Flash Actionscript Actionscript and Objects Control Structures

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Flash Actionscript Actionscript and
Objects Control Structures
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ActionScript and Objects

• It should be apparent by now that ActionScript is
  to some extent an object oriented language.
• When we have an instance of a symbol on the stage
  this is effectively an object. You can think of
  the symbol itself as being like a class (i.e. we
  can create new instances/objects from this
  class).
• As with any object there are two types of things
  associated with it
• 1. Properties
• 2. Methods

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ActionScript and Objects

• So when we create a circle symbol and then drag
  in onto the stage and call the instance ball we
  now have access to various properties of ball.
• For example it has a number of frames, a
  position, and so on. We have already seen an
  example of directly altering its position.
• ball.x mouseX

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ActionScript and Objects

• There are also various commands and actions we
  can issue (i.e. ask the object to perform) e.g
• ball.stop()
• These commands and actions are equivalent to
  methods.

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The Math Object

• Lets look at another commonly used object, the
  Math object.
• Go back to the text input example movie and
  change the ActionScript for the enter button to
  the following

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The Math Object

• This shows the use of the sqrt method of the Math
  Object
• Its also a good example of Type conversions

• function updateOutput(eventMouseEvent)void
• var pressedKeyString entryText.text
• var calcValueint int(pressedKey)
• var myCalculation Math.sqrt(calcValue)
• var finalOutputString String(myCalculation)
• outputText.text finalOutput
• entryButton.addEventListener(MouseEvent.CLICK,upda
  teOutput) )

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The Math Object

• This does pretty much what you would expect it to
  
• If you have a look at the top level Actions
  window you will see all the other methods that
  the Math object offers

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ActionScript Control Structures

• Three basic control structures for programming
  languages
• sequence
• selection
• iteration
• These structures are necessary and sufficient to
  implement any algorithm.

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ActionScript Control Structures

• To make the job easier most programming languages
  provide higher-level structures for grouping and
  organising them
• functions/methods
• objects
• Syntax of the three basic control structures is
  the same for ActionScript as it is for many other
  programming languages such as C, Java etc

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Sequence

• Sequence simply means the ability to execute a
  set of statements in order.
• This is done by writing them in the order in
  which you wish them to be executed and separating
  them by semicolons e.g.
• S1S2S3
• Usually these are placed on separate lines e.g.
• S1
• S2
• S3

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Sequence

• For example
• ball.x 20
• ball.y 30
• ball.x ball.x 1
• ball.y ball.y 1

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Selection

• Selection means the ability to choose between two
  (or more) different statements depending on the
  value of some condition.
• It takes the general form
• if (condition)
• //statements
• It is worth looking briefly at how ActionScript
  handles conditions.

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Selection

• Conditions are formed by the use of a set of
  conditional operators. These are
• lt less than
• gt greater than
• lt less than or equal to
• gt greater than or equal to
• equal to
• ! not equal to

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Selection

• These operators can be used to compare the values
  of variables and/or literals. The result is that
  a Boolean value, true or false, is returned.
  E.g.
• x lt 10 10 lt 11
• x gt y x y

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Selection

• These conditions could be used in selection (IF)
  statements as follows
• if (x lt 10)
• x
• else
• x-- // and so on !
• Compound conditions can be formed by combining
  conditions using the logical operators. These
  are
• ! Logical NOT
• Logical AND
• Logical OR

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Selection

• Logical NOT inverts the value of a condition. So
  if xlty is true then !(xlty) will be false.
• A condition formed by the use of will be true
  if and only if both of its constituent conditions
  are true.
• A condition formed by the use of will be true
  if one of its constituent conditions is true.
• So suppose x has the value 5 and y has the value
  0 then .
• (xlt10) (ygt0) false
• (xlt10) (ygt0) true
• !(xlt10) (ylt0) ?

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Selection

• If statements can be formed without the else and
  they can also contain else if clauses also
• if (xlt10)
• x
• else if (xlt20)
• x x 2
• else
• x 30

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Selection

• There are three general forms of the if selection
  structure. They are

if (condition) // statements
if (condition1) // statements else if
(condition2) // statements . .
. else // statements
if (condition) // statements else //
statements
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Selection

• Switch
• The switch statement is useful if you have
  several execution paths that depend on the same
  condition expression.
• It provides functionality similar to a long
  series of ifelse if statements, but is somewhat
  easier to read.
• Blocks of code begin with a case statement and
  end with a break statement.
• For example, the following switch statement
  prints the day of the week, based on the day
  number returned by the Date.getDay() method

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• var someDateDate new Date()
• var dayNumuint someDate.getDay()
• switch(dayNum)
• case 0
• trace("Sunday")
• break
• case 1
• trace("Monday")
• break
• case 2
• trace("Tuesday")
• break
• case 3
• trace("Wednesday")
• break
• case 4
• trace("Thursday")
• break

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Selection

• Lets look at some examples of the use of
  selection statements.
• Example 1 Space Invaders

I have a game of space invaders. The players
ship is allowed to fire bullets at the aliens.
Suppose the bullet has just been fired and this
is signified by the Boolean variable bulletFired
true. Also the bullet is moving up the screen
at a speed signified by the variable bulletSpeed.
I want to move it further up the screen if it
hasnt already reached the top. If its at the
top I want to stop moving the bullet and remove
it from the screen. In this case I also want to
set bulletFired to false , which Flash will see
as a sign that the player is allowed to fire
again.
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Selection

• Lets draw a picture ...

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Selection

• Players ship is at the bottom and our bullet is
  at some point moving upwards.
• In Flash the top-left hand corner of the screen
  is (0,0) so as we increase the x coordinate of an
  object it moves to the right and as we increase
  the y coordinate it moves down.
• The bullet moves up in a straight line so only
  its y-coordinate decreases as it moves upwards
  eventually reaching a value of 0 at the top.

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Selection

• Suppose the instance of the ship on the screen is
  called ship, then its x coordinate is ship.x and
  its y coordinate is ship.y.
• Similarly if the bullet is called bullet then its
  coordinates are bullet.x and bullet.y.
• So bullet.y is positive as it is moving up the
  screen but if it is lt0 then it has moved off the
  top of the screen.

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Selection

• So as long as bullet.y is positive we want to
  keep subtracting from it which will move it
  upwards.
• As soon as bullet.y is less than zero we no
  longer need to animate the bullet, although we
  need it to disappear.
• Easiest way to do this is to place it off the
  screen, say by setting bullet.y -100.

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Selection

• When the bullet is off the screen, the player
  would be allowed to fire another bullet, so we
  need to tell Flash this by making bulletFired
  false. (This would have been set to true after
  the bullet was fired).
• So the decision becomes
• If (the bullet is being fired) and (its y
  position is greater than or equal to 0)
• move the bullet
• If (the bullet is being fired) and (its y
  position is less than or equal to 0)
• hide the bullet and let the user fire another.

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Selection

• This becomes

if ((bulletFiredtrue) (bullet.ygt0))
bullet.y bullet.y - speed else if
(bulletFiredtrue (bullet.ylt0))
bulletFired false bullet.y -100
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Selection

• We need a bit more ActionScript to start thinking
  about coding this fully so instead lets look at a
  password screen.
• Example 2 Password Field
• Suppose we want to password protect our Flash
  website.
• We could do this by dividing it into two scenes.
  Scene 1 is a screen which puts up a text field
  and invites the user to enter a password.
• If this password is correct then the movie jumps
  to scene 2. If not it doesnt.

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Selection

• So this requires a simple if statement to
  implement (along with the textfields we have
  already seen).
• We need two elements besides the static graphics.
• 1. A textfield.
• 2. A button (execute).
• The idea is that when the user hits the execute
  button the system reads what is in the textfield
  and if it matches the password jumps to scene 2.
• Assume we have already named the password
  textfield instance to be password and the button
  instance to be enterButton.

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Selection

• We need to attach the following action to frame
  one of our actions layer in scene 1.

stop() function checkPassword(eventMouseEvent)v
oid if (password.text"mypassword")
gotoAndPlay(1,"Scene 2")
enterButton.addEventListener(MouseEvent.CLICK,che
ckPassword)
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Selection

• And the following to frame one of the actions
  layer in scene 2.

stop()
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Selection

• Example 3 Galaxians
• Imagine we are writing a piece of code which
  controls
• how a diving space invader
• attacks the player as it
• advances down the screen.
• It should do different things
• depending on how close it
• gets to the players ship.

50
25
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Selection

• We might write the decision like this
• Remember it will only try the first else if
  branch if the if branch is false.
• It will only try the second else if branch if the
  if and the first else if are false.

if (ship.y - alien.y gt 50) make alien stay
in formation else if (ship.y alien.y gt 25)
make alien start to fire on the ship and
dive in a random manner else if (ship.y
alien.y gt 10) execute kamikaze tactics
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Selection

• What would happen if we changed the password code
  to the following?

stop() function checkPassword(eventMouseEvent)v
oid if (password.text"user1")
gotoAndPlay(1,"Scene 2") else if
(password.text"user2") gotoAndPlay(1,"Sce
ne 3") enterButton.addEventListener(M
ouseEvent.CLICK,checkPassword)
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Looping

• In any programming language you essentially have
  two different situations when employing a loop.
• Either (a) you know how many times the loop has
  to be executed in advance or (b) you dont.

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Looping

• An example of (a) would be adding up all the
  elements of an array. We know how many times we
  have to loop because we know how many elements
  there are in the array.
• An example of (b) would be a situation where we
  want to keep repeating some action until the user
  presses a key indicating that we should stop.
• We tend to use while loops for (b) and for loops
  for (a).

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Looping

• Lets start with the while loop. Its general form
  is
• The condition is tested. If it is true then the
  statements are executed.
• The condition is tested again. If true, the
  statements are executed.
• This sequence continues until the condition is
  found to be false.

while (condition) statement(s)
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Looping

• In Flash these loops are used to obtain
  repetitive behaviour within a single frame.
• This is different to the loops between frames
  which we have already seen by the use of the
  gotoAndPlay() action.
• Lets look at a substantial example of the use of
  while loops in Flash.

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Looping

• Example Brownian Motion
• We are going to build a brownian motion
  simulator. Gases are made up of lots of little
  particles that bounce around in a random motion.
  This is called brownian motion and is caused by
  the particles constantly hitting each other and
  shooting off in random directions after each
  collision. The effect we want to achieve is ...

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Looping
1. Open a new movie and give it a black
background. Create a graphic symbol called
plasma that consists of a white circle with a
radial gradient that fades to black at the
edges. 2. Now create a new movie clip called
randomplasma. This should be blank at present!.
Drag the plasma graphic symbol onto the centre
of the stage and use the Properties panel to
centre it at 0.0, 0.0. Create a new layer called
Actions. 3. In frame 1 of the Actions layer (of
the movie clip random plasma) add the following
code in the Frame Actions window this.x
this.x (Math.random()4)-2 this.y this.y
(Math.random()4)-2 4. Add a keyframe in frame
2 of the Actions layer and add a gotoAndPlay(1)
action. 5. Return to Scene 1 and drag three
instances of the random plasma movie clip onto
the stage. Test the movie (Plasma01.fla).
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Looping
6. Delete two of the instances. Bring up the
properties panel and give the movieclip an
instance name, particle. 7. Go to the root
timeline (e.g. Scene 1). Add a new layer called
Actions.

• In this new layer we are going to clone our
  particle so we have lots of them in order to form
  a gas cloud.
• We are going to use the new keyword to do this.

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Loading Symbols Dynamically

• In order for this to work you must turn on the
  Export for ActionScript option in the Linkage
  properties for the target symbol.
• Right click on the symbol in the Library and
  choose Linkage. Check Export for Actionscript.
• Because you can no longer add an identifier, you
  have to handle everything instead using the class
  name. If you don't have a custom class for the
  symbol you want to attach, use the symbol name as
  inserted by default. In this case, Flash creates
  a skeleton class for you using the name of the
  symbol.

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Loading Symbols Dynamically

• This is the general form of the code

var myInstance new MyClassName()
addChild(myInstance)
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Looping
8. In frame 1 of the new layer actions add the
following code
var counterint 10 while (countergt0) var
newParticle new randomPlasma() addChild(newPar
ticle) newParticle.x particle.x newParticle.
y particle.y counter-- stop()
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Looping

• Something wrong with this
• We were supposed to make the movie clip symbol
  transparent to some degree ...
• This works a bit better.
• Try it out with 100 instances instead .

9. Edit the symbol plasma. Using the color mixer
set it so the radial gradient goes from
white/alpha 100 to white/alpha 0