The Alien Sprite

The Player Sprite

PlayerSprite represents the player and is a subclass of TiledSprite. The statechart for PlayerSprite in Figure 13-14 shows that the sprite performs three concurrent activities.

The move( ) and tryPickup( ) transitions are triggered by the user from the keyboard. The hitByAlien( ) transition is initiated by the WorldDisplay object when an alien tells it that it has hit the player.

The transitions in Figure 13-14 are labeled with method names; this is a practice that I'll use when there's a direct mapping from a transition to a method call. This makes it easier to see the mapping from the statechart to the corresponding code.

Figure 13-14. PlayerSprite statechart

Moving (and Standing Still)

A PlayerSprite TRies to move when the user presses one of the quadrant keys (9, 3, 1, or 7):

     public void move(int quad)     {       Point newPt = tryMove(quad);       if (newPt == null) {   // move not possible         clipsLoader.play("slap", false);         standStill( );       }       else {    // move is possible         setTileLoc(newPt);    // update the sprite's tile location         if (quad == NE)           setImage("ne");         else if (quad == SE)           setImage("se");         else if (quad == SW)           setImage("sw");         else // quad == NW           setImage("nw");         world.playerHasMoved(newPt, quad);       }     }  // end of move( )

The attempt is handled by TiledSprite's inherited tryMove( ) method, and the sprite's tile location is updated if it's successful. The move is dressed up with an image change for the sprite and the playing of a sound effect if the move is blocked.

The player can press 5 to make the sprite stand still, which only changes its associated image. Normally, the sprite is poised in a running position, pointing in one of the quadrant directions.

     public void standStill( )     { setImage("still"); }

Drawing the Player

The statechart includes a draw state, triggered by a draw( ) transition. The draw activity is implemented by using the setPosition( ) and draw( ) methods inherited from Sprite. The drawing isn't initiated by code in PlayerSprite but is by WorldDisplay's draw( ) method:

     public void draw(Graphics g)     // in WorldDisplay     { g.drawImage(floorIm, xOffset, yOffset, null); // draw floor image       wItems.positionSprites(player, aliens);       // add sprites       wItems.draw(g, xOffset, yOffset);             // draw things       wItems.removeSprites( );                          // remove sprites     }

As explained earlier, all the sprites, including the player, are added to WorldItems temporarily so they can be drawn in the correct z-order. Each sprite is stored as a TileOccupier object, and setPosition( ) and draw( ) are called from there.

Being Hit by an Alien

PlayerSprite maintains a hit counter, which is incremented by a call to hitByAlien( ) from the WorldDisplay object:

     public void hitByAlien( )     { clipsLoader.play("hit", false);       hitCount++;       if (hitCount == MAX_HITS)    // player is dead         atPanel.gameOver( );     }

When hitCount reaches a certain value (MAX_HITS), it's all over. The sprite doesn't terminate though; it only notifies AlienTilePanel. This allows AlienTilesPanel to carry out "end of game" tasks, which in this case are reporting the game score and playing a sound clip of applause. AlienTilesPanel could do a lot more, such as ask users if they wanted to play another game. These kinds of game-wide activities should be done at the game panel level and not by a sprite.

Trying to Pick Up a Pickup

The user tries to pick up an item by pressing 2 on the numbers keypad. The hard work here is determining if the sprite's current tile location contains a pickup and to remove that item from the scene. The two operations are handled by WorldDisplay methods:

     public boolean tryPickup( )     {       String pickupName;       if ((pickupName = world.overPickup( getTileLoc( ))) == null) {         clipsLoader.play("noPickup", false);     // nothing to pickup         return false;       }       else {     // found a pickup         clipsLoader.play("gotPickup", false);         world.removePickup(pickupName);    // tell WorldDisplay         return true;       }     }

The name of the pickup on the current tile is obtained and used in the deletion request. If the tile is empty, a sound clip will be played instead.