        updateBackendState()
+        
+        // Have the backend scan right away, which will make it detect that we disconnected.
+        // Otherwise there would be up to a second of delay (the backend device manager scan interval).
+        MobileserverUsbUpdate()
+        
+        restartScan()
+    }
 
     // This method gets called if the peripheral disconnects
     func centralManager(_ central: CBCentralManager, didDisconnectPeripheral peripheral: CBPeripheral, error: Error?) {
         print("BLE: peripheral disconnected")
-        connectedPeripheral = nil;
-        pReader = nil;
-        pWriter = nil;
-        pProduct = nil;
-
-        restartScan()
+        handleDisconnect()
     }
 
     func readBlocking(length: Int) -> Data? {