Observer pattern in PHP - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare the Observer interface.

PHP

<?php
interface [1] {
    public function update();
}
?>

Drag options to blanks, or click blank then click option'

AObservable

BListener

CSubject

DObserver

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Subject' instead of 'Observer' as interface name.

Naming the interface 'Listener' which is not standard here.

The Observer pattern requires an Observer interface that defines the update() method.

2fill in blank

medium

Complete the code to add an observer to the subject.

PHP

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

    public function [1](Observer $observer) {
        $this->observers[] = $observer;
    }
}
?>

Drag options to blanks, or click blank then click option'

Aattach

BaddObserver

CregisterObserver

Dadd

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'addObserver' which is common in other languages but not here.

Using 'add' which is too generic.

The method to add an observer is commonly named attach in PHP Observer implementations.

3fill in blank

hard

Fix the error in the notifyObservers method to call update on each observer.

PHP

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

    public function notifyObservers() {
        foreach ($this->observers as $observer) {
            $observer->[1]();
        }
    }
}
?>

Drag options to blanks, or click blank then click option'

Anotify

Bupdate

Crefresh

Dcall

Attempts:

3 left

💡 Hint

Common Mistakes

Calling a non-existent method like 'notify' or 'refresh'.

Using 'call' which is not a method name.

The update() method is the standard method called on observers to notify them of changes.

4fill in blank

hard

Fill both blanks to complete the observer class that implements the Observer interface.

PHP

<?php
class ConcreteObserver implements [1] {
    public function [2]() {
        echo "Observer notified!";
    }
}
?>

Drag options to blanks, or click blank then click option'

AObserver

BSubject

Cupdate

Dnotify

Attempts:

3 left

💡 Hint

Common Mistakes

Implementing 'Subject' instead of 'Observer'.

Naming the method 'notify' instead of 'update'.

The class must implement the Observer interface and define the update() method.

5fill in blank

hard

Fill all three blanks to complete the Subject class with attach, detach, and notify methods.

PHP

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

    public function [1](Observer $observer) {
        $this->observers[] = $observer;
    }

    public function [2](Observer $observer) {
        $index = array_search($observer, $this->observers, true);
        if ($index !== false) {
            unset($this->observers[$index]);
        }
    }

    public function [3]() {
        foreach ($this->observers as $observer) {
            $observer->update();
        }
    }
}
?>

Drag options to blanks, or click blank then click option'

Aattach

Bdetach

CnotifyObservers

DremoveObserver

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'addObserver' or 'removeObserver' instead of 'attach' and 'detach'.

Naming notify method simply 'notify' instead of 'notifyObservers'.

The Subject class uses attach to add, detach to remove, and notifyObservers to notify all observers.

Practice

1. What is the main purpose of the Observer pattern in PHP programming?

easy

A. To allow objects to watch and react to changes in another object

B. To speed up the execution of PHP scripts

C. To store data in a database efficiently

D. To handle user input validation

Solution

1. Step 1: Understand the Observer pattern concept

   The Observer pattern is designed to let one object notify others about changes.

2. Step 2: Match the purpose with the options

   To allow objects to watch and react to changes in another object correctly describes this notification and reaction mechanism.

3. Final Answer:

   To allow objects to watch and react to changes in another object -> Option A

4. Quick Check:

   Observer pattern = objects watch and react [OK]

Hint: Observer pattern = watch and react to changes [OK]

Common Mistakes:

• Confusing Observer with database or input handling
• Thinking it speeds up code execution
• Mixing it with unrelated design patterns

2. Which of the following is the correct way to attach an observer to a subject in PHP using the Observer pattern?

easy

A. $subject->addObserver($observer);

B. $observer->attach($subject);

C. $subject->attach($observer);

D. $observer->addSubject($subject);

Solution

1. Step 1: Recall the standard method to add observers

   In PHP's Observer pattern, the subject calls attach() to add observers.

2. Step 2: Verify method and object roles

   The subject manages observers, so $subject->attach($observer); is correct.

3. Final Answer:

   $subject->attach($observer); -> Option C

4. Quick Check:

   Subject attaches observer = $subject->attach($observer) [OK]

Hint: Subject attaches observer with attach() method [OK]

Common Mistakes:

• Calling attach() on observer instead of subject
• Using non-standard method names like addObserver
• Mixing up subject and observer roles

3. Given the following PHP code using the Observer pattern, what will be the output?

$subject = new Subject();
$observer = new ConcreteObserver();
$subject->attach($observer);
$subject->setState('Hello');

Assuming ConcreteObserver prints the new state when notified.

medium

A. Hello

B. State changed

C. No output

D. Error: method not found

Solution

1. Step 1: Understand the flow of notification

   The subject changes state and notifies the observer, which prints the new state.

2. Step 2: Identify the observer's reaction

   The observer prints the state 'Hello' when notified.

3. Final Answer:

   Hello -> Option A

4. Quick Check:

   Observer prints new state = Hello [OK]

Hint: Observer prints state when notified after attach [OK]

Common Mistakes:

• Assuming no output without calling notify explicitly
• Confusing printed message with generic text
• Expecting errors without method definitions

4. What is wrong with this PHP Observer pattern code snippet?

class Subject {
  private $observers = [];
  public function attach($observer) {
    $this->observers[] = $observer;
  }
  public function notify() {
    foreach ($this->observers as $observer) {
      $observer->update();
    }
  }
}

$subject = new Subject();
$subject->notify();

medium

A. attach() method is missing

B. Observers array is private and inaccessible

C. notify() method should not call update()

D. No observers attached before notify is called

Solution

1. Step 1: Check observer attachment before notify

   The code calls notify() without attaching any observers first.

2. Step 2: Understand effect of no observers

   Since no observers are attached, notify() loops over an empty array and does nothing.

3. Final Answer:

   No observers attached before notify is called -> Option D

4. Quick Check:

   Notify without observers = no action [OK]

Hint: Attach observers before notify() to trigger updates [OK]

Common Mistakes:

• Thinking private array causes error
• Assuming attach() method is missing
• Believing notify() should not call update()

5. You want to implement an Observer pattern in PHP where multiple observers react differently to the same state change. Which approach best supports this requirement?

hard

A. Use a single observer class and change its behavior with if-else inside update()

B. Create different observer classes implementing a common interface and attach them to the subject

C. Attach only one observer and switch its internal state manually

D. Avoid using Observer pattern and use global variables instead

Solution

1. Step 1: Identify how to support multiple reactions

   Different observer classes allow each to react uniquely to the same notification.

2. Step 2: Evaluate options for flexibility and design

   Create different observer classes implementing a common interface and attach them to the subject uses a common interface but different classes, which is clean and scalable.

3. Final Answer:

   Create different observer classes implementing a common interface and attach them to the subject -> Option B

4. Quick Check:

   Multiple observers = different classes with common interface [OK]

Hint: Use multiple observer classes for different reactions [OK]

Common Mistakes:

• Using one observer with complex if-else logic
• Attaching only one observer limits reactions
• Ignoring design principles by using globals