Getting Started

We start by implementing a simple calculator app in Android and iOS

Android

On Android, we use a standard project with a single Activity.

class MainActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        calculateButton.setOnClickListener {

            val numberA = inputA.text.toString().toDoubleOrNull() ?: 0.0
            val numberB = inputB.text.toString().toDoubleOrNull() ?: 0.0
            val resultValue = numberA + numberB
            result.text = "Result: ${resultValue}"

        }
    }
}

Android Calculator

On iOS we use Xcode and Swift UI to implement the same calculator

struct ContentView: View {
    
    @State private var inputA = ""
    @State private var inputB = ""
    @State private var result = 0.0

    var body: some View {
        
        VStack {
            HStack {
                TextField("Number A", text: $inputA)
            .padding([.leading, .bottom])
            .keyboardType(.numberPad)
            }
            HStack {
                TextField("Number B", text: $inputB)
            .padding([.leading, .bottom])
            .keyboardType(.numberPad)
            }
            Button(action: {
                let numberA = Double(self.inputA) ?? 0.0
                let numberB = Double(self.inputB) ?? 0.0
                self.result = numberA + numberB
            }) {
                Text("Calculate")
            }
            .padding(.bottom)
            
            HStack {
                Text("Result: \(self.result)")
            }

        }
        
    }
}

iOS Calculator

Comparing the “Business Logic” of both implementations we see that they are in fact quite similar, and it would be great to have them in one place only.

    val numberA = inputA.text.toString().toDoubleOrNull() ?: 0.0
    val numberB = inputB.text.toString().toDoubleOrNull() ?: 0.0
    val resultValue = numberA + numberB
    result.text = "Result: ${resultValue}"

    let numberA = Double(self.inputA) ?? 0.0
    let numberB = Double(self.inputB) ?? 0.0
    self.result = numberA + numberB

So let’s try to set up a Kotlin Multi Platform project and refactor that part into a common Kotlin library.