SwiftUI View not updating when #ObservedObject changes using GCD - ios

Question

I'm still in the early stages with SwiftUI. Everything's been fine up until now, but I'm running into problems with using GCD - never worked with multi-threading before.
I am getting data from various sensors over TCP continuously running in the background. All this seems fine, I can see the data coming across and it appears to be getting stored where I want it...
Raw data
$AIMTW,26.1,C*1E
Contents of struct
(src: "AI", timeStamp: 2021-09-11 10:43:01 +0000, temp: 26.1, units: "ºC")
I think I'm getting the data processing happening on the main thread where it should update my View, but it doesn't update. I expect I've missed something pretty fundamental, but I just can't work out what it is. Any help would be appreciated.
This should hopefully be enough to reproduce the problem. Originally the code would be reading in a constant stream of data from a TCP source; I have modified it to read from a text file. The View should update after each line of the text file.
import Foundation
struct NMEA{
// Mean Temperature of Water
var mtw = (src: "", // Source
timeStamp: Date(), // Generated Time Stamp
temp: 0.0, // temperature
units: "") // unit of measurement, Celsius (C)
}
import SwiftUI
struct ContentView: View {
#ObservedObject var nmeaHandler = NMEAhandler()
var body: some View {
HStack{
Text("Water Temperature: ")
Text(String(nmeaHandler.nmea.mtw.temp))
Text(nmeaHandler.nmea.mtw.units)
}
.onAppear{
nmeaHandler.fetchNMEA()
}
}
}
import Foundation
class NMEAhandler: ObservableObject{
#Published var nmea = NMEA()
func fetchNMEA() {
DispatchQueue.global(qos: .userInitiated).async { self.receiveNMEA()
}
}
private func receiveNMEA() {
do {
let path = Bundle.main.path(forResource: "reprex-input", ofType: "txt") ?? "" // file path for file "data.txt"
let message = try String(contentsOfFile: path, encoding: String.Encoding.utf8)
print(message)
message.enumerateLines(invoking: { (line, stop) -> () in
print("Line = \(line)")
})
DispatchQueue.main.async {
self.prepareSentences(data: message)
print("The View updates now.")
}
} catch {
print("Error:", error)
}
}
private func prepareSentences(data: String){
var messages: [String] = []
var fields: [String] = []
if data != "-"{
messages = data.components(separatedBy: "\r\n")
for message in messages{
fields = message.components(separatedBy: ",")
let lastField = fields.last
let splitLast = lastField?.components(separatedBy: "*") ?? [""]
fields.removeLast(1)
fields.append(contentsOf: splitLast)
let valid = checksum(message: message, checksum: fields.last ?? "")
if valid{
parseNMEA(fields: fields)
// The View should be updated by now
}
}
return
}
}
private func checksum(message: String, checksum: String) -> Bool {
let messageLength = message.count
let firstCharacter = message.first
if messageLength < 5 || firstCharacter != "$"{
return false
}
let actualMessage = message.slice(1, messageLength - 4)
var calcChecksum = 0
for c in actualMessage.utf8 {
calcChecksum = calcChecksum ^ Int(c)
}
if calcChecksum == Int(checksum, radix: 16) ?? 999{
return true
}
return false
}
private func parseNMEA(fields: [String]){
switch fields[0].slice(3, 5){
case "MTW":
nmea.mtw.src = fields[0].slice(1, 2)
nmea.mtw.timeStamp = Date()
nmea.mtw.temp = Double(fields[1]) ?? 0
nmea.mtw.units = "º" + fields[2]
print("MTW: \(nmea.mtw)")
default:
print("Missing sentence: \(fields[0].slice(1, 2))")
return
}
}
}
import Foundation
extension StringProtocol {
func slice(_ start: Int, _ end: Int) -> String {
if self.count >= end {
let lower = index(self.startIndex, offsetBy: start)
let upper = index(lower, offsetBy: end - start)
return String(self[lower...upper])
} else {
return self as! String
}
}
}
Text file used for input:
$AIMTW,24.7,C*1A
$AIMTW,24.9,C*14
$AIMTW,24.8,C*15
$SDMTW,19.4,C*08
$AIMTW,24.9,C*14
$SDMTW,19.4,C*08
$SDMTW,19.4,C*08
$AIMTW,24.7,C*1A
$SDMTW,19.6,C*0A
$AIMTW,23.9,C*13