Real World Example
Imagine you are using some drawing application, you choose the paint brush to draw. Now the brush changes its behavior based on the selected color i.e. if you have chosen red color it will draw in red, if blue then it will be in blue etc.
In Plain Words
It lets you change the behavior of a class when the state changes.
Wikipedia Says
The state pattern is a behavioral software design pattern that implements a state machine in an object-oriented way. With the state pattern, a state machine is implemented by implementing each individual state as a derived class of the state pattern interface, and implementing state transitions by invoking methods defined by the pattern’s superclass.
The state pattern can be interpreted as a strategy pattern which is able to switch the current strategy through invocations of methods defined in the pattern’s interface.
Programmatic Example
Let’s take an example of a phone. First of all we have our state interface and some state implementations:
interface PhoneState {
public function pickUp(): PhoneState;
public function hangUp(): PhoneState;
public function dial(): PhoneState;
}
// states implementation
class PhoneStateIdle implements PhoneState {
public function pickUp(): PhoneState {
return new PhoneStatePickedUp();
}
public function hangUp(): PhoneState {
throw new Exception("already idle");
}
public function dial(): PhoneState {
throw new Exception("unable to dial in idle state");
}
}
class PhoneStatePickedUp implements PhoneState {
public function pickUp(): PhoneState {
throw new Exception("already picked up");
}
public function hangUp(): PhoneState {
return new PhoneStateIdle();
}
public function dial(): PhoneState {
return new PhoneStateCalling();
}
}
class PhoneStateCalling implements PhoneState {
public function pickUp(): PhoneState {
throw new Exception("already picked up");
}
public function hangUp(): PhoneState {
return new PhoneStateIdle();
}
public function dial(): PhoneState {
throw new Exception("already dialing");
}
}
class Phone {
private $state;
public function __construct() {
$this->state = new PhoneStateIdle();
}
public function pickUp() {
$this->state = $this->state->pickUp();
}
public function hangUp() {
$this->state = $this->state->hangUp();
}
public function dial() {
$this->state = $this->state->dial();
}
}
$phone = new Phone();
$phone->pickUp();
$phone->dial();
Key Points
- Encapsulates state-specific behavior in separate classes
- Makes state transitions explicit
- Eliminates large conditional statements
- Makes it easy to add new states without changing existing code
The State pattern is particularly useful when an object’s behavior depends on its state, and it must change its behavior at runtime depending on that state. It’s commonly used in implementing state machines, game character behaviors, and workflow systems.