unit 6
object-oriented simulation
Object-oriented simulation
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Used for simulation of physical systems
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Key idea: the physical world consists of objects
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Chemical plants
Air combat
Ecologies
Objects have their own state
Objects act on one another
Make a simulation which has one data object per physical object
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Give each object fields that mirror the state of the physical objects
Give objects methods to implement the forces/actions that change
objects
Object-oriented programming
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Means many things to many people
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Least common denominator:
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Divide data into objects with fields
Provide some mechanism to allow objects to specify their own methods
for implementing different procedures ( a.k.a. “messages”)
OOP with class-based inheritance
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Objects are divided into classes (types)
Fields and methods determined are by an object’s class
Subclasses inherit fields and methods from parent classes
But can selectively override particular methods of the parent class
OOP in Meta
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[class [Name fields …]
Parent more-fields …]
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Makes a new object of type Type
And sets its fields based on args
Defines a method for initializing an
object of type Type before it’s returned
by new
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Makes a procedure that can be
specialized with methods
[define-method [generic [Type arg] …]
body …]
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[define-method [initialize [Type arg]]
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[generic-procedure]
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[new Type args …]
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Creates a new class named Name
With parent class Parent
And fields fields … and more-fields …
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Adds/modifies a method for generic
procedure generic
Matches only those calls whose args
have their corresponding Types
When a match is found, executes body
[call-next-method]
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When used inside a method, calls the
method of the parent class
Adventure games
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One of the oldest
computer game genres
Based on
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wandering through
a set of rooms
interacting with
objects and characters
Solving puzzles
or mysteries
Subgenres
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Point-and-click adventures (Myst, Syberia)
Text adventure games (Zork, Infocom games)
RPGs (Final Fantasy, Everquest, Baldur’s gate)
Ontology of adventure games
Things
Persons
Player
Items
Monsters
NPCs
Portals
Game state:
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Spatiality
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Things have locations (places)
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Places have contents (sets of things)
Inventory
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Equipment
Consumables (health, gold, potions, etc.)
Key items
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“Unlock” parts of the game
Allowing the narrative to progress
Key items
Supplies
Equipment
Places
Inventory
Rooms
An adventure game in Meta
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Supports
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Pictures (for point-and-click style)
Text output
Sound
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You can decide which of these you want to use
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Basic ontology
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Things (have name, location)
Places (have name, contents)
Player
Portals (have name, location, destination)
The Thing class
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Things have:
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Name
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Location
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A string describing the
object
May be null
Place in which the Thing
can be found
May be null
Adjectives
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List of strings describing
the object
May be null
[define Thing
[class [Thing name location]
Object
adjectives]]
[define-method [initialize [Thing x]]
[move x x.location]]
The Place class
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Places have
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Contents
A list of Things inside them
Sound
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Sound-loop?
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If true, the sound plays
continuously
Image
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Buffer to play when the user
enters the room
Can be null if no sound
Bitmap to display when the user
enters the room
Hotspots
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A list of places in the image the
player can click and handlers
(procedures to call) for when
they’re clicked
[define Place
[class [Place name]
Thing
contents hotspots
sound sound-loop?
image]]
[define-method [initialize [Place p]]
[call-next-method]
[p.contents ← [new ArrayList]]]
Trivial example
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(Based on a randomly chosen picture from
my hard drive)
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One room
Two Things in it
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The Player
A group of Nerds
Trivial example
[define trivial-example
[→ [with great-hall = [new Place “the Great Hall”]
[great-hall.adjectives ← [list “big” “pink”]]
[great-hall.image ← [image-file “Aaron talk.jpg”]]
[great-hall.sound ← [sound-file “dog-bark4.wav”]]
[great-hall.hotspots ←
[list [list 640 320 760 450
[→ [print “Aaron says Mythica was great until they killed the project.”]]]]]
[the-player ← [new Player “Emily” great-hall]]
[with nerds = [new Nerds null great-hall]
[nerds.adjectives ← [list “many”]]]
[start-game “Sample game”
800
600 100 100]]]]
Trivial example
Make the Place object
[define trivial-example
[→ [with great-hall = [new Place “the Great Hall”]
[great-hall.adjectives ← [list “big” “pink”]]
[great-hall.image ← [image-file “Aaron talk.jpg”]]
[great-hall.sound ← [sound-file “dog-bark4.wav”]]
[great-hall.hotspots ←
[list [list 640 320 760 450
[→ [print “Aaron says Mythica was great until they killed the project.”]]]]]
[the-player ← [new Player “Emily” great-hall]]
[with nerds = [new Nerds null great-hall]
[nerds.adjectives ← [list “many”]]]
[start-game “Sample game”
800
600 100 100]]]]
Trivial example
Set up its attributes
[define trivial-example
[→ [with great-hall = [new Place “the Great Hall”]
[great-hall.adjectives ← [list “big” “pink”]]
[great-hall.image ← [image-file “Aaron talk.jpg”]]
[great-hall.sound ← [sound-file “dog-bark4.wav”]]
[great-hall.hotspots ←
[list [list 640 320 760 450
[→ [print “Aaron says Mythica was great until they killed the project.”]]]]]
[the-player ← [new Player “Emily” great-hall]]
[with nerds = [new Nerds null great-hall]
[nerds.adjectives ← [list “many”]]]
[start-game “Sample game”
800
600 100 100]]]]
Trivial example
Make Player object
[define trivial-example
[→ [with great-hall = [new Place “the Great Hall”]
[great-hall.adjectives ← [list “big” “pink”]]
[great-hall.image ← [image-file “Aaron talk.jpg”]]
[great-hall.sound ← [sound-file “dog-bark4.wav”]]
[great-hall.hotspots ←
[list [list 640 320 760 450
[→ [print “Aaron says Mythica was great until they killed the project.”]]]]]
[the-player ← [new Player “Emily” great-hall]]
[with nerds = [new Nerds null great-hall]
[nerds.adjectives ← [list “many”]]]
[start-game “Sample game”
800
600 100 100]]]]
Trivial example
Make Nerds object
[define trivial-example
[→ [with great-hall = [new Place “the Great Hall”]
[great-hall.adjectives ← [list “big” “pink”]]
[great-hall.image ← [image-file “Aaron talk.jpg”]]
[great-hall.sound ← [sound-file “dog-bark4.wav”]]
[great-hall.hotspots ←
[list [list 640 320 760 450
[→ [print “Aaron says Mythica was great until they killed the project.”]]]]]
[the-player ← [new Player “Emily” great-hall]]
[with nerds = [new Nerds null great-hall]
[nerds.adjectives ← [list “many”]]]
[start-game “Sample game”
800
600 100 100]]]]
Trivial example
Open window
and start game
[define trivial-example
[→ [with great-hall = [new Place “the Great Hall”]
[great-hall.adjectives ← [list “big” “pink”]]
[great-hall.image ← [image-file “Aaron talk.jpg”]]
[great-hall.sound ← [sound-file “dog-bark4.wav”]]
[great-hall.hotspots ←
[list [list 640 320 760 450
[→ [print “Aaron says Mythica was great until they killed the project.”]]]]]
[the-player ← [new Player “Emily” great-hall]]
[with nerds = [new Nerds null great-hall]
[nerds.adjectives ← [list “many”]]]
[start-game “Sample game”
800
600 100 100]]]]
Moving objects
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We need to make sure that
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We make generic procedures for
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If a thing T says place P is its
location
Then P lists T in its contents
And vice-versa
Adding and removing objects from
locations
Moving objects between locations
This allows us to define special
behavior for particular kinds of
Places and Things
[define move [generic-procedure]]
[define remove [generic-procedure]]
[define add [generic-procedure]]
[define-method [move thing place]
[remove thing thing.location]
[add thing place]]
Adding and removing
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Some Things can have null
as their location
[define-method [remove thing [null place]]
«Do nothing»
null]
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So we need to be careful to
specialize the behavior of
add and remove
[define-method [remove thing [Place
place]]
[thing.location ← null]
[place.contents.Remove thing]]
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Fortunately, you can specify
null as the class of an
argument
[define-method [add thing [null place]]
[thing.location ← null]]
[define-method [add thing [Place place]]
[thing.location ← place]
[place.contents.Add thing]]
Describing objects
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For text adventures
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We need a way to generate
text to describe objects
[define description “Returns a string describing the object”
[generic-procedure]]
[define-method [description [Thing t]]
[with type-name = [[type-of t].Name.ToLower]
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The name of the object (if any)
modifiers = [maybe-pad
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It’s type
[comma-separate t.adjectives
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Its adjectives (if any)
false]]
name-string = [if [null? t.name]
“”
Examples
[concat “ named ” t.name]]
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[description [new Cat]]
[concat “a ”
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“a cat”
modifiers
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[description [new Cat “Harry” null]]
type-name
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“a cat named Harry”
name-string]]]
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[with h = [new Cat “Harry” null]
We want to base it on
[h.adjectives ← [list “big” “black”]]
[describe h]]
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“a big, black cat named Harry”
Notes
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Concat is a procedure that appends strings together
Stuff in gray is black magic we haven’t taught you about yet
Describing scenes
[define-method [view-description [Place p]]
[with stuff = [filter [t → [≠ t the-player]] «Everything in the room except the player»
p.contents]
[concat “You are in ”
[description p]
“. ”
[cond [[= [length stuff]
1]
[concat “You see ”
[description [first stuff]]
“.”]]
[[empty? stuff]
“”]
[else
[concat “ You see ”
[comma-separate [map description stuff]
true]
“.”]]]]]]
Describing scenes
[describe great-hall]
“You are in a big, pink place named the
Great Hall. You see a many nerds.”
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Choosing the sound to play
[define background-sound
“Returns the sound buffer for the background noise of this Place, if any”
[generic-procedure]]
[define-method [background-sound [Place p]]
p.sound]
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The window code calls background-sound on the player’s location
to choose what sound to play
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Why did we do it this way?
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Why not just have it play p.sound directly?
Choosing the image to show
[define view-image [generic-procedure]]
[define-method [view-image [Place p]]
p.image]
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Again: why bother with a procedure like this?
Handling clicks of the mouse
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The hotspots of a Place is a field
with a list of lists:
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[list left top
right bottom
proc]
Where:
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Left and top give the
coordinates of the upper-left
corner of an area of the image
Right and bottom give the
coordinates of its lower-right
corner
Proc gives a procedure to call
when the user clicks there
But again, you can change the
behavior of clicking by
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Making a new kind of place (i.e.
a subclass of Place)
and defining a new method for it
[define click
[generic-procedure]]
[define-method [click [Place p] mouseEventArgs]
[with x = mouseEventArgs.X
y = mouseEventArgs.Y
[unless [null? p.hotspots]
[for-each [hotspot →
[when [and [≥ x [first hotspot]]
[≥ y [second hotspot]]
[≤ x [third hotspot]]
[≤ y [fourth hotspot]]]
[[fifth hotspot]]]]
p.hotspots]]]]