»»» Object Collision Detection with Flash 5 part 2

To test this out, we are going to add a paddle to the game, as shown in Figure C. You will be able to control the movement of the paddle with your arrow keys. The paddle will hit the ball and bounce it off the walls.

Figure C: The Bouncing Ball
The paddle bounces the ball off the walls The following script adds the movement:

           for(thisPad=0;thisPad<2;thisPad++){
           var thePaddle = ThePlayer[thisPad];
           if (this.MovieClip.hitTest(thePaddle) && this.deltaY > 0) {
                  this.paddleHit(thePaddle);
           }
     }

Here, the movie clip method “hitTest” is used. The hitTest method allows an object to physically collide with another. In this case, the ball can collide with the paddle. Once a collision has occurred an event can be triggered. In this instance, the value to the variable deltaY is triggered. This causes the ball to be sent back up the screen.
Some more script can be added to remove a life from the player is they miss the ball with the paddle. That script is:

           if (this.MovieClip._y>(GamePlayArea[3]+20)) {
                           theBallsList[this.name] = 0;
                           removeMovieClip(this.MovieClip);
                           CurrentBallNum--;
                           if (CurrentBallNum<=0) {
                                           ballsLeft--;
                                           if (ballsLeft<0) {
                                                           ballsLeft = 0;
                                                           gameOn = 0;
                                                           gotoAndStop (5);
                                           } else {
                                           addNewBall();
                                           }
                           }
          }

What we see in this scipt is that Flash is looking for the current position of the ball in relationship to the GamePlayArea. If the ball falls below a specific point, ie: where the paddle is, then player automatically looses a life.

Hitting the Blocks
The blocks across the screen are controlled programmatically. Each block is an instance of the same block. A color effect is applied to each to change the color of the blocks. In addition, the following script is also applied to each block:

onClipEvent (load) {
           startX = this._x
}

This script will be used to tell the block to hide. The only difference between each block is the instance name. Each block has a different name. The names for each all start the same Tile and then followed by a number between 1-35. At the top of the script you tell the game how many blocks are on the stage:
TileNum = 36;

You then tell Flash how to calculate that the object you have just hit is a block:

function maketargetList (numOfTiles) {
           for (i=0; i<numOfTiles; i++) {
                           TargetList[i] = String("tile"+(i+1));
           }
}

The final step is to add the script that will do something when a block when it is hit. That is done with:
           //check if ball hits a tile
           var theTileNum = TargetList.length;
           for (i=0; i<theTileNum; i++) {
                          var thisTile = TargetList[i];
                           if (this.MovieClip.hitTest(_root[thisTile])) {
                                           this.increaseSpeed();
           Total += (10 * CurrentBallNum);
           _root[thisTile]._x += -1000;
                                           TargetList.splice(i, 1);
                                           this.flipY();
                if (TargetList.length == 0) {
                                                           resetGame();
                                           }break;
                           }
           }

Here the script verifies that a target is hit and then increases the games score by 10 points after the collision.
Now you have the basics to a breakout game. The complete code can be downloaded from the web site. The key to any game is colliding with an object. This may be in the form a ball, a laser gun or a rampant Pac-Man eating powerpills. The key is to fully understand how to do this programmatically through ActionScript. Here you have learnt how to use the HitTest method and programmatically define the area of a game. The next step if for you to develop the game further to allow for more sophisticated control and scoring.

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Matthew David is a long standing evangelist for Macromedia’s product line. He has contributed to some of the best selling Macromedia books, such as The Dreamweaver Bible (IDG/Hungry Minds), Flash 5 Magic (New Riders) and Inside Flash MX (New Riders). He has also written Flash MX Magic (New Riders) and Building Great Games with Flash MX (Wiley).

He is a regular contributor to Macromedia’s Designer/Developer site, InformIT.com, Element K Journals’ Macromedia Solutions and MX Insite Magazines.
http://www.matthewdavid.ws

Matthew works as a freelance consultant. He can be contacted at 262-488-0026
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