Hi so my conversion app is working as intended (almost!) I would like to pop an alert when a valid Double is not provided as well as setting the default value to 0. I have almost achieved this but I believe I am changing my showingAlert variable to true in the wrong place. I get the warning:

Modifying state during view update, this will cause undefined behaviour

Can anyone help?

//
//  ContentView.swift
//  Converter
//
//  Created by Lee Casey on 09/08/2020.
//  Copyright © 2020 Lee Casey. All rights reserved.
//
//  C -> F = (0°C × 9/5) + 32 = 32°F
//  F -> C = (0°F − 32) × 5/9
//  C -> K = 0°C + 273.15 = 273.15K

import SwiftUI

struct ContentView: View {
    @State private var source = ""
    @State private var sourceSelection = 0
    @State private var resultSelection = 0
    @State private var showingAlert = false

    let units = ["Celcius", "Farenheit", "Kelvins"]

    //work all calculations from C
    func calcBase() -> Double {
        //let x = Double(source) ?? 0.0
        var x: Double

        if let unwrapped = Double(source) {
            x = unwrapped
        } else {
            x = 0.0
            showingAlert = true
        }

        if (sourceSelection == 1) {
           return (x - 32) * 5/9
        }

        else if (sourceSelection == 2) {
            return x - 273.15
        }

        return x
    }

    func calculate() -> Double {
        if (resultSelection == 0) {
            return Double(calcBase())
        }
        else if (resultSelection == 1) {
            return (Double(calcBase()) * 9/5) + 32
        }
        else {
            return (Double(calcBase()) + 273.15)
        }
    }

    var body: some View {
        NavigationView {
            Form {
                Section(header: Text("Please enter the amount you want to convert")) {
                    TextField("Enter here", text: $source)
                    .keyboardType(.decimalPad)
                    Picker("Unit", selection: $sourceSelection) {
                        ForEach(0 ..< units.count) {
                            Text("\(self.units[$0])")
                        }
                    }
                    .pickerStyle(SegmentedPickerStyle())
                    .alert(isPresented: $showingAlert) {
                           Alert(title: Text("ERROR"), message: Text("Please enter a valid number"), dismissButton: .default(Text("OK")))
                       }
                }
                Section(header: Text("Results")) {
                    Text("\(calculate(), specifier: "%.2f")")
                    Picker("Unit", selection: $resultSelection) {
                        ForEach(0 ..< units.count) {
                            Text("\(self.units[$0])")
                        }
                    }
                    .pickerStyle(SegmentedPickerStyle())
                }
            }
            .navigationBarTitle("Converter")
        }
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}

Your call to calculate() changes the value of showingAlert, but calculate() is being called inside body{} so SwiftUI can't determine how to render the View reliably.

showingAlert needs to be set outside the View itself, in a modifier of some sort or perhaps a button. Hopefully someone more experienced can offer a specific solution!

Actually, there is a solution on how to fix this on this site :) https://www.hackingwithswift.com/quick-start/swiftui/how-to-fix-modifying-state-during-view-update-this-will-cause-undefined-behavior

hi,

i'd recommend that you not try to pop up an Alert any time there's an illegal input in a live edit/convert like this -- it will be pretty annoying, and it just won't happen that often if you have the decimal keypad on screen. (by the way, entering a negative temperature is a problem on the decimal pad.)

instead, just replace the computation of the output converstion string so that it shows there's an error. just replace Text("\(calculate(), specifier: "%.2f")") with a basic function call like this:

Text(conversionResult())

the conversionResult could look something like this:

    func conversionResult() -> String {
        guard source.count > 0 else {
            return "0.0"
        }
        guard var inputValue = Double(source) else {
            return "Input Error"
        }
        // convert to C
        if (sourceSelection == 1) {
            inputValue = (inputValue - 32) * 5/9
        } else if (sourceSelection == 2) {
            inputValue -= 273.15
        }
        // convert to desired output base
        var convertedValue: Double
        if (resultSelection == 0) {
            convertedValue = inputValue
        }
        else if (resultSelection == 1) {
            convertedValue = (inputValue * 9/5) + 32
        }
        else {
            convertedValue = inputValue + 273.15
        }
        return String(format:"%.2f", convertedValue)
    }

that should do it for you.

a comment to consider: the logic of conversion could be simplified if you took a look at Foundation's support for Measurement. Paul has some information on this.

hope that helps,

DMG

Thank you everyone. Much appreciated.