Extensions.kt

fun Activity.hasOverlayPermission(): Boolean =
    if (Build.VERSION.SDK_INT >= 23) Settings.canDrawOverlays(this) else true

fun Activity.requestOverlayPermission(requestCode: Int) {
    if (Build.VERSION.SDK_INT >= 23) {
        val intent = Intent(
                Settings.ACTION_MANAGE_OVERLAY_PERMISSION, Uri.parse("package:$packageName"))
        startActivityForResult(intent, requestCode)
    }
}

Permission.xml

<!-- Build.VERSION.SDK_INT < 23 -->
<!-- 顯示系統窗口權限 -->
<uses-permission android:name="android.permission.SYSTEM_ALERT_WINDOW"/>

<!-- 在最頂部顯示 -->
<uses-permission android:name="android.permission.SYSTEM_OVERLAY_WINDOW" />

SystemLevelDialog.kt

private val REQUEST_OVERLAY_PERMISSION = 77

fun checkOverlayPermission() {
    if (hasOverlayPermission()) {
        showSystemDialog()
    } else {
        requestOverlayPermission(REQUEST_OVERLAY_PERMISSION)
    }
}

override fun onActivityResult(requestCode: Int, resultCode: Int, data: Intent?) {
        super.onActivityResult(requestCode, resultCode, data)
        if (resultCode == RESULT_OK) {
            when (requestCode) {
                REQUEST_OVERLAY_PERMISSION -> showSystemDialog()
            }
        }
    }

fun showSystemDialog() {
    val dialogBuilder: AlertDialog.Builder = AlertDialog.Builder(this)
    val dialog: AlertDialog = dialogBuilder
        .setIcon(R.drawable.ic_launcher_background)
        .setTitle(R.string.app_name)
        .setMessage("msg")
        .setPositiveButton("ok", object : DialogInterface.OnClickListener {
            override fun onClick(p0: DialogInterface?, p1: Int) {
                p0?.dismiss()
            }
        }).create()
    if (Build.VERSION.SDK_INT > Build.VERSION_CODES.O) {
        dialog.window?.setType(WindowManager.LayoutParams.TYPE_APPLICATION_OVERLAY)
    } else {
        dialog.window?.setType(WindowManager.LayoutParams.TYPE_PHONE)
    }
    dialog.window?.setFlags(
        WindowManager.LayoutParams.FLAG_NOT_TOUCH_MODAL,
        WindowManager.LayoutParams.FLAG_NOT_FOCUSABLE
    )
    dialog.show()
}