Finite State MachineMobile Game Programming

jintaeks@gmail.comDivision of Digital Contents, Dongseo Univ.

October 6, 2016

Action states

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states of the mon-ster– Idle– Moving– Attack– Hurt– Dead

Simplification Step1: How to model the problem to find a solu-

tion. Step2: How to implement the model.

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Step1: Model

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Model: Object State Diagram The State Diagram is a well known data structure

in computer science. In compiler theory, we learn State Diagram which

is used in automata. In database, there is an entity-relation model

which is similar to the state diagram. In game programming, there is a state machine

which is based on the state diagram. We apply this data structure to an object in

game.– It is called ‘Object State Diagram’.

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State DiagramState

A State is represented as a circle.

Start state has a incoming arrow.

Final state has a double circle.

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State DiagramState Action A State can have action(s). An action is described in text after

slash symbol. An action describes the dynamic be-

havior of an object in that state.

Example:

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State/Action

State DiagramTransition A transition is represented as a

directed solid arrow. A transition means the state

change between two different states.

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A state transition between A and B.

It can be read, ‘State A goes to B’.

State DiagramTransition Input/Action A transition can have input(s)

and action(s). An input describes a condition

which trigger the state change. An action describes the dynamic

behavior of an object when it goes to other state.

Example:

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Monster State Diagram For simplicity, we only select 3 states.

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Final Monster State Diagram

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Question In Moving state, I want to add more detailed mo-

tions for the monster.– Crawl– Walk– Run

How to add this feature?

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(Answer to Question) We can build the ‘Moving State Diagram’. Add this sub state diagram into inner state dia-

gram of Moving State.

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Step2: Implementation

Step1: How to model the problem to find solu-tion.

Step2: How to implement the model.

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Simple Game Client

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Coding Conventions

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Adding States

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Adding Properties

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m_fIdleTimer

m_dwIdleMotion

m_dwDistanceToTarget

m_fSpellGauge

m_dwAttackMotion

m_dwTargetId

m_dwBboxId

m_fPrepareAttackTimer

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Adding State Transitions

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State transition: Idle state

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State transition: Moving state

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State transition: Attack state

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Test routine

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Demo The Simple Console application project using

Microsoft Visual Studio 2010.

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User press a ‘1' key.

User press a 'Esc' key.

Question: There is an output sequence of 1 and 0 after first 1 is printed. Why?

(Answer to question)

In Moving state, the condition of first if-statement is always true. Because the string expression “target is out in range” always returns a non-zero value.

In C++, a non-zero value evaluated to true for a boolean expression.

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(Implementation Optimization) Our C++ implementation can be structured to

more elegant form. We can use well-known design patterns. Use the pointer to member functions!

– I will show it in the next lecture.

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More Complex Example

Most programmer are aware of this concept. But they cannot use it freely, why?

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Implementing Simple Desktop Calculator.◇Input ◇Output• 1• +• 2• +• 13• =• +• 3• -• …

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1

2

116

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3

16

13

1

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How to Find States? Let’s focus on number.

1 + 2 + 13 = + 3 -

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first operand

second operandsecond operand

Can you find a hidden state?

1 + 2 + 13 = + 3 -

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The result can be used as first operand, but also it can be dis-carded.

3 States for Calculator problem

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Homework: Complete the Calculator State Model.

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Q&A

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