Hello World Tutorial

Overview

The HelloWorld example shows how to develop a simple Swift class in Xcode, compile it to a native Android library, and very easily call the code in Android Studio.

Functionality demonstrated	Technology demonstrated	Github Project
• Call methods of a Swift class • Support of closures	• Swift Android compiler • Swift4j	• n.a

The HelloWorld tutorial is extremely detailed. Please ensure you understand each step. In other tutorials, we will omit some instructions to avoid instruction bloat.

Develop in Xcode

Create Xcode project directory

Create a directory HelloWorld that contains your Swift library. Let’s refer to this as the project directory.
Terminal window
```
./Projects % mkdir HelloWorld
```

Create Xcode project

Create an Xcode project that contains your shared code:

swift package init --type library --name HelloWorld

Make sure to use it from inside your project directory:

./Projects/HelloWorld  % swift package init --type library --name HelloWorld

As a result, you see:

Creating library package: HelloWorld
Creating Package.swift
Creating .gitignore
Creating Sources/
Creating Sources/HelloWorld/HelloWorld.swift
Creating Tests/
Creating Tests/HelloWorldTests/
Creating Tests/HelloWorldTests/HelloWorldTests.swift

Open project in Xcode and add first Swift class

Open the project in Xcode.
Create a Swift class called GreetingService:
```
public class GreetingService {
    // applies the function fct to the String parameter name and returns the result
    func greet(name: String, _ fct:  (String) -> String) -> String {
      return fct(name)
  }
}
```
- This is a very simple class with a function greet.
- Function greet has two arguments:
  - name: a variable of type String
  - fct: a variable of type closure
- The function executes the closure using the name argument as an input and returns a String.

Add @jvm to compile the class to Android

Now we make the changes to be able to compile the class to Android.

Add @jvm to the classes you want to compile to Android. In our case here, we add @jvm to the GreetingService class.

Import Swift4j using import Swift4j (pay attention to the lowercase j). The @jvm annotation is part of the Swift4j library.

import Swift4j

@jvm
public class GreetingService {
    // applies the function fct to the String parameter name and returns the result
    func greet(name: String, _ fct:  (String) -> String) -> String {
      return fct(name)
  }
}

As you can see, Xcode is complaining about the missing module Swift4j (“No such module Swift4j”).

Add Swift4j dependency in package.swift

To fix the compile errors and compile the Swift project, you now need to add the necessary dependencies and modify the Package.swift as described here.

The steps describe the necessary changes. Either carefully make the changes yourself for learning and training purposes to the existing Package.swift or alternatively copy and replace the Package.swift from the code snippet at the bottom.

Change 1: Add a platform with minimum version (not included by default):
```
   platforms: [
        .macOS(.v13)
       ],
```
This ensures a minimum version and avoids compiler errors.
Change 2: You need to set the library target type to dynamic:
```
   type: .dynamic,
```
This is due to limitations of JNI. Only Swift targets that are part of dynamic libraries can be accessed from Java/Kotlin.

Change 3: Add a dependency on the Swift4j package into the Package.swift file:

  dependencies: [
    .package(url: "https://github.com/scade-platform/swift4j.git", from: "1.3.0")
  ]

Change 4: Add the Swift4j package as a dependency to our target:

.target(
  ...
  dependencies: [
    .product(name: "Swift4j", package: "swift4j")
  ]
)

The entire updated Package.swift file should now look like below:

// The swift-tools-version declares the minimum version of Swift required to build this package.

import Foundation
import PackageDescription

let package = Package(
    name: "HelloWorld",

    platforms: [
        .macOS(.v13)
       ],

    products: [
        .library(
            name: "HelloWorld",
            type: .dynamic,
            targets: ["HelloWorld"])
    ],

    dependencies: [
        .package(url: "https://github.com/scade-platform/swift4j.git", from: "1.3.0")
    ],

    targets: [
        .target(
            name: "HelloWorld",
            dependencies: [
              .product(name: "Swift4j", package: "swift4j")
            ]
        )
    ]
)

Build should now be successful

Your build should now be successful. In some cases, you also need to give permission to access the Swift4j library. You can do this in Xcode by clicking on the error message and approving the library.

Reuse Swift code on Android

Create new Android project

Choose File > New Project
Choose Empty Activity
Call the project HelloWorld
- Choose Kotlin DSL as build configuration language

Setup the gradle config file in the app directory

Open the build.gradle.kts file in the app directory

There is a 2nd build.gradle.kts outside of the app directory. Don’t use that one.

Add the SCADE Swift Product Manager plugin under plugins:

id("io.scade.gradle.plugins.android.swiftpm") version "1.3.0"

Add the SCADE Swift Product Manager configuration at the bottom of the file:
```
swiftpm {
    path = file("<fullpath>/Projects/HelloWorld")
    product = "HelloWorld"
    javaVersion = 8
    scdAutoUpdate = true
}
```
- The path needs to point to the directory of the Swift project (see step 1).
- Set the product to “HelloWorld”.

Build and run the project to see it’s working

Build the project. When you run the build for the first time, it downloads all dependencies and compiles the source code. This could take 2–3 minutes. Subsequent builds will be much faster.

Once the build has succeeded, you can find the Java interop code here:
To test the setup., run the Android app in the simulator. You have to assign a simulator in Android Studio and configure a run file. This step is not described in detail here, as these are basic Android development steps.

If your run button in Android Studio is grayed out, this is an Android Studio issue and you need to restart Android Studio.

Bind Swift logic to Android UI control

By now, we compiled Swift code to Android. We included the resulting library called HelloWorld in the Android project and had SCADE generate the interop code. We now can bind the logic to the UI.

Import the code using the import command. Add:
```
import HelloWorld.GreetingService
```
If Android Studio cannot find the namespace or class and autocompletion does not work, use the Sync Project with Gradle File menu from Android Studio.

Add the line System.loadLibrary(“HelloWorld”) in the onCreate method to load the HelloWorld library.

class MainActivity : ComponentActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        enableEdgeToEdge()

        System.loadLibrary("HelloWorld")

This loads your Swift code into memory and the code is ready to use.

Finally, call the Swift code. SCADE produces extremely powerful interop code. Calling our greet method developed in Swift code is extremely easy. Add the following code in the next line:
```
val greeting = GreetingService().greet("rocks")
   { name: String -> "SCADE AppLogic " + name.uppercase() + "!!!" }
```
- First, we instantiate the GreetingService.
- Then we call the greet method with parameter name set to "rocks" and pass in the closure:
  - The closure takes a String as input, converts it to uppercase, and concatenates it with two String constants.
  - The closure then returns the result.
- We store the result in a variable greeting.
Great! So we coded our first call to a Swift method in Android.

Bind the greeting variable into the UI text label. In the setContent code, set name = greeting.

setContent {
    HelloWorldTheme {
        Scaffold(modifier = Modifier.fillMaxSize()) { innerPadding ->
            Greeting(
                name = greeting,
                modifier = Modifier.padding(innerPadding)
            )
        }
    }
}

As a final (optional) step, we make the UI more beautiful by centering the label. Replace the Greeting function with the following code:

@Composable
fun Greeting(name: String, modifier: Modifier = Modifier) {
    Box(
        modifier = modifier.fillMaxSize(),
        contentAlignment = Alignment.Center
    ) {
        Text(text = "$name!")
    }
}

This centers the text control by introducing a Box control.

We are done coding !

Here’s the full code for MainActivity.kt:

package com.example.helloworld

import androidx.compose.foundation.layout.Box
import androidx.compose.foundation.layout.fillMaxSize
import androidx.compose.ui.Alignment
import android.os.Bundle
import androidx.activity.ComponentActivity
import androidx.activity.compose.setContent
import androidx.activity.enableEdgeToEdge
import androidx.compose.foundation.layout.padding
import androidx.compose.material3.Scaffold
import androidx.compose.material3.Text
import androidx.compose.runtime.Composable
import androidx.compose.ui.Modifier
import androidx.compose.ui.tooling.preview.Preview
import com.example.helloworld.ui.theme.HelloWorldTheme
import HelloWorld.GreetingService

class MainActivity : ComponentActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        enableEdgeToEdge()

        System.loadLibrary("HelloWorld")

     // Call Swift Code easily
        val greeting = GreetingService().greet("rocks")
            { name: String -> "SCADE AppLogic " + name.uppercase() + "!!!" }

        setContent {
            HelloWorldTheme {
                Scaffold(modifier = Modifier.fillMaxSize()) { innerPadding ->
                    Greeting(
                        name = greeting,
                        modifier = Modifier.padding(innerPadding)
                    )
                }
            }
        }
    }
}

@Composable
fun Greeting(name: String, modifier: Modifier = Modifier) {
    Box(
        modifier = modifier.fillMaxSize(),
        contentAlignment = Alignment.Center
    ) {
        Text(text = "$name!")
    }
}

@Preview(showBackground = true)
@Composable
fun GreetingPreview() {
    HelloWorldTheme {
        Greeting("Android")
    }
}

Run the native Swift app on Android

Run your first cross-platform native Swift app. Press the run button and see the application running: