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Enhancing Sprites in the Game Engine

If you're a particularly inquisitive person, you might have wondered how exactly the explosion sprites will work in the game. The animation part is simple enough because the Sprite class in the game engine now allows you to flip through a series of animation images. However, an explosion sprite must cycle through its animation frames and then immediately go away, which sounds simple but presents a problem for the game engine. The problem is that there currently is no mechanism for a sprite to hide or kill itself automatically when it is no longer needed. Granted, you can kill a sprite from within the game code, but how would you keep track of when the frame animation for an explosion is finished?

The real problem I'm talking about here is that of allowing a sprite to animate through one cycle and then go away. The game engine doesn't currently support this feature, but it's not too difficult to add. Without this feature, there is no straightforward way to use a sprite such as an explosion that must cycle through its animation once and then exit the game. The key to adding this feature to the game engine is including a couple of new variables to the Sprite class:

BOOL m_bDying;
BOOL m_bOneCycle;

The m_bDying member variable determines whether a sprite has been flagged as dying. In other words, normal sprites have their m_bDying variable set to FALSE, whereas a sprite on the way out has it set to TRUE. The cool thing about this variable is that it allows you to kill a sprite at any time by simply setting the variable to TRUE. Of course, this requires some additional code in both the Sprite and GameEngine classes to make it actually work.

The second member variable, m_bOneCycle, indicates whether a sprite should animate once through its frames and then kill itself. Because this variable only makes sense within the context of a frame animated sprite, it is set when you call the SetNumFrames() method. You see how the SetNumFrames() method is modified to account for the m_bOneCycle variable in a moment.

For now, let's take a look at how the two new Sprite member variables are initialized in the Sprite() constructors:

m_bDying = FALSE;
m_bOneCycle = FALSE;

As you might expect, the default value of both variables is FALSE, which makes sprites behave normally.

The m_bDying member variable can be set to TRUE through the Kill() method, which is really just an accessor method because all it does is set the variable:

void Kill() { m_bDying = TRUE; };

This method now gives you a clean and efficient means of killing any sprite—with the security of knowing that it will be properly removed from the sprite list maintained by the game engine. This is a crucial aspect of destroying a sprite because the sprite list in the game engine would go haywire if you were to simply delete a sprite from memory and not remove it from the list. The Kill() method provides a clean interface for carrying out this task.

Whereas the Kill() method provides an immediate way to kill a sprite that can be useful in some situations, the more elegant approach is to allow a sprite to kill itself when it finishes cycling through its frame animations. The UpdateFrame() method now supports this feature by examining the m_bOneCycle variable, and then setting the m_bDying variable accordingly. The original version of this method simply set the m_iCurFrame variable to 0 so that the animation starts over, which this method still does if the m_bOneCycle variable is FALSE. However, if the m_bOneCycle variable is TRUE, the m_bDying variable is set to TRUE, which starts the sprite on a path to destruction.

The m_bOneCycle variable is set in the SetNumFrames() method, which now looks like this:

void SetNumFrames(int iNumFrames, BOOL bOneCycle = FALSE);

As you can see, the SetNumFrames() method now includes a second argument for setting the m_bOneCycle member variable. To help make the transition to using the new version of the method easier, the bOneCycle argument is given a default value of FALSE. This allows you to use the method just as you've already grown accustomed. However, if you want to create a sprite that cycles through its animation once and then goes away, just pass TRUE as the second argument to SetNumFrames().

Getting back to the killing of a sprite via the m_bDying member variable, the place where the murder plot unfolds is in the Update() method, which is shown in Listing 19.1.

Listing 19.1 The Sprite::Update() Method Kills a Sprite if It Is Flagged as Dying

 1: SPRITEACTION Sprite::Update()
 2: {
 3:  // See if the sprite needs to be killed
 4:  if (m_bDying)
 5:   return SA_KILL;
 7:  // Update the frame
 8:  UpdateFrame();
10:  ...
11: }

If you recall, the Sprite::Update() method is actually a large method, so I'm only showing you the beginning of it here because this is all that has changed. The method now checks the value of the m_bDying member variable (line 4), and then returns SA_KILL if it is set to TRUE (line 5). If you recall from earlier in the book, SA_KILL is a sprite action you created that notifies the game engine when a sprite needs to be killed. Prior to now, it was only used to kill a sprite when it encounters a boundary.

Simply killing a sprite isn't quite sufficient when it comes to improving the game engine for future games. You'll find out later in the hour that it can be incredibly useful to know when a sprite is being destroyed—regardless of why it is being destroyed. For example, when a meteor sprite is destroyed, you can create an explosion sprite to show the destruction of the meteor. The notification of a sprite being killed is made possible through the SpriteDying() function, which is called whenever a sprite is dying:

void SpriteDying(Sprite* pSprite);

Understand that the SpriteDying() function is a function that you must provide as part of a game. In other words, it is designed to house game-specific code that responds to particular types of sprites dying within a game.

The final change to the game engine to support the new sprite killing features appears within the GameEngine class in the UpdateSprites() method, which now includes a call to SpriteDying() that notifies a game of a sprite being destroyed and removed from the sprite list. This gives the game a chance to respond to the sprite's demise and carry out any appropriate actions.

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