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Design

Ian Holyer

Ian Holyer

Case Study: Stage

●

Design

Design is too varied and personal to be able to develop principles

is too varied and personal to be able to develop principles

very easily

very easily

●

We will look at some OO Design issues and Design Patterns later

We will look at some OO Design issues and Design Patterns later

on, which help with design, but only

on, which help with design, but only after

after you already know what

you already know what

it is

it is

●

For now, to explore what design really

For now, to explore what design really is

is , we will just look at one

, we will just look at one

case study

case study

●

This is the Stage project, where we have already done the analysis

This is the Stage project, where we have already done the analysis

●

The things to remember during the case study are (a) we are trying

The things to remember during the case study are (a) we are trying

to design an engine, not a program and (b) we are trying to follow

to design an engine, not a program and (b) we are trying to follow

the KISS principle

the KISS principle

Where to Start the Design

●

Before OO, the standard way to start a design was with

Before OO, the standard way to start a design was with

functional decomposition, i.e. knowing what the program

functional decomposition, i.e. knowing what the program

does overall, break the work into major pieces

does overall, break the work into major pieces

●

This suits a non-OO program that ends up as procedures

This suits a non-OO program that ends up as procedures

calling procedures, probably monolithic

calling procedures, probably monolithic

●

With OO, one way to start is by trying to identify nouns

With OO, one way to start is by trying to identify nouns

that might make good classes

that might make good classes

●

In this case, without making any decisions, we have

In this case, without making any decisions, we have

Player, Command, Word, Noun, Verb, Place, Direction,

Player, Command, Word, Noun, Verb, Place, Direction,

Thing, Description, Puzzle, World, Game

Thing, Description, Puzzle, World, Game

●

Let's start in the middle with Place and Thing

Let's start in the middle with Place and Thing

A First Attempt

●

Perhaps the most obvious approach is for a Place to

Perhaps the most obvious approach is for a Place to

store places you can go, and Things at that place

store places you can go, and Things at that place

class Place

{

Place north, east, south, west;

List things;

}

class Place

{

Place north, east, south, west;

List things;

}

class Thing

{ ...

}

class Thing

{ ...

}

●

This looks natural, but has two serious problems for us

This looks natural, but has two serious problems for us

The Flexibility Problem

●

The second problem concerns creators of new games

The second problem concerns creators of new games

●

We don't want to restrict a creator's imagination, and

We don't want to restrict a creator's imagination, and

adventure games are about fantasy where anything goes

adventure games are about fantasy where anything goes

●

Before

Before

looking at the next slide, try to think of:

looking at the next slide, try to think of:

●

other directions

●

things inside things

●

places inside places

●

a thing which can be in two places at once

●

something which is a thing and a place at the same time

●

something which is not a thing nor a place

●

something you haven't got the imagination to predict

The Flexibility Problem

●

Here are some possible answers:

Here are some possible answers:

●

other directions: northeast, forward, down

●

things inside things: contents of box, bag, egg

●

places inside places: cupboard/room/building, car/road

●

things in two places: river, door, hole in wall, teleport

●

a thing and a place: tent, wormhole

●

not thing nor place: monsters, other players

●

something you haven't got the imagination to predict: ...

A Metaphor

●

It is often useful to use metaphors, which is like finding

It is often useful to use metaphors, which is like finding

extra nouns to play with to help design classes

extra nouns to play with to help design classes

●

A good metaphor in this case is Java itself, particularly

A good metaphor in this case is Java itself, particularly

Object and Field, though those won't make good class

Object and Field, though those won't make good class

names, because they will clash with Java

names, because they will clash with Java

●

A Place is like an Object (actually, it already

A Place is like an Object (actually, it already is

is )

●

Directions are like Fields (actually, they already are)

Directions are like Fields (actually, they already are)

●

The Things that are currently in a Place are a bit like

The Things that are currently in a Place are a bit like

Fields (but they change dynamically)

Fields (but they change dynamically)

●

A Container is like an Object with Fields which refer to

A Container is like an Object with Fields which refer to

other Things as contents, and so on

other Things as contents, and so on

Place and Thing Objects

●

Our example looks like this, as a UML object diagram,

Our example looks like this, as a UML object diagram,

showing fields linking objects

showing fields linking objects

beach

tree

sand

spade

spade

south north

sand

here

●

The field names are the nouns used in the game

The field names are the nouns used in the game

tree.spade = spade

tree.south = beach

beach.sand = sand

beach.north = tree

The Entity Class

●

Let's have a class called Entity

Let's have a class called Entity

●

Let's decide that it represents a place, or a thing, or a

Let's decide that it represents a place, or a thing, or a

container, or a vehicle, or a monster, or ...

container, or a vehicle, or a monster, or ...

●

Let's decide that an Entity has a set or list of named

Let's decide that an Entity has a set or list of named

links to other entities, which represent directions, or

links to other entities, which represent directions, or

contents, or any other one-way relationships

contents, or any other one-way relationships

●

Links can come and go during play

Links can come and go during play

●

The link names are the nouns that the player uses, and

The link names are the nouns that the player uses, and

we don't want to hard-wire nouns into the

we don't want to hard-wire nouns into the Stage

Stage engine,

engine,

so north/south etc. are not going to be special

so north/south etc. are not going to be special

●

Question: does an Entity have a name?

Question: does an Entity have a name?

No Names for Entities

●

An Entity can be known by different names when

An Entity can be known by different names when

viewed from different perspectives

viewed from different perspectives

city river field

east west

●

The river is known as "east" when you are in the city,

The river is known as "east" when you are in the city,

and as "west" when you are in the field

and as "west" when you are in the field

●

The player often doesn't know exactly what the entity is

The player often doesn't know exactly what the entity is

that is being referred to:

that is being referred to:

cave dynamite

candle

●

Only world creators seem to need global names for

Only world creators seem to need global names for

entities, and even they can be links

entities, and even they can be links

Commands

●

We want to get back to our overall design wanderings

We want to get back to our overall design wanderings

and think about commands, "go south" or "take spade"

and think about commands, "go south" or "take spade"

●

We have decided that we don't want nouns like "south"

We have decided that we don't want nouns like "south"

or "spade" to be special, but invented by the creator

or "spade" to be special, but invented by the creator

●

We can take the same attitude to verbs, "go" and "take"

We can take the same attitude to verbs, "go" and "take"

●

But how do we know if an entity is a place or a thing,

But how do we know if an entity is a place or a thing,

how do we know "go spade" and "take south" won't

how do we know "go spade" and "take south" won't

work, and how do we get "go" and "take" to work?

work, and how do we get "go" and "take" to work?

●

There must be a way forward from here, mustn't there?

There must be a way forward from here, mustn't there?

Hints

●

We can go back to the Object, Field metaphor, but as

We can go back to the Object, Field metaphor, but as

before we want more flexibility

before we want more flexibility

●

We can make use of our new understanding of OO,

We can make use of our new understanding of OO,

"delegate the work to the object with the data"

"delegate the work to the object with the data"

●

Does this suggest a way forward?

Does this suggest a way forward?

Answered Questions

●

This leads to the idea that an Entity has a collection of

This leads to the idea that an Entity has a collection of

named Actions (say) associated with it

named Actions (say) associated with it

●

A "method call"

A "method call"

spade.take()

spade.take()

is implemented by

is implemented by

looking up a "take" action in the "spade" entity

looking up a "take" action in the "spade" entity

●

How do you know an entity is a thing that can be

How do you know an entity is a thing that can be picked

picked

up

up ? Answer, because it has a "take" action

? Answer, because it has a "take" action

●

How do you know an entity is a place? Answer,

How do you know an entity is a place? Answer,

because it has a "go" action

because it has a "go" action

●

The command "go south" becomes

The command "go south" becomes south.go()

south.go() so it is

so it is

the

the

place

place

that is responsible for getting you there (this is

that is responsible for getting you there (this is

unusual, bit it fits

unusual, bit it fits these

these circumstances)

circumstances)

Actions

●

Our example now looks something like this:

Our example now looks something like this:

beach

tree

sand

spade

spade

south north

sand

here

take

take

go

go

go

dig

dig

drop

drop