Mobile App Dev 2021W: Tutorial 2

This tutorial is still being developed.

In this tutorial you'll be playing with Convert2Cmd and Converter2. Convert2Cmd and Converter2 both make use of the unit converters defined in Converter.swift; Convert2Cmd provides a simple command line, menu-based interface to the converters, while Converter2 uses them in a simple SwiftUI-based iOS application. By studying two interfaces for the same functionality, you can see how SwiftUI-based interfaces compare with command line ones.

Key Concepts

Below are the key concepts covered in this tutorial, with links to (mostly) Apple documentation on them.

• Swift functions & closures
• Swift dictionaries & arrays
• Writing simple command line synchronous interface in Swift
• Declaring custom views in SwiftUI
• SwiftUI @State and @Binding. (This article by Shen Sheng is also helpful.
• SwiftUI ForEach (hackingwithswift.com)

You can supplement these with other web resources. In particular, the WWDC videos related to SwiftUI are good, such as this 2020 introduction to SwiftUI. There are also videos from WWDC 2019 and before, such as this video on SwiftUI Essentials. While you do not need to consult outside material to do the assignments in this class, another perspective can be helpful.

Questions

Part A: Converter.swift

1. If you changed Conversions:6 to var convert: (from: Double) -> Double? (removing the underscore), what else would have to be changed to make this declaration correct? Why?
2. The "inch to cm" conversion is coded differently from the others. What is different about it? How would you change it to be similar to the rest?
3. When is the init method of the Converter struct (Conversions:16) called?
4. Is the type declaration in Conversions:28 (the declaration of the conversions dictionary) necessary? Why?

Part B: main.swift (Convert2Cmd)

1. What type is convAvail?
2. What is the i+1 for on main:8?
3. How could you change the conversion message to be of the form "Boss, I think 0 Celsius is 32 Farenheit!" with only making changes to main.swift, not Conversions.swift?
4. Why is the test on main:25 needed? Why main:26?

Part C: ContentView.swift (Converter2)

1. Can you combine all of the separate views into one view, getting rid of ConvMenu and ConvOutput? Show your attempt and say what happens.
2. Why is there an ! on ContentView:43? How could you get rid of it?
3. Add a menu to the bottom of the screen, identical to the one at the top. How hard was this to do?
4. How could you have values from previous conversions used for new conversions? For example, if you entered in 32 for Fahrenheit, how could you keep that 32 when you switched to inches?
5. Could the ForEach() on ContentView:27 have been used in main? Explain.

Code

• Convert2Cmd.zip
• Converter2.zip

Conversions.swift

//  Conversions.swift

struct Converter {
    var convFrom = "From Type"
    var convTo = "To Type"
    var convert: (_ from: Double) -> Double?
    
    func formatConversion(_ from: Double) -> String {
        if let to = self.convert(from) {
            return "\(from) \(self.convFrom) is \(to) \(self.convTo)."
        } else {
            return "Converting \(from) \(convFrom) to \(self.convTo) failed."
        }
    }
    
    init(_ from: String, _ to: String, _ f: @escaping (_ from: Double) -> Double?) {
        self.convFrom = from
        self.convTo = to
        self.convert = f
    }
}

func InToCM(_ inch: Double) -> Double {
    let cm = 2.54 * inch
    return cm
}

let conversions: [String: Converter] =
    ["F to C": Converter("Farenheit", "Celsius",
                         {F in return ((F - 32.0)*(5/9))}),
     "C to F": Converter("Celsius", "Farenheit",
                         {C in return ((9/5)*C + 32.0)}),
     "km to mi": Converter("kilometers", "miles",
                           {k in
                            let m = k * 0.6213712
                            return m
                           }),
     "inch to cm": Converter("inches", "centimeters",
                             InToCM)
    ]

main.swift (Convert2Cmd)

//  main.swift (Convert2Cmd)

var convAvail = conversions.keys.sorted()

func getChoice() -> String? {
    print("Available Conversions:")
    for i in 0..<convAvail.count {
        print(" \(i+1). \(convAvail[i])")
    }
    
    print("Enter choice: ", terminator: "")
    if let choice = readLine() {
        if let c = Int(choice) {
            if ((c > 0) && (c <= convAvail.count)) {
                return convAvail[c-1]
            }
        }
    }
    return nil
}

let getConversion = {(_ choice: String) in
    let conv = conversions[choice]!
    print("\nEnter \(conv.convFrom): ", terminator: "")
    if let vs = readLine() {
        if let v = Double(vs) {
            let res = conv.formatConversion(v)
            print(res)
            return
        } else {
            print("You didn't enter a number!")
        }
    }
}

if let choice = getChoice() {
    getConversion(choice)
} else {
    print("You didn't make a valid choice!")
}

ContentView.swift (Converter2)

// ContentView.swift (Converter2)

import SwiftUI

struct ContentView: View {
    @State var convMode: String? = nil
    @State var fromS = ""
    var body: some View {
        VStack{
            ConvMenu(convMode: $convMode,
                     fromString: $fromS)
            Spacer()
            ConvOutput(convMode: $convMode,
                       fromString: $fromS)
            Spacer()
            Spacer()
        }
    }
}

struct ConvMenu: View {
    @Binding var convMode: String?
    @Binding var fromString: String
    var body: some View {
        let availConversions = [String](conversions.keys)
        Menu("Conversions menu") {
            ForEach(availConversions, id: \.self) {
                conversion in
                Button(conversion, action: {
                    convMode = conversion
                    fromString = ""
                })
            }
        }
    }
}

struct ConvOutput: View {
    @Binding var convMode: String?
    @Binding var fromString: String
    var body: some View {
        if let mode = convMode {
            let conv = conversions[mode]!
            TextField("Enter \(conv.convFrom)", text: $fromString)
            Spacer()
            if let from = Double(fromString) {
                Text(conv.formatConversion(from))
            }
        } else {
            Text("Please select a conversion")
            Text("type from the menu above.")
        }
    }
}

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