JavaFX Canvas Update

Question

I've been working on switching my applications from Swing to JavaFX. I've been working on a room escape game which displays a description of the item on which the user clicks. In Swing, I'd subclass JComponent and override the paintComponent(Graphics) method. I could draw the text there, knowing that the method is constantly called to update the screen. However, using the JavaFX Canvas, there is no method that is called constantly, which makes this task harder. I attempted save()ing the GraphicsContext after I drew the images and called restore() when I wanted to remove the text, but to no avail. Here's the important code:

package me.nrubin29.jescape;

import javafx.application.Platform;
import javafx.scene.canvas.Canvas;
import javafx.scene.canvas.GraphicsContext;
import javafx.scene.shape.Rectangle;

import java.util.Timer;
import java.util.TimerTask;

public class RoomPane extends Canvas {

    private Room room;

    private Toast toast;

    public RoomPane() {
        super(640, 480);

        setOnMouseClicked(e -> {
            for (JObject o : room.getObjects()) {
                if (o.getBounds().contains(e.getX(), e.getY())) {
                    toast = new Toast(o.getDescription());
                }
            }
        });

        new Timer().schedule(new TimerTask() {
            @Override
            public void run() {
                if (toast == null) {
                    return;
                }

                if (toast.decrement()) { // Decrements the internal counter. If the count is 0, this method returns true.
                    toast = null;
                    Platform.runLater(() -> getGraphicsContext2D().restore());
                }

                else {
                    Platform.runLater(() -> getGraphicsContext2D().strokeText(toast.getText(), 300, 100));
                }
            }
        }, 0, 1000);
    }

    public void changeRoom(Room room) {
        this.room = room;

        GraphicsContext g = getGraphicsContext2D();

        g.drawImage(room.getBackground(), 0, 0);

        for (JObject o : room.getObjects()) {
            g.drawImage(o.getImage(), getCenterX(o.getBounds()), getCenterY(o.getBounds()));
        }

        g.save();
    }
}

Answer 1

I attempted save()ing the GraphicsContext after I drew the images and called restore() when I wanted to remove the text, but to no avail.

save and restore have nothing to with removing things like text, what they do is save in a stack the state of various settings like a stroke or fill to use to draw shapes and allow them to be popped off the stack for application later. Those routines don't effect the pixels drawn on the canvas at all.

To remove something from a GraphicsContext, you can either draw over the of it, or clear it. For your code, what you could do is snapshot the canvas node where you are trying to save it, then draw your snapshot image onto the canvas where you are trying to restore it. It is probably not the most efficient way of handling drawing (a smarter routine which just draws only damaged area where the text is would be better, but probably not required for your simple game).

However, using the JavaFX Canvas, there is no method that is called constantly

Rather than using a timer to trigger canvas calls, use a AnimationTimer or a Timeline. The AnimationTimer has a callback method which is invoked every pulse (60 times a second, or as fast as JavaFX can render frames, whichever is the lesser), so it gives you an efficient hook into the JavaFX pulse based rendering system. The Timeline can have keyframes which are invoked at user specified durations and each keyframe can have an event handler callback which is invoked at that duration.

Using the built-in JavaFX animation framework, you don't have to worry about multi-threading issues and doing things like Platform.runLater which overly complicate your code and can easily lead to subtle and serious errors.

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On a kind of unrelated note, for a simple game like this, IMO you are probably better off recoding it completely to use the JavaFX scene graph rather than a canvas. That way you will be working at a higher level of abstraction rather than clip areas and repainting damaged paint components.