Game:

In this section, you will learn what each line of code found in the previous page accomplishes.

The Button Code - on (release)
Let's take a look at the code a section at a time:

on (release) {
	_root.fire = 1;
	_root.i += 1;
	duplicateMovieClip(_root.bullet, "bullet"+i, i);
}

This chunk of code is responsible for telling the movie clip that a bullet should be fired. It does that by setting a conditional statement for bullet fire to true by using the value 1. _root.fire = 1 enables a if statement that, while not used in this section of code, is used in the movie clip in the  next few section. You will see why once you get there.

_root.i += 1 initializes a counter and increments its current value by 1. This is used in the next line where the duplicateMovieClip action is. The variable i (_root.i) is incremented because each new movie clip name that is duplicated has the variable i included in it. For example, "bullet" + i. This topic has been covered extensively on the site, and doing a simple search for duplicateMovieClip should yield plenty of good results.

All of the above code is executed ONLY when the mouse button is released.

The Code - onClipEvent (load)
Let's tackle the first section of code for our bullet movie clip:

onClipEvent (load) {
	this._visible = 0;
	if (_root.fire == 1) {
		this._x = _root.crab._x;
		this._y = _root.crab._y;
	}
}

In the next line, there is a conditional if statement that sets the x and y position of the bullet to the x and y position of the crab. The statement is conditional because it is executed ONLY when _root.fire ==1. If you recall in the previous section, we set the value of fire to 1 when the mouse is released.

If we don't use an if statement, the bullet would simply follow the crab until the mouse is released. Since in the next section we discuss the enterFrame actions, you will notice that the bullet, once fire is set to 1, will fly toward the top of the screen. Failing to synchronize the default position of the bullet to the position of the crab will cause the bullet to fire from a random location that is different from the crab's position.

The Code - onClipEvent (enterFrame)
What's up with the next section of code?

onClipEvent (enterFrame) { if (this != _level0.bullet) { if (_root.fire == 1) { this._y -= 5; this._visible = 1; } if (this._y<0) { this.removeMovieClip(); } } }

This section of code is where the bullet is actually displayed, moved, and removed. Ignore the first if statement that contains this != _level0.bullet. I will cover why that if statement is displayed in greater detail in the colored table a few paragraphs down.

The variable fire makes another guest appearance! If the value of fire is equal to 1, the bullet is set to be visible by this._visible = 1. Located in this section of code is the material needed to make the bullet move up. this._y -= 5 reduces the value of y by 5 pixels. Reducing the y value actually makes the bullet go up. Flash's Cartesian co-ordinate system is a little different than the one you may have learned in school.

We do not want the bullet to be moving indefinitely. We need an exit strategy for the bullet movie clip. The third if statement, if (this._y <0), becomes active when the bullet's y position becomes less than zero. When the bullet (the current movie clip) goes below a y position of 0, the bullet gets removed via the this.removeMovieClip() code.

Why if (this != _level0.bullet) is Used

When you look at the code in the previous section, you will notice that a movie clip that reaches the top boundary (y < 0) is removed by this.removeMovieClip. While this code looks handy to remove a movie clip from the stage, there is one caveat. The movie clip that is going to be removed must be a movie clip that has been duplicated. A movie clip that is not duplicated cannot be removed with the removeMovieClip function. Now, the bullet movie clip you created in the previous page - that is not a duplicated movie clip. That is the source movie clip from which the rest of the movie clips are duplicated from. So, trying to remove the source bullet movie clip using removeMovieClip will not work. Why not let the bullet go beyond the screen? Nobody will see it go beyond the stage. While nobody will see it go beyond the stage and up into the stratosphere, Flash and your system still recognize it. As the movie clip keeps going up, the actions in the onClipEvent(enterFrame) continue to be executed. Not only is the execution unnecessary, the further the movie clip goes, the larger the numbers used in calculating its y position, the greater the amount of system resources needed, and the more frustrated one will get when their computer starts to behave sluggishly after a few minutes of playing. Therefore, I set an if statement that checks if the name of the movie clip. If the name of the movie clip matches the name of the original source movie, the actions simply don't execute. How did I find the name of original movie? It's not the instance name. I simply ran a trace(this._name) statement in the onLoad handler that gave me the name of the movie clip. With that information handy, _level0.bullet, I was on my way to preventing the movie clip from executing any actions by placing it as a condition in an if statement. All subsequent movie clips have different names and are duplicated, so they will initialize, move, and be removed like they are supposed to.

Replay
Now, let's have a quick review of what happens. First, the crab button is pressed. The variable for fire is initialized to 1. The movie clip is duplicated and has a number equivalent to the value of i attached to its instance name. The movie clip's onLoad handler tells it, after being duplicated, to position itself on the same x and y co-ordinates as that of the crab.

When you reach onEnterFrame, Flash checks to make sure you are a duplicated movie clip by ensuring your name is not the same name as that of the source bullet movie clip you created. Once that test is passed, you check to ensure that the variable fire is still 1. With that done, the movie clip's visibility is set to 1 so you can see the bullet as it streaks upward. The streaking upward action is controlled by the this._y -=5 action. Remember, in Flash, subtracting from a y value makes the object go up on the stage.

Once the bullet movie clip reaches the top of the animation, it is removed because a duplicated movie can be easily removed with the removeMovieClip() function. Repeat everything mentioned under Replay each time for each time the mouse button is depressed. Quite amazing isn't it?

You are finished with this tutorial! I hope you found the explanations helpful. I tried to make sure you understand why each and every line of code works the way it does. Some of the sections, such as checking to see if the movie clip is the original movie clip you created, are nitpicky. It is not designed to make this more complicated. All of these little details, in the end, help make you a better programmer and, hopefully, a good game designer.

Here is the source code for the final animation:

download source for flash mx