From Swift it is mainly Apple's CoreBluetooth API calls that power the Nordic Uart Service (nus). You can find the source to nrfToolbox here: github.com/.../IOS-nRF-Toolbox, and you can use it to connect to a nordic device running the ble_app_uart example from the SDK. NUS is just a series of GATT Notifications that move the bytes between devices. A slight caution when reading the sdk example code, it effectively contains two uarts -- one a physical uart, and one a virtual uart that transports over btle.
nrfToolbox contains a *lot* of functionality. Take a look at NORBluetoothManager.swift, and NORUartViewController.swift.
