How to Fire - Code Explanation
In this section, you will learn what each
line of code found in the previous page
The Button Code - on (release)
Let's take a look at the code a section at a
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
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)
In the onLoad event, the goal was to set the
initial properties of the bullet. Since we do not want the
bullet to be visible before firing, it is kept invisible
Let's tackle the first section of code
for our bullet movie clip:
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
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
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?
This section of code is where the bullet is
actually displayed, moved, and removed. Ignore the first if
statement that contains
_level0.bullet. I will cover why that
is displayed in greater detail in the colored table a few
another guest appearance! If the value of
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,
<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
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
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.
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?
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:
If you have a question about this or any other topic, the easiest thing is to drop by our forums where a bunch of the friendliest people you'll ever run into will be happy to help you out!
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