Java2D: Capturing an event on a Line object

Question

I have a JPanel which has a line, circle etc..Now when I click on the line, will the event get reported as a line event or a general jframe event. I need to be able to move the line, if the user clicks on the line and moves it. Is this possible in Java2d.

Thank you.

Answer 1

Yes, but you will need to do some work (see java.awt.Shape). Basically you need to track a list of Shapes. The JPanel will recieve a mouse event, which you can translate to (x,y) coordinates. You can then call Shape.contains(x,y) to see if your various shapes were clicked on.

This will work well for Circle, Polygon, Arc, etc; however in the case of Line2D, it won't work as easily, but you can use Line2D.intersects() with a small rectangle around the mouse click (this is also good UI since you don't want to force the user to click exactly on a pixel that is hard to see).

Answer 2

There is no such concept as a "line event" unless you decide to implement one.

I would suggest adding a MouseListener and a MouseMotionListener to the Canvas or JPanel onto which your geometric shapes are drawn. Use the MouseListener's mousePressed(MouseEvent) callback to determine whether a given shape has been clicked on. Once you've established this, use MouseMotionListener's mouseDragged(MouseEvent) method to move and redraw the shape as the mouse cursor is moved.

Answer 3

Here's a simple example that demonstrates some of the techniques adduced in other answers.

Answer 4

I created a canvas markup library in Java a few years back and if you don't need to worry about transforms on the canvas (scaling, rotation, etc.) it is very easy to do.

Basically you just need to maintain a collection of the canvas shapes in a List (not a Set because Z order is probably important). The mouse listener will be on your canvas, and not on individual shapes. Add new items to the beginning of your collection (or iterate the list backwards later).

When the canvas receives a mouse down event iterate through your collection of shapes until you find one that is underneath your mouse coordinates. The easiest way to do this is to have your shapes implement an interface that defines some sort of hitPoint(int x, int y) method. That way your rectangles can implement a contains(), lines can do intersects() or graphics paths, you can account for some hit padding, etc.

Taking it one step further, your shapes should define their own draw(Graphics2D g) method so that you can easily do things like selection boxes, or set the paint mode to XOR to make shape 'moving' easier. The paintComponent method of your canvas would just have to iterate through your collection of shapes, calling shape.draw(g) on each one, passing in the graphics instance provided to the paintComponent method.