A simple communications protocol

simpleprotocol.swift

let START_BYTE: UInt8 = 0x42 // or whatever
let ACTION_ON: UInt8 = 0x01 // or whatever
let ACTION_SLIDER: UInt8 = 0x02 // same here

var state = 0
var code: UInt8 = 0
var expected_count = 0
var data = [UInt8]()

func serialDidReceiveBytes(bytes: [UInt8]) {
    for b in bytes {
        switch state {
        case 0:
            // expect start byte
            if b == START_BYTE {
                // yay! a new beginning
                state += 1
                continue
            } else {
                // fail. better luck next time
                continue
            }
            
        case 1:
            // expect a code byte
            switch b {
            case ACTION_ON:
                // no data bytes needed, perform action and reset
                do_action_on()
                reset()
                continue
                
            case ACTION_SLIDER:
                // two data bytes needed
                code = ACTION_SLIDER
                expected_count = 2
                state += 1
                continue
                
            default:
                // error! reset!
                reset()
                continue
            }
            
        case 2:
            // expect a data byte
            data.append(b)
            
            if data.count == expected_count {
                // perform action and reset
                switch code {
                case ACTION_SLIDER:
                    do_action_slider_with_data(data)
                default:
                    // error
                    break
                }
                reset()
            }
            continue
        }
    }
}

func reset() {
    state = 0
    code = 0
    expected_count = 0
    data = []
}