Implement graceful shutdown

src/main/kotlin/dev/nuculabs/imagetagger/ui/MainPage.kt

@@ -5,12 +5,17 @@ import javafx.application.Application
 import javafx.fxml.FXMLLoader
 import javafx.scene.Scene
 import javafx.stage.Stage
+import java.util.logging.Logger
 
 class MainPage : Application() {
+    private val logger: Logger = Logger.getLogger("MainPage")
+
+    private lateinit var fxmlLoader: FXMLLoader
+
     override fun start(stage: Stage) {
         ImageTagsPrediction.getInstance()
 
-        val fxmlLoader = FXMLLoader(MainPage::class.java.getResource("main-window-view.fxml"))
+        fxmlLoader = FXMLLoader(MainPage::class.java.getResource("main-window-view.fxml"))
         val scene = Scene(fxmlLoader.load(), 640.0, 760.0)
         stage.title = "Image Tagger"
         stage.scene = scene
@@ -18,6 +23,12 @@ class MainPage : Application() {
         stage.minHeight = 760.0
         stage.show()
     }
+
+    override fun stop() {
+        logger.info("Stop called.")
+        val controller = fxmlLoader.getController<MainPageController>()
+        controller.shutdown()
+    }
 }
 
 fun main(args: Array<String>) {

src/main/kotlin/dev/nuculabs/imagetagger/ui/MainPageController.kt

@@ -18,6 +18,8 @@ import javax.imageio.ImageIO
 
 
 class MainPageController {
+    private val logger: Logger = Logger.getLogger("MainPageController")
+
     /**
      * The thread pool worker pool.
      */
@@ -49,8 +51,6 @@ class MainPageController {
     private val imageTagsPrediction = ImageTagsPrediction.getInstance()
 
 
-    private val logger: Logger = Logger.getLogger("MainPageController")
-
     @FXML
     private lateinit var progressBar: ProgressBar
 
@@ -143,4 +143,11 @@ class MainPageController {
             logger.info("Finished processing images.")
         }
     }
+
+    /**
+     * Shuts down the MainPageController.
+     */
+    fun shutdown() {
+        workerPool.shutdownNow()
+    }
 }