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Adding Sprite object references to the list

Listing 8 shows the beginning of an if statement in which a new Sprite objects reference will be added to the list every eighth iteration (frame) of the game loop until all 24 sprites have been added.

Note that a frame counter named frameCnt is declared and initialized to zero in Listing 5 and is incremented near the end of the overridden Update method in Listing 12 .

Instantiate new Sprite objects

The code in Listing 9 uses the modulus (%) operator to identify every eighth iteration of the game loop and to instantiate a new Sprite object during those iterations.

Listing 9 . Instantiate new Sprite objects.

if(frameCnt % 8 == 0) { //Instantiate a new sprite every 8th frame.if((sprites.Count) % 2 == 0) { //Even numbered spritessprites.Add(new Sprite("blueball",Content)); }else { //Odd numbered spritessprites.Add(new Sprite("redball",Content)); }//end else}//end if on frameCnt }//end if on sprites.Count

The modulus operator

In case you have forgotten, the modulus operator returns the remainder of a division instead of returning the quotient. If an integer value is divided by 8,the returned value is 0 only when the integer value is a multiple of 8. (Also by definition, 0 % 8 returns 0.)

Every eighth iteration

Therefore, the conditional expression in the first if statementin Listing 9 will allow the statements contained in the body of the if statement, (which instantiate new Sprite objects), to be executed only during every eighth iteration of the game loop.

Further, the conditional expression in Listing 8 will not allow the code in Listing 9 to be executed after 24 Sprite objects have been instantiated.

Even and odd sprite images

The conditional expression in the second if statement in Listing 9 causes the new Sprite objects that are instantiated to alternate between the "blueball" and "redball" images shown in Figure 1 .

Make all the existing sprites move

Listing 5 declares an instance variable named moveLim and sets its value to 200. The code in Listing 10 causes all of the existing sprites to move to the right and down if the value of the frame counter is less than 200.

Listing 10 . Make all the existing sprites move.

if(frameCnt<moveLim) { for(int cnt = 0;cnt<sprites.Count;cnt++) { sprites[cnt].Position = new Vector2( 10 * cnt + frameCnt,10 * cnt + frameCnt);}//end for loop }//end if

Use a for loop

The code in Listing 10 uses a for loop to access each of the Sprite object references currently stored in the list, iterating from 0 to one less than the count of references stored in the listgiven by sprites.Count .

Put a new value in the Position property

Once a Sprite object's reference has been accessed, Listing 10 sets the Position property stored in the object to a new Vector2 object for which the X and Y values have been modified on the basis of the frame counter.

The new X and Y values cause the object to be drawn a little further down and to the right the next time it is drawnrelative to its current position. This causes the entire diagonal line of Sprite objects to move down and to the right in the game window.

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Source:  OpenStax, Xna game studio. OpenStax CNX. Feb 28, 2014 Download for free at https://legacy.cnx.org/content/col11634/1.6
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