The state is a behavioural design pattern that let’s an object alter it’s behaviour when it’s internal state changes.
Finite State Machine
A system in which:
- There’s a fixed set of states
- There can be only one active state at a time
- A sequence of inputs or events is sent to the machine
- Each state has a set of transitions - defining what state it can change to after what input
State Pattern
This pattern calls for representing each state as a class and extracting all state-specific behaviour into these classes. Each of them should follow the same State interface
The original object (called \context) stores a reference to one of the state objects and delegates work.
To transition the context into another state, replace the object with another that represents new state.
Difference with Strategy Pattern
In the State pattern, the particular states may be aware of each other and initiate transitions from one state to another, whereas strategies almost never know about each other.
Usage
When to Use
- When an object behaves differently depending on its state, there’s considerable number of states and the state-specific code is prone to changes
- When you need to use massive conditionals that alter class behaviour
- When there’s a lot of duplicated code across similar states and transitions of condition-based state machine
When not to Use
- When a state machine has not a lot of states or rarely changes