import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

/**
 * AnimationPanel.java
 *
 * An AnimationPanel is responsible for two things.  First, it needs
 * to know at all times how to draw the current picture.  As the
 * images/frames of the animation go by, the AnimationPanel knows
 * everything there is to know about the current image.  Thus, any
 * time we explicitly call paint or the JVM's event thread calls
 * paint, paint will draw the current image of the animation.
 *
 * Second, the AnimationPanel is responsible for knowing how to
 * advance the animation from the current image to the next image.
 * AnimationPanel's step method knows how to move the Ball and adjust
 * the Ball's velocity if a bounce is needed.  (Whether this
 * knowledge should be stored here or in some other class is an
 * interesting design question.  For our short-term purposes, I have
 * left this knowledge in AnimationPanel for simplicity.)
 */
public class AnimationPanel extends JPanel
{
    private Ball ball;

    public AnimationPanel()
    {
        super();
    }

    public void setBall(Ball b)
    {
        ball = b;
    }

    public Ball getBall()
    {
        return ball;
    }

    public void step()
    {
        // Move the ball and draw it.
        ball.step();
        repaint();

        // Adjust the velocity in preparation for the next step of the animation.
        int vx = ball.getVelocityX();
        int vy = ball.getVelocityY();
        Dimension panelSize = getSize();

        if (ball.getX() - ball.getRadius() < 0 || ball.getX() + ball.getRadius() > panelSize.width) {
            vx = -vx;
        }

        if (ball.getY() - ball.getRadius() < 0 || ball.getY() + ball.getRadius() > panelSize.height) {
            vy = -vy;
        }

        ball.setVelocity(vx, vy);
    }

    public void paint(Graphics g)
    {
        // Fill the panel with the background color.  That is,
        // erase the whole thing.
        //
        // Note that we are not calling JPanel's default version
        // of paint, which allows us to control everything about
        // the paint operation.  (In my experiments, replacing the
        // following three lines with "super.paint(g);" produces
        // the same behavior, however.)
        Dimension size = getSize();
        g.setColor(getBackground());
        g.fillRect(0, 0, size.width, size.height);

        // Draw the Ball.
        ball.draw(g);
    }
}