Service ready to display complete view (with the right context to access the window manager and layout inflater if needed, but also access to a saved state registry and a view model store owner). Then a compose overlay service, that use the first one to push a compose view as system overlay, and also proposing a simple draggable box that can be …

ComposeOverlayViewService.kt

import android.content.Intent
import android.graphics.PixelFormat
import android.os.IBinder
import android.view.Gravity
import android.view.WindowManager
import androidx.compose.foundation.gestures.detectDragGestures
import androidx.compose.foundation.layout.Box
import androidx.compose.foundation.layout.BoxScope
import androidx.compose.runtime.Composable
import androidx.compose.runtime.getValue
import androidx.compose.runtime.mutableStateOf
import androidx.compose.runtime.setValue
import androidx.compose.ui.Modifier
import androidx.compose.ui.geometry.Offset
import androidx.compose.ui.input.pointer.consumeAllChanges
import androidx.compose.ui.input.pointer.pointerInput
import androidx.compose.ui.platform.ComposeView
import androidx.lifecycle.ViewTreeLifecycleOwner
import androidx.lifecycle.ViewTreeViewModelStoreOwner
import androidx.savedstate.setViewTreeSavedStateRegistryOwner
import kotlin.math.roundToInt

/**
 * Service that is ready to display compose overlay view
 * @author Quentin Nivelais
 */
abstract class ComposeOverlayViewService : ViewReadyService() {

    // Build the layout param for our popup
    private val layoutParams by lazy {
        WindowManager.LayoutParams(
            WindowManager.LayoutParams.WRAP_CONTENT,
            WindowManager.LayoutParams.WRAP_CONTENT,
            WindowManager.LayoutParams.TYPE_APPLICATION_OVERLAY,
            WindowManager.LayoutParams.FLAG_NOT_FOCUSABLE,
            PixelFormat.TRANSLUCENT
        ).apply {
            gravity = Gravity.TOP or Gravity.CENTER_HORIZONTAL
        }
    }

    // The current offset of our overlay composable
    private var overlayOffset by mutableStateOf(Offset.Zero)

    // Access our window manager
    private val windowManager by lazy {
        overlayContext.getSystemService(WindowManager::class.java)
    }

    // Build our compose view
    private val composeView by lazy {
        ComposeView(overlayContext)
    }

    override fun onBind(intent: Intent): IBinder? = null

    override fun onCreate() {
        super.onCreate()

        // Bound the compose lifecycle, view model and view tree saved state, into our view service
        ViewTreeLifecycleOwner.set(composeView, this)
        ViewTreeViewModelStoreOwner.set(composeView) { viewModelStore }
        composeView.setViewTreeSavedStateRegistryOwner(this)

        // Set the content of our compose view
        composeView.setContent { Content() }

        // Push the compose view into our window manager
        windowManager.addView(composeView, layoutParams)
    }

    override fun onDestroy() {
        super.onDestroy()
        // Remove our compose view from the window manager
        windowManager.removeView(composeView)
    }

    @Composable
    abstract fun Content()

    /**
     * Draggable box container (not used by default, since not every overlay should be draggable)
     */
    @Composable
    internal fun OverlayDraggableContainer(modifier: Modifier = Modifier, content: @Composable BoxScope.() -> Unit) =
        Box(
            modifier = modifier.pointerInput(Unit) {
                detectDragGestures { change, dragAmount ->
                    change.consumeAllChanges()

                    // Update our current offset
                    val newOffset = overlayOffset + dragAmount
                    overlayOffset = newOffset

                    // Update the layout params, and then the view
                    layoutParams.apply {
                        x = overlayOffset.x.roundToInt()
                        y = overlayOffset.y.roundToInt()
                    }
                    windowManager.updateViewLayout(composeView, layoutParams)
                }
            },
            content = content
        )
}

MyComposeDraggableOverlayService.kt

class MyComposeOverlayService : ComposeOverlayViewService() {

    override fun onBind(intent: Intent): IBinder? = null

    @Composable
    override fun Content() = OverlayDraggableContainer {
        Text("My super component")
    }
}

ViewReadyService.kt

import android.content.Context
import android.hardware.display.DisplayManager
import android.view.Display
import android.view.WindowManager
import androidx.lifecycle.LifecycleService
import androidx.lifecycle.ViewModelStore
import androidx.lifecycle.ViewModelStoreOwner
import androidx.savedstate.SavedStateRegistry
import androidx.savedstate.SavedStateRegistryController
import androidx.savedstate.SavedStateRegistryOwner

/**
 * Service that is ready to display view, provide a ui context on the primary screen, and all the tools needed to built a view with state managment, view model etc
 * @author Quentin Nivelais
 */
abstract class ViewReadyService : LifecycleService(), SavedStateRegistryOwner, ViewModelStoreOwner {

    /**
     * Build our saved state registry controller
     */
    private val savedStateRegistryController: SavedStateRegistryController by lazy(LazyThreadSafetyMode.NONE) {
        SavedStateRegistryController.create(this)
    }

    /**
     * Build our view model store
     */
    private val internalViewModelStore: ViewModelStore by lazy {
        ViewModelStore()
    }

    /**
     * Context dedicated to the view
     */
    internal val overlayContext: Context by lazy {
        // Get the default display
        val defaultDisplay: Display = getSystemService(DisplayManager::class.java).getDisplay(Display.DEFAULT_DISPLAY)
        // Create a display context, and then the window context
        createDisplayContext(defaultDisplay)
            .createWindowContext(WindowManager.LayoutParams.TYPE_APPLICATION_OVERLAY, null)
    }

    override fun onCreate() {
        super.onCreate()
        // Restore the last saved state registry
        savedStateRegistryController.performRestore(null)
    }

    override fun onDestroy() {
        super.onDestroy()
    }

    override val savedStateRegistry: SavedStateRegistry
        get() = savedStateRegistryController.savedStateRegistry

    override fun getViewModelStore(): ViewModelStore = internalViewModelStore
}

This is an incredibly useful gist, @KONFeature! Would you mind attaching a license to it? Thanks 👍