public CharacterEntity (GraphicsDevice graphicsDevice)
		{
			if (characterSheetTexture == null)
			{
				using (var stream = TitleContainer.OpenStream ("Content/charactersheet.png"))
				{
					characterSheetTexture = Texture2D.FromStream (graphicsDevice, stream);
				}
			}

			walkDown = new Animation ();
			walkDown.AddFrame (new Rectangle (0, 0, 16, 16), TimeSpan.FromSeconds (.25));
			walkDown.AddFrame (new Rectangle (16, 0, 16, 16), TimeSpan.FromSeconds (.25));
			walkDown.AddFrame (new Rectangle (0, 0, 16, 16), TimeSpan.FromSeconds (.25));
			walkDown.AddFrame (new Rectangle (32, 0, 16, 16), TimeSpan.FromSeconds (.25));

			walkUp = new Animation ();
			walkUp.AddFrame (new Rectangle (144, 0, 16, 16), TimeSpan.FromSeconds (.25));
			walkUp.AddFrame (new Rectangle (160, 0, 16, 16), TimeSpan.FromSeconds (.25));
			walkUp.AddFrame (new Rectangle (144, 0, 16, 16), TimeSpan.FromSeconds (.25));
			walkUp.AddFrame (new Rectangle (176, 0, 16, 16), TimeSpan.FromSeconds (.25));

			walkLeft = new Animation ();
			walkLeft.AddFrame (new Rectangle (48, 0, 16, 16), TimeSpan.FromSeconds (.25));
			walkLeft.AddFrame (new Rectangle (64, 0, 16, 16), TimeSpan.FromSeconds (.25));
			walkLeft.AddFrame (new Rectangle (48, 0, 16, 16), TimeSpan.FromSeconds (.25));
			walkLeft.AddFrame (new Rectangle (80, 0, 16, 16), TimeSpan.FromSeconds (.25));

			walkRight = new Animation ();
			walkRight.AddFrame (new Rectangle (96, 0, 16, 16), TimeSpan.FromSeconds (.25));
			walkRight.AddFrame (new Rectangle (112, 0, 16, 16), TimeSpan.FromSeconds (.25));
			walkRight.AddFrame (new Rectangle (96, 0, 16, 16), TimeSpan.FromSeconds (.25));
			walkRight.AddFrame (new Rectangle (128, 0, 16, 16), TimeSpan.FromSeconds (.25));

			// Standing animations only have a single frame of animation:
			standDown = new Animation ();
			standDown.AddFrame (new Rectangle (0, 0, 16, 16), TimeSpan.FromSeconds (.25));

			standUp = new Animation ();
			standUp.AddFrame (new Rectangle (144, 0, 16, 16), TimeSpan.FromSeconds (.25));

			standLeft = new Animation ();
			standLeft.AddFrame (new Rectangle (48, 0, 16, 16), TimeSpan.FromSeconds (.25));

			standRight = new Animation ();
			standRight.AddFrame (new Rectangle (96, 0, 16, 16), TimeSpan.FromSeconds (.25));

		}
        public void Update(GameTime gameTime)
        {
            var velocity = GetDesiredVelocityFromInput ();

            X += velocity.X * (float)gameTime.ElapsedGameTime.TotalSeconds;
            Y += velocity.Y * (float)gameTime.ElapsedGameTime.TotalSeconds;

            // We can use the velocity variable to determine if the
            // character is moving or standing still
            bool isMoving = velocity != Vector2.Zero;
            if (isMoving) {
                // If the absolute value of the X component
                // is larger than the absolute value of the Y
                // component, then that means the character is
                // moving horizontally:
                bool isMovingHorizontally = Math.Abs (velocity.X) > Math.Abs (velocity.Y);
                if (isMovingHorizontally) {
                    // No that we know the character is moving horizontally
                    // we can check if the velocity is positive (moving right)
                    // or negative (moving left)
                    currentAnimation = (velocity.X > 0) ? walkRight : walkLeft;
                } else {
                    // If the character is not moving horizontally
                    // then it must be moving vertically. The SpriteBatch
                    // class treats positive Y as down, so this defines the
                    // coordinate system for our game. Therefore if
                    // Y is positive then the character is moving down.
                    // Otherwise, the character is moving up.
                    currentAnimation = (velocity.Y > 0) ? walkDown : walkUp;
                }
            } else {
                // This else statement contains logic for if the
                // character is standing still.
                // First we are going to check if the character
                // is currently playing any walking animations.
                // If so, then we want to switch to a standing animation.
                // We want to preserve the direction that the character
                // is facing so we'll set the corresponding standing
                // animation according to the walking animation being played.
                if (currentAnimation == walkRight)
                    currentAnimation = standRight;
                else if (currentAnimation == walkLeft)
                    currentAnimation = standLeft;
                else if (currentAnimation == walkUp)
                    currentAnimation = standUp;
                else if (currentAnimation == walkDown)
                    currentAnimation = standDown;
                else if (currentAnimation == null)
                    currentAnimation = standDown;
            }

            currentAnimation.Update (gameTime);
        }