JavaFX Tip 1: Resizable Canvas

Dirk LemmermannApril 11th, 2014Last Updated: April 10th, 2014

0 941 1 minute read

While working on FlexGanttFX I had to deal a lot with the JavaFX Canvas node. I am using it to render activities on a timeline. Each row in the Gantt chart is a Canvas node. The user has the option to resize each row individually. So I had to figure out the best way to resize a canvas, which out-of-the-box is not resizable. The listing below shows how this can be accomplished.

The main steps needed are:

Create a subclass of Canvas.
Override the isResizable() method and return true.
Override the prefWidth() and prefHeight() methods. Return the values of Canvas.getWidth() and Canvas.getHeight().
Add listeners to the width and height properties of Canvas in order to trigger a redraw when the size of the canvas changes.
Bind the width and height properties of Canvas to the width and height properties of the parent pane.

import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.canvas.Canvas;
import javafx.scene.canvas.GraphicsContext;
import javafx.scene.layout.StackPane;
import javafx.scene.paint.Color;
import javafx.stage.Stage;

/**
 * Tip 1: A canvas resizing itself to the size of
 *        the parent pane.
 */
public class Tip1ResizableCanvas extends Application {

	class ResizableCanvas extends Canvas {

		public ResizableCanvas() {
			// Redraw canvas when size changes.
			widthProperty().addListener(evt -> draw());
			heightProperty().addListener(evt -> draw());
		}

		private void draw() {
			double width = getWidth();
			double height = getHeight();

			GraphicsContext gc = getGraphicsContext2D();
			gc.clearRect(0, 0, width, height);

			gc.setStroke(Color.RED);
			gc.strokeLine(0, 0, width, height);
			gc.strokeLine(0, height, width, 0);
		}

		@Override
		public boolean isResizable() {
			return true;
		}

		@Override
		public double prefWidth(double height) {
			return getWidth();
		}

		@Override
		public double prefHeight(double width) {
			return getHeight();
		}
	}

	@Override
	public void start(Stage stage) throws Exception {
		ResizableCanvas canvas = new ResizableCanvas();

		StackPane stackPane = new StackPane();
		stackPane.getChildren().add(canvas);

		// Bind canvas size to stack pane size.
		canvas.widthProperty().bind(
                       stackPane.widthProperty());
		canvas.heightProperty().bind(
                       stackPane.heightProperty());

		stage.setScene(new Scene(stackPane));
		stage.setTitle("Tip 1: Resizable Canvas");
		stage.show();
	}

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

When run you should see the following: