How to drag and drop button onto GridPane?

Question

I\'m working on a school project creating a simple battleship game and i want to use the Drag And Drop function to drag buttons from HBox on the bottom of the screen to the Grid

Answer 1

The plan here is to start a drag event with the data needed to create your button when the drag event finished. In your code, it looks like you have added the Buttons into the Grid already. That means you need to only transfer a String to change the Button's text. In my code, I am using a StackPane when creating the Grid. I then create and add the Buttons later. You approach may be better. I have looked that far. I have added an MCVE (Altered code from here):

import javafx.application.Application;
import javafx.geometry.Insets;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.input.*;
import javafx.scene.layout.GridPane;
import javafx.scene.layout.HBox;
import javafx.scene.layout.StackPane;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;

/**
 * Demonstrates a drag-and-drop feature.
 */
public class HelloDragAndDrop extends Application
{

    @Override
    public void start(Stage stage)
    {
        //Source Buttons.
        final Button boat1 = new Button("boat1");
        final Button boat2 = new Button("boat2");
        final Button boat3 = new Button("boat3");
        final Button boat4 = new Button("boat4");

        //Adding OnDragDetected to source Buttons.
        setOnDragDetected(boat1);
        setOnDragDetected(boat2);
        setOnDragDetected(boat3);
        setOnDragDetected(boat4);

        //Adding onDragDone to source Buttons.
        setOnDragDone(boat1);
        setOnDragDone(boat2);
        setOnDragDone(boat3);
        setOnDragDone(boat4);

        //Creating GridPane
        GridPane gridPane = new GridPane();
        gridPane.setVgap(5);
        gridPane.setHgap(5);
        gridPane.setPadding(new Insets(5, 5, 5, 5));
        gridPane.setStyle("-fx-background-color: black;");
        //Adding StackPane to every Cell in the GridPane and Adding the Target Events to each StackPane.
        for (int i = 0; i < 6; i++) {
            StackPane stackPane = new StackPane();
            stackPane.setPrefSize(150, 50);
            stackPane.setStyle("-fx-background-color: yellow;");
            setOnDragOver(stackPane);
            setOnDragEntered(stackPane);
            setOnDragExited(stackPane);
            setOnDragDropped(stackPane);

            gridPane.add(stackPane, i / 3, i % 3);
        }

        HBox root = new HBox(new VBox(boat1, boat2, boat3, boat4), gridPane);
        stage.setTitle("Hello Drag And Drop");
        Scene scene = new Scene(root, 400, 200);
        stage.setScene(scene);
        stage.show();
    }

    public static void main(String[] args)
    {
        Application.launch(args);
    }

    //source events handlers
    public void setOnDragDetected(Button source)
    {
        source.setOnDragDetected((MouseEvent event) -> {
            /* drag was detected, start drag-and-drop gesture*/
            System.out.println("onDragDetected");

            /* allow any transfer mode */
            Dragboard db = source.startDragAndDrop(TransferMode.ANY);

            /* put a string on dragboard */
            ClipboardContent content = new ClipboardContent();
            content.putString(source.getText());
            db.setContent(content);

            event.consume();
        });
    }

    public void setOnDragDone(Button source)
    {
        source.setOnDragDone((DragEvent event) -> {
            /* the drag-and-drop gesture ended */
            System.out.println("onDragDone");
            /* if the data was successfully moved, clear it */
//            if (event.getTransferMode() == TransferMode.MOVE) {
//                source.setText("");
//            }

            event.consume();
        });
    }

    //target event handlers
    public void setOnDragOver(StackPane target)
    {
        target.setOnDragOver((DragEvent event) -> {
            /* data is dragged over the target */
            System.out.println("onDragOver");

            /* accept it only if it is  not dragged from the same node
            * and if it has a string data */
            if (event.getGestureSource() != target
                    && event.getDragboard().hasString()) {
                /* allow for both copying and moving, whatever user chooses */
                event.acceptTransferModes(TransferMode.COPY_OR_MOVE);
            }

            event.consume();
        });
    }

    public void setOnDragEntered(StackPane target)
    {
        target.setOnDragEntered((DragEvent event) -> {
            /* the drag-and-drop gesture entered the target */
            System.out.println("onDragEntered");
            /* show to the user that it is an actual gesture target */
            if (event.getGestureSource() != target
                    && event.getDragboard().hasString()) {
                target.setStyle("-fx-background-color: green;");
            }

            event.consume();
        });
    }

    public void setOnDragExited(StackPane target)
    {
        target.setOnDragExited((DragEvent event) -> {
            /* mouse moved away, remove the graphical cues */
            target.setStyle("-fx-background-color: transparent;");

            event.consume();
        });
    }

    public void setOnDragDropped(StackPane target)
    {
        target.setOnDragDropped((DragEvent event) -> {
            /* data dropped */
            System.out.println("onDragDropped");
            /* if there is a string data on dragboard, read it and use it */
            Dragboard db = event.getDragboard();
            boolean success = false;
            if (db.hasString()) {
                //target.setText(db.getString());
                Button tempBoat = new Button(db.getString());
                tempBoat.setMaxSize(Double.MAX_VALUE, Double.MAX_VALUE);
                target.getChildren().clear();
                target.getChildren().add(tempBoat);
                success = true;
            }
            /* let the source know whether the string was successfully
            * transferred and used */
            event.setDropCompleted(success);

            event.consume();
        });
    }    
}

In this code, the Buttons that represent the Boats all have onDragDetected and onDragDone event handlers attached. They are considered Source/start of a Drag event. For every cell in the GridPane, a StackPane is added. These StackPanes are considered the Target/areas to drop drag events. Each StackPane has onDragOver, onDragEntered, onDragDropped, and onDragExited attached. Once a drag event is complete, the StackPane/Target that receives the event gets a new Button as a child with the same name as the Source Button.