Read.msc

msc {
  hscale = "2";
 
  CM[label="App"], CBC[label="CBCentralManager"],
  BLE[label="BLE"],
  CBP[label="CBPeripheralManager"],PM[label="Simulated Device\n(Another App)"];
  
  
  ---     [label = "Read Request", ID="*" ];
  CM=>>CBC[label = "connectPeripheral:X options:"];
  CBC-xBLE[label = "Listen for Connectable Peripheral X"];
  PM=>>CBP[label = "addService:Y[Characteristic=Z]"];
  ...;
  ...     [label = "Time Passes"];
  PM=>>CBP[label = "startAdvertising:"];
  CBP->BLE[label = "Connectable & Scannable Advertisement"];
  BLE->CBC[label = "Connected Peripheral X found"];
  CBC=>CM [label = "centralManager:didConnectPeripheral:X"];
  
  CM=>>CBC[label = "X.discoverServices:[Y]"];
  CBC->CBP[label = "GAP Discover Service UUID Y"];
  CBP->CBP[label = "Lookup Service Y"];
  CBP->CBC[label = "GAP Service Handle"];
  CBC=>CM [label = "peripheral:X didDiscoverServices: Y"];

  CM=>>CBC[label = "X.discoverCharacteristics:Z forService: Y"];
  CBC->CBP[label = "GAP Discover Characteristic Z for service Y"];
  CBP->CBP[label = "Lookup Characteristic Z in service Y"];
  CBP->CBC[label = "GAP Characteristic Handle"];
  CBC=>CM [label = "peripheral:X didDiscoverCharacteristicsForService: Y\n\n"];

  CM=>>CBC[label = "X.readValueForCharacteristic:Z"];
  CBC->CBP[label = "GAP Read Characteristic Z"];

  CBP=>PM [label = "peripheralManager:didReceiveReadRequest:"];
  PM=>>CBP[label = "respondToRequest:withResult:(Payload)"];
  CBP->CBC[label = "GAP Update Characteristic Z(Payload)"];
  CBC=>CM [label = "peripheral:X didUpdateValueForCharacteristic: Z\n\n"];
  CM=>CM  [label = "Decode (Payload)"];
}