Kotlin Native Win32 GUI Sample

build.gradle

plugins {
    id 'org.jetbrains.kotlin.multiplatform' version '1.3.41'
}
repositories {
    mavenCentral()
}
kotlin {
    // For ARM, should be changed to iosArm32 or iosArm64
    // For Linux, should be changed to e.g. linuxX64
    // For MacOS, should be changed to e.g. macosX64
    // For Windows, should be changed to e.g. mingwX64
    mingwX64("mingw") {
        binaries {
            executable {
                // Change to specify fully qualified name of your application's entry point:
               entryPoint = 'sample.main'
                // Specify command-line arguments, if necessary:
                runTask?.args('')
                linkerOpts("-Wl,--subsystem,windows")
            }
        }
    }
    sourceSets {
        // Note: To enable common source sets please comment out 'kotlin.import.noCommonSourceSets' property
        // in gradle.properties file and re-import your project in IDE.
        mingwMain {
        }
        mingwTest {
        }
    }
}

// Use the following Gradle tasks to run your application:
// :runReleaseExecutableMingw - without debug symbols
// :runDebugExecutableMingw - with debug symbols

SampleMingw.kt

package sample

import kotlinx.cinterop.*
import platform.windows.*

@ExperimentalUnsignedTypes
fun WndProc(hwnd: HWND?, msg: UINT, wParam: WPARAM, lParam: LPARAM) : LRESULT
{
    // This switch block differentiates between the message type that could have been received. If you want to
    // handle a specific type of message in your application, just define it in this block.
    when(msg)
    {
        // This message type is used by the OS to close a window. Just closes the window using DestroyWindow(hwnd);
        WM_CLOSE.toUInt() -> DestroyWindow(hwnd)

        // This message type is part of the WM_CLOSE case. After the DestroyWindow(hwnd) function is called, a
        // WM_DESTROY message is sent to the window, which actually closes it.
        WM_DESTROY.toUInt() -> PostQuitMessage(0)

        // This message type is an important one for GUI programming. It symbolizes an event for a button for example.
        WM_COMMAND.toUInt() ->
        {
            // To differentiate between controls, compare the HWND of, for example, the button to the HWND that is passed
            // into the LPARAM parameter. This way you can establish control-specific actions.
            //if (lParam == button.objcPtr().toLong() && (wParam == BN_CLICKED.toULong()))
            //{
            //    // The button was clicked, this is your proof.
            //    MessageBoxA(hwnd, "Button is pressed!", "test", MB_ICONINFORMATION);
            //}
        }

        // When no message type is handled in your application, return the default window procedure. In this case the message
        // will be handled elsewhere or not handled at all.
        else -> return (DefWindowProc!!)(hwnd, msg, wParam, lParam)
    }

    return 0;
}

@ExperimentalUnsignedTypes
fun main() {
    
    println("Hello, Kotlin/Native!")

    
    memScoped {
        
        val hInstance = (GetModuleHandle!!)(null)
        val lpszClassName = "GijSoft"

        // In order to be able to create a window you need to have a window class available. A window class can be created for your
        // application by registering one. The following struct declaration and fill provides details for a new window class.
        val wc = alloc<WNDCLASSEX>();

        wc.cbSize        = sizeOf<WNDCLASSEX>().toUInt();
        wc.style         = 0u;
        wc.lpfnWndProc   = staticCFunction(::WndProc)
        wc.cbClsExtra    = 0;
        wc.cbWndExtra    = 0;
        wc.hInstance     = hInstance;
        wc.hIcon         = null;
        wc.hCursor       = (LoadCursor!!)(hInstance, IDC_ARROW);
        //wc.hbrBackground = HBRUSH(COLOR_WINDOW+1);
        wc.lpszMenuName  = null;
        wc.lpszClassName = lpszClassName.wcstr.ptr
        wc.hIconSm       = null;

        // This function actually registers the window class. If the information specified in the 'wc' struct is correct,
        // the window class should be created and no error is returned.
        if((RegisterClassEx!!)(wc.ptr) == 0u.toUShort())
        {
            println("Failed to register!")
            return
        }

        // This function creates the first window. It uses the window class registered in the first part, and takes a title,
        // style and position/size parameters. For more information about style-specific definitions, refer to the MSDN where
        // extended documentation is available.
        val hwnd = CreateWindowExA(WS_EX_CLIENTEDGE, lpszClassName, "Win32 C Window application by evolution536",
            (WS_OVERLAPPED or WS_CAPTION or WS_SYSMENU or WS_MINIMIZEBOX).toUInt(),
            CW_USEDEFAULT, CW_USEDEFAULT, 320, 125, null, null, hInstance, NULL
        )

        // Everything went right, show the window including all controls.
        ShowWindow(hwnd, 1);
        UpdateWindow(hwnd);


        // This part is the "message loop". This loop ensures the application keeps running and makes the window able to receive messages
        // in the WndProc function. You must have this piece of code in your GUI application if you want it to run properly.
        val Msg = alloc<MSG>();
        while((GetMessage!!)(Msg.ptr, null, 0u, 0u) > 0)
        {
            TranslateMessage(Msg.ptr);
            (DispatchMessage!!)(Msg.ptr);
        }
    }
    
}

C++ equivalent code: https://www.unknowncheats.me/wiki/C%2B%2B:Creating_a_Simple_Win32_GUI_using_C/C%2B%2B

how to set wc.hbrBackground?

error:

Cannot access '<init>': it is internal in 'CPointer'

Is this application possible to be debugged using a debugger tool?

Is this application possible to be debugged using a debugger tool?

Yes, https://kotlinlang.org/docs/native-debugging.html

how to set wc.hbrBackground?

error:

Cannot access '<init>': it is internal in 'CPointer'

You can set the background color like this:
wc.hbrBackground = CreateSolidBrush(0xFF00FF)