Call repaint from another class in Java?

Question

I'm probably doing this wrong, so please be nice. I'm developing a Java game, and I'm at the stage of testing character movement / animation.

The "person" can move up down left and right on a grid. The class the grid is drawn in is the gamePanel class. The buttons are in the gameControlPanel class.

I have a button which spawns a person on the grid. I then have a button to move the person up down left and right.

When the move up button is pressed, it calls the move up method from the person class. (At the moment, I'm only testing one "person" at a time.) In that method is the following code...

int move = 10;
while(move!=0)
{
    setTopLeftPoint(new Point((int)getTopLeftPoint().getX(),
                              (int)getTopLeftPoint().getY() - 3));

    try
    {
        Thread.sleep(300);
    } catch (InterruptedException e)
    {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }   
    move-=1;
}

The problem is that I can't seem to call the repaint method for the gamePanel class from within the Person class. To get around this, I created a timer in the gamePanel class which repaints every 20ms.

When I press the up button after the person is spawned, the button remains pressed down until the cycles of the while loop have been completed, and then the circle representation of the person is displayed in the grid square above.

I will try to answer any questions regarding this.

Answer 1

repaint() does not immediately repaint the GUI. Rather, it posts a message to the AWT thread telling it to paint at the next convenient opportunity. When it gets the chance, it will repaint your app. However, if you do this in an event handler, then the AWT thread is already busy and you need to exit the handler to give control back to the AWT handler.

As a general rule of thumb, you don't want to do any long-running calculations on the AWT thread (including in event handlers) since these will stop the app from responding to other events until your calculations are done. This will often appear to the user as stuck buttons like you described. To get around this, use a SwingWorker which can do the calculations on a separate thread.

Finally, something to be aware (but not necessarily to change) is that timers and sleeps do not guarantee when they will awaken. Rather, they guarantee that they will not waken before the amount of time elapses, but can theoretically sleep indefinitely. Also, not all machines have 1 ms timer resolution. In particular, on many windows machines the timers only have 55 ms resolution, so a 20 ms timer may give weird results.

Answer 2

If you want to repaint at a certain interval, javax.swing.Timer is probably the class for you. In the specific case of repaint you can call it from a non-EDT thread, but you may get yourself into difficulty as you are now dealing with multiple threads.

Answer 3

I don't have much experience making games but having a loop to control all animation is a fundamental aspect of game programming. Most simple 2d games only have 1 loop to render most of its animation.

In my experience a good way to render a whole bunch of stuff is to have a collection of all the entities in your game in one place and just loop over this collection passing the Graphics object to each entity.

This will allow each entity to draw itself onto the graphics object. Although this is just one way to do it.

synchronized ( entities ) {
    for ( Entity e : entities ) {
        e.draw( g );
        e.doAction();
    }
}

Answer 4

Is the button press handled by the event-dispatch thread of your GUI?

If this is a case, then the repaint method on the GUI will not fire until the event dispatch thread ends (ie when the button is released and it breaks out of the loop). I had this problem recently, and the best solution I can suggest is to make the class with the movement algorithm threadable and fire off a thread when the keypress is detected. This allows the event-dispatch thread to finish and therefore allows the gui to repaint.

For more information on threading see Starting a thread .