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This method causes the sprite to move in the direction of the direction vector if the elapsed time since the last move exceeds the elapsed time targetbased on the specified speed.
The Move method begins in Listing 6 .
Listing 6 . Beginning of the Move method.
public void Move(GameTime gameTime) {
//Accumulate elapsed time since the last move.elapsedTime +=
gameTime.ElapsedGameTime.Milliseconds;if(elapsedTime>elapsedTimeTarget){
//It's time to make a move. Set the elapsed// time to a value that will attempt to produce
// the specified speed on the average.elapsedTime -= elapsedTimeTarget; The sprite doesn't necessarily move every time the Move method is called. Instead, it uses the incoming parameter to compute theelapsed time since the last time that it actually moved.
If that elapsed time exceeds the target that is based on the specified speed in moves/millisecond , then it reduces the elapsed time value by the target value and makes an adjustment to the position value. Changing the position value will cause the sprite to move in the game window the next time it is drawn.
By reducing the elapsed time by the target time instead of setting it to zero, the sprite attempts to achieve the target speed on the average . For example, assume that for some reason, there is a long delay between calls tothe Move method and the elapsed time value is two or three times greater than the target time. This means that the sprite has gotten behind and is not in the position thatit should be in. In that case, the sprite will move every time the Move method is called for several successive calls to the Move method. (In other words, the sprite will experience a short spurt in speed.)This should cause it to catch up and be in the correct position once it does catch up.
Of course, if the elapsed time between calls to the Move method is greater than the target time over the long term, the sprite will neverbe able to keep up.
If the conditional expression for the if statement in Listing 6 returns true, then the last statement in Listing 6 along with the remainder of the body of the if statement will be executed. Otherwise, that statement and the remaining body of the if statement will be skipped.
The remaining body of the if statement begins in Listing 7 .
One of the advantages of treating the position and the direction as 2D vectors based on the structure named Vector2 is that the Vector2 structure provides various methods that can be used to manipulate vectors.
The code in Listing 7 calls the Add method of the Vector2 class to add the direction vector to the position vector returning the sum of the two vectors. The sum is saved as the new positionvector.
Listing 7 . Add the direction vector to the position vector.
position = Vector2.Add(position,direction); In case you are unfamiliar with the addition of 2D vectors, if you add a pair of 2D vectors, the X component of the sum is the sum of the X components and theY component of the sum is the sum of the Y components.
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