Observer pattern in PHP - Step-by-Step Execution

Concept Flow - Observer pattern

Subject state changes

↓

Notify all observers

↓

Each observer updates

↓

Observers react to change

↓

End

The subject changes state, then notifies all observers, which then update themselves accordingly.

Execution Sample

PHP

<?php
class Subject {
  private $observers = [];
  private $state;

  public function attach($observer) {
    $this->observers[] = $observer;
  }

  public function setState($state) {
    $this->state = $state;
    $this->notify();
  }

  public function notify() {
    foreach ($this->observers as $obs) {
      $obs->update($this->state);
    }
  }
}

class Observer {
  public function update($state) {
    echo "Observer notified with state: $state\n";
  }
}

$subject = new Subject();
$observer1 = new Observer();
$observer2 = new Observer();
$subject->attach($observer1);
$subject->attach($observer2);
$subject->setState('ON');
?>

This PHP code shows a subject notifying two observers when its state changes.

Execution Table

Step	Action	Subject State	Observers Notified	Output
1	Create Subject	null	none
2	Create Observer1	null	none
3	Create Observer2	null	none
4	Attach Observer1	null	none
5	Attach Observer2	null	none
6	Set Subject state to 'ON'	ON	Observer1, Observer2	Observer notified with state: ON Observer notified with state: ON
7	Notify observers	ON	Observer1, Observer2
8	Observer1 update called	ON	Observer1	Observer notified with state: ON
9	Observer2 update called	ON	Observer2	Observer notified with state: ON
10	End	ON	all notified	Notifications complete

💡 All observers notified after subject state changed to 'ON'

Variable Tracker

Variable	Start	After Step 5	After Step 6	After Step 10
subject->state	null	null	'ON'	'ON'
subject->observers	[]	[Observer1, Observer2]	[Observer1, Observer2]	[Observer1, Observer2]

Key Moments - 3 Insights

Why do observers get notified only after the subject's state changes?

Because the notify() method is called inside setState(), so observers update only when the state changes (see execution_table step 6).

Does attaching observers change the subject's state?

No, attaching observers just adds them to the list; the state remains unchanged until setState() is called (see steps 4 and 5).

Why does each observer print the notification message separately?

Because notify() calls update() on each observer individually, so each observer reacts and outputs its own message (see steps 8 and 9).

Visual Quiz - 3 Questions

Look at the execution_table, what is the subject's state after step 5?

A 'ON'

B null

C empty string

D undefined

💡 Hint

Check the 'Subject State' column at step 5 in the execution_table.

At which step do observers start getting notified?

A Step 5

B Step 7

C Step 6

D Step 10

💡 Hint

Look at the 'Observers Notified' column and output messages in execution_table.

If we add a third observer before step 6, how would the 'Observers Notified' column change at step 7?

A Observer1, Observer2, Observer3

B Observer1, Observer2

C Observer3 only

D No observers

💡 Hint

The subject notifies all attached observers, so adding one more means all three get notified.

Concept Snapshot

Observer pattern:
- Subject holds list of observers
- Observers attach to subject
- When subject state changes, it calls notify()
- notify() calls update() on each observer
- Observers react to changes independently

Full Transcript

The Observer pattern lets one object (subject) tell many others (observers) when something changes. Here, the subject keeps a list of observers. When its state changes, it calls notify(), which calls update() on each observer. Each observer then reacts, for example by printing a message. This way, observers stay updated automatically without the subject needing to know details about them.