Mobile Game Programming for Beginners: Part 1 of 4

package com.bornander.games.basics;

import java.io.IOException;
import javax.microedition.lcdui.Canvas;
import javax.microedition.lcdui.Font;
import javax.microedition.lcdui.Graphics;
import javax.microedition.lcdui.Image;

/**
 *
 * @author Fredrik Bornander
 */
public class MainCanvas extends Canvas implements Runnable {
    
    // A reference to the midlet that created this class, used to closing the application.
    private BasicsMIDlet owner;
    
    // The thread that will hold the main game loop.
    private Thread thread = new Thread(this);
    
    // The two images
    private Image avatar;
    private Image target;
    
    // The coordinates of the player.
    private int x = 0;
    private int y = 0;
    
    // The target's coordinates.
    private int targetX;
    private int targetY;
    
    // Flags indicating which buttons are pressed.
    private boolean up = false;
    private boolean down = false;
    private boolean left = false;
    private boolean right = false;
    
    // The gameloop will continue as long as this is true.
    private boolean shouldRun = true;
    
    // This gets set to true when the game is completed.
    private boolean completed = false;
    
    public MainCanvas(BasicsMIDlet owner) throws IOException {
        this.owner = owner;
        avatar = Image.createImage(getClass().getResourceAsStream("/com/bornander/games/basics/resources/avatar.png"));
        target = Image.createImage(getClass().getResourceAsStream("/com/bornander/games/basics/resources/target.png"));
        
        targetX = getWidth() - target.getWidth();
        targetY = getHeight() - target.getHeight();
        
        thread.start();
    }

    /**
     * Detects if the game has been completed (i.e. the avatar has navigated to the target).
     * @return <code>true</code> if the game is completed.
     */
    private boolean isGameCompleted() {
        return x == targetX && y == targetY;
    }
    
    /**
     * Moves the avatar according to the pressed keys.
     */
    private void moveAvatar() {
        if (up)
            --y;
        if (down)
            ++y;
        if (left)
            --x;
        if (right)
            ++x;

        if (y < 0)
            y = 0;
        if (y > getHeight() - avatar.getHeight())
            y = getHeight() - avatar.getHeight();

        if (x < 0)
            x = 0;
        if (x > getWidth() - avatar.getWidth())
            x = getWidth() - avatar.getWidth();        
    }    
    
    /**
     * Renders the game state to screen.
     * This is a overridden method from <code>Canvas>/code>, it is called 
     * automatically for us when we call <code>Canvas.repaint()</code>.
     * @param graphics The graphics object to draw onto.
     */
    protected void paint(Graphics graphics) {

        int w = getWidth();
        int h = getHeight();
        
        graphics.setColor(0x00007F);
        graphics.fillRect(0, 0, w, h);
        
        
        graphics.drawImage(target, targetX, targetY, 0);
        graphics.drawImage(avatar, x, y, 0);

        if (completed) {
            graphics.setColor(0xA0A0FF);
            graphics.setFont(Font.getDefaultFont());
            graphics.drawString("Game Over", w / 2, h / 2, Graphics.BASELINE | Graphics.HCENTER);
        }        
    }

    /**
     * The game's main loop. This will run until the player presses the fire button.
     * It checks for game completed, then moves the avatar, then updates the screen
     * and then sleeps for a little while, and then starts from the beginning again.
     */
    public void run() {
        while(shouldRun) {
            completed = isGameCompleted();

            if (!completed) {
                moveAvatar();
            }            
            
            repaint();
            
            try {
                Thread.sleep(20);
            } 
            catch (InterruptedException ex) {
            }
        } 
        owner.exit();
    }

    /**
     * This gets called for us whenever a key is pressed.
     * @param key The pressed key.
     */
    protected void keyPressed(int key) {
        int gameKey = getGameAction(key);
        switch(gameKey) {
            case Canvas.UP: up = true; break;
            case Canvas.DOWN: down = true; break;
            case Canvas.LEFT: left = true; break;
            case Canvas.RIGHT: right = true; break;
            case Canvas.FIRE: shouldRun = false; break;
        } 
    }

    /**
     * This gets called for us whenever a key is released.
     * @param key The released key.
     */
    protected void keyReleased(int key) {
        int gameKey = getGameAction(key);
        switch(gameKey) {
            case Canvas.UP: up = false; break;
            case Canvas.DOWN: down = false; break;
            case Canvas.LEFT: left = false; break;
            case Canvas.RIGHT: right = false; break;
        } 
    }
}