Time for action – updating the Sprite

1. Add the Update() method to the Sprite class:

   Public Overridable Sub Update(gameTime as GameTime)
       Dim elapsed as Single
       elapsed = CSng(gameTime.ElapsedGameTime.TotalSeconds)
   
       timeForCurrentFrame += elapsed
   
       If (timeForCurrentFrame >= FrameTime) Then
           currentFrame = (currentFrame + 1) Mod (frames.Count)
           timeForCurrentFrame = 0.0
       End If
   
       _location += (_velocity * elapsed)
   End Sub

What just happened?

When Update() is called for the sprite, the standard timing mechanism, we have been using is implemented to determine when the frame should be updated. When it is, the frame is set to (currentFrame + 1) Mod (frames.Count). This is a short-hand method of saying, "Add 1 to the current frame, divide the total by the number of frames in the animation and return the remainder.".

As an example, consider an animation with five frames (numbered zero through four). When currentFrame needs to go from zero to one, the assignment looks...