How to build click through components in Java swing?

Question

I have built a custom component that shows only a line. The line is drawn from the top left corner to the bottom right corner as a Line2D at the paint method. The background is transparent. I extended JComponent. These line components are draggable and change their line color when the mouse pointer is located max. 15 pixels away from the drawn line. But if I have multiple of these components added to another custom component that extends JPanel they sometimes overlap. I want to implement that if the mouse pointer is more than 15 pixels away from the line the mouse events should fall through the component. How to let it fall through is my problem. Is that even possible?

Thanks in advance!

score 3 · Answer 1 · answered Aug 30 '11 at 13:56

I want to implement that if the mouse pointer is more than 15 pixels away from the line the mouse events should fall through the component.

If your child component has a mouse listener, then it will intercept every mouse event occurring over it. If you want to forward the MouseEvent to the parent Component you should manually do it. For example you can implement your custom mouse listener extending MouseAdapter:

public class yourMouseListener extends MouseAdapter{

    //this will be called when mouse is pressed on the component
    public void mousePressed(MouseEvent me) { 
         if (/*do your controls to decide if you want to propagate the event*/){
              Component child = me.getComponent();
              Component parent = child.getParent();

              //transform the mouse coordinate to be relative to the parent component:
              int deltax = child.getX() + me.getX();
              int deltay = child.getY() + me.getY();

              //build new mouse event:
              MouseEvent parentMouseEvent =new MouseEvent(parent, MouseEvent.MOUSE_PRESSED, me.getWhen(), me.getModifiers(),deltax, deltay, me.getClickCount(), false) 
              //dispatch it to the parent component
              parent.dispatchEvent( parentMouseEvent);
         }
    }
}

That works great for the parent of the component... But I need the event to go to the next component, that is placed under the clicked one, not to the parent that all components are added to... — tk--, Aug 30 '11 at 14:05
Well. you can forward the mouse event to the parent like showed in my example, and then inside the parent mouse handler decide to which children the event should be passed to. — Heisenbug, Aug 30 '11 at 14:21

score 2 · Answer 2 · answered Aug 30 '11 at 14:06

2

It's been a while since I touched Swing, but I think you will need to handle your mouse events in the parent component and then loop through the child components with lines and determine which one of them should handle the event (well, the logic of deciding should still remain in the line component, but parent will explicitly invoke that logic until one of the components takes the event).

answered Aug 30 '11 at 14:06

MK.

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+1 for adding the listener to the parent instead of trying to pass events up to the parent. – camickr Aug 30 '11 at 15:18
@camickr: if a child component could be configured to intercept events and not forwarding them, then having a mouse listener for every child could be a good option, instead of complicating the logic inside parent component. – Heisenbug Aug 30 '11 at 15:42
@Overbose, why have logic in two places? If the child component can't handle the request it needs to forward the event to the parent, so some code is with the child and some is with the parent. I would rather keep all the code in one place at the parent since the parent is the only component that knows about all the children. – camickr Aug 30 '11 at 15:48
@camickr: maybe you are right. But if the child component could be of different types, then for adding a new type you should modify the parent component code too. – Heisenbug Aug 30 '11 at 16:01
@0verbose you don't, you just create an interface ClickHandler and make sure all your components implement it. – MK. Aug 30 '11 at 17:15
@MK: or instead of make all children implement a ClickHandler interface, they simply implements a mouse listener interface, that's pretty the same thing. – Heisenbug Aug 30 '11 at 17:30
@0verbose except it is not going to work, because Swing will not pass these events to any of the components by the topmost one. Anyways, most likely he doesn't need these shapes to be components in the first place. – MK. Aug 30 '11 at 17:34

score 2 · Accepted Answer · answered Aug 30 '11 at 14:58

For my final year project at university I did a whiteboard program and had the same problem. For each shape the user drew on the board I created a JComponent, which was fine when they were drawing rectangles, but more difficult with the free form line tool.

The way I fixed it in the end was to do away with JComponents altogether. I had a JPanel which held a Vector (I think) of custom Shape objects. Each object held its own coordinates and line thicknesses and such. When the user clicked on the board, the mouse listener on the JPanel fired and went through each Shape calling a contains(int x, int y) method on each one (x and y being the coordinates of the event). Because the Shapes were added to the Vector as they were drawn I knew that the last one to return true was the topmost Shape.

This is what I used for a straight line contains method. The maths might be a bit iffy but it worked for me.

public boolean contains(int x, int y) {

    // Check if line is a point
    if(posX == endX && posY == endY){
        if(Math.abs(posY - y) <= lineThickness / 2 && Math.abs(posX - x) <= lineThickness / 2)
            return true;
        else
            return false;
    }

    int x1, x2, y1, y2;

    if(posX < endX){
        x1 = posX;
        y1 = posY;
        x2 = endX;
        y2 = endY;
    }
    else{
        x1 = endX;
        y1 = endY;
        x2 = posX;
        y2 = posY;
    }


    /**** USING MATRIX TRANSFORMATIONS ****/

    double r_numerator = (x-x1)*(x2-x1) + (y-y1)*(y2-y1);
    double r_denomenator = (x2-x1)*(x2-x1) + (y2-y1)*(y2-y1);
    double r = r_numerator / r_denomenator;

    // s is the position of the perpendicular projection of the point along
    // the line: s < 0 = point is left of the line; s > 0 = point is right of
    // the line; s = 0 = the point is along the line
    double s =  ((y1-y)*(x2-x1)-(x1-x)*(y2-y1) ) / r_denomenator;

    double distance = Math.abs(s)*Math.sqrt(r_denomenator);

    // Point is along the length of the line
    if ( (r >= 0) && (r <= 1) )
    {
            if(Math.abs(distance) <= lineThickness / 2){
                return true;
            }
            else
                return false;
    }
    // else point is at one end of the line
    else{
        double dist1 = (x-x1)*(x-x1) + (y-y1)*(y-y1); // distance to start of line
        double dist2 = (x-x2)*(x-x2) + (y-y2)*(y-y2); // distance to end of line
        if (dist1 < dist2){
            distance = Math.sqrt(dist1);
        }
        else{
            distance = Math.sqrt(dist2);
        }
        if(distance <= lineThickness / 2){
            return true;
        }
        else
            return false;
    }
    /**** END USING MATRIX TRANSFORMATIONS****/

}

posX and posY make up the coordinates of the start of the line and endX and endY are, yep, the end of the line. This returned true if the click is within lineThickness/2 of the centre of the line, otherwise you have to click right along the very middle of the line.

Drawing the Shapes was a case of passing in the JPanel's Graphics object to each Shape and doing the drawing with that.

Oh, yes, if you can, do drop a component per shape approach, it is wasteful. — MK., Aug 30 '11 at 15:40

score 1 · Answer 4 · answered Aug 30 '11 at 13:32

1

I believe that the easiest way is to catch the event and call parent.processEvent(). So, you component will be transparent for events because it will propagate them to parent.

answered Aug 30 '11 at 13:32

AlexR

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The problem is, that my parent is the canvas all components are added to... Not the component that lays "under" the clicked one... – tk-- Aug 30 '11 at 13:59
So use parent.getComponentAt(x, y). The x, y get from the mouse event and recalculate them relatively to the parent. – AlexR Aug 30 '11 at 14:16
@AlexR getComponentAt(x,y) will only give you the topmost child. – MK. Aug 30 '11 at 14:30

score 0 · Answer 5 · answered Apr 12 '13 at 11:12

I was struggling with this sort of question and tried all the stuff with parents and glasspane until i realised that override of contains method does just what you want. Because when parent fires some sort of getcomponent your 'Line' will reply to it: 'no, its not me, i'm not there!' and the loop will check other components. Also, when you need to setup a complex depth to your draggable object, you can use JLayeredPane descendant.

How to build click through components in Java swing?

5 Answers5