TUTORIAL: Building games in Flash 5
Part 3: Enemies and collisions.

Author: David Doull
Date: 11/04/01
Download source files: tut3.zip

Step 2: Getting the enemies moving

Ok, we have created our enemy movie clip and have set up its start position, now lets get it moving.

If you don't have it still open, open up the actions window for the enemy movie clip.

Underneath the code from the previous step type the following code for the enemies enterFrame clip event:

onClipEvent (enterFrame) {

  if (_root.spaceship.scrollStart){
	    this._x-=enemySpeed+_root.mainGround.groundSpeed;
  } else {
	    this._x-=enemySpeed;
  }

 
  if (this._x<-10) {
	    reset();
  }

}

This code does two things; it moves the enemy across the stage by reducing its x-coordinate and it resets the enemy if it has moved off the left edge of the stage.

The first if statement checks to see if our variable scrollStart is true?
If it is true then the enemy ships x-coordinate is decreased by enemySpeed+_root.mainGround.groundSpeed otherwise it is decreased by just enemySpeed. So if the ground isn't scrolling the enemy will be moved left by the random number assigned to enemySpeed. If the ground is scrolling the enemy's x-coordinate will be decreased by enemySpeed plus the speed at which the ground is scrolling. We need to do this to ensure the movement looks realistic. If the ground started scrolling and the enemy just kept moving at the same speed it wouldn't look right (try it out if you are not convinced).

The second if statement checks to see if the enemy movie clip has moved off the left of the stage (i.e.: it's x-coordinate is less
than -10). If this is true then the reset function is called. The reset function will reposition the movie clip on the right of the stage and assign a new random speed and y-coordinate.

If you test your flash file you should now have a enemy spaceship moving across the stage and resetting itself after it moves off the left side of the stage.

More enemies

But one enemy isn't going to be a very challenging game, so lets use duplicateMovieClip to add some more enemies. We will put the code to duplicate the enemy in a new layer on the main timeline.

Control Layer

Create a new layer on the main timeline and call it control. Click on the first frame of the control layer and open up the actions window (either Window > Actions or right-click and pick actions).

Type the following code in the actions window.

numEnemy=3;
for (i=2; i<=numEnemy; i++){
	enemy1.duplicateMovieClip( "enemy"+i, i+100 );

}

The first line creates a new variable called numEnemy and sets it to 3. This variable will be the number of enemies on the stage at any one point in time, if you want to make the game harder just try increasing this number.

The next three lines are a for loop. The loop duplicates the enemy1 movie clip.

What is a for loop?

For loops are used when you want to repeat some code a set number of times. For loops use a counter, in our case its called i, which is typically increased by one each time flash goes through the loop.

The line for (i=2; i<=numEnemy; i++){ is actually doing quite a lot.
The i=2 bit sets a variable called i equal to 2 when the loop starts. The i++ bit tells flash to increase i by one everytime the loop is repeated and the i<=numEnemy bit tells flash to keep looping while i is less than or equal to numEnemy. So it will stop looping when i is greater than numEnemy.

The result of this is that two duplicates of enemy1 will be created, called enemy2 and enemy3. If you increase the value of numEnemy then the number of duplicate enemies will increase accordingly.

Now, close the actions window and on the main time line add an extra frame to each of the layers (as shown). Select the second frame of the control layer and insert a keyframe (press F6). Open up the actions window for this frame (either Window > Actions or right-click and pick actions) and type the following line:

stop();

This just stops the main timeline.

You might wonder how the game plays if the main timeline is stopped? Well, even though the main timeline is stopped the laser, spaceship and ground movieclips are still playing. The timelines of movieclips run independent of the main timeline, they will only stop if you specifically tell each one of them to stop.

You might also be wondering about why we used the control layer.
We could have just put the code in any one of the other four layers. However its much better practice to include all frame code in a separate layer. This makes it much easier to debug your flash file and for other people to understand your flash file. It also means that if you delete a layer to remove a graphic you wont accidentally also remove some code. It has become a standard in the flash programming community to call this layer control.

If you test your flash file now you should have three enemy spaceships flying across the screen.

If you don't want to type in all that code you can download the fla: tutorial3_partb.fla .

Next >>