Method_added hook in Ruby - Step-by-Step Execution

Concept Flow - Method_added hook

Define class

↓

Define method

↓

Ruby detects new method

↓

Trigger method_added hook

↓

Execute method_added code

↓

Continue defining class/methods or finish

When a new method is defined in a Ruby class or module, Ruby automatically calls the method_added hook if it is defined, allowing custom code to run right after method creation.

Execution Sample

Ruby

class MyClass
  def self.method_added(name)
    puts "Added method: #{name}"
  end

  def greet
    puts 'Hello'
  end
end

This code shows how Ruby calls method_added hook after defining the greet method, printing the method name.

Execution Table

Step	Action	Method Name Detected	Hook Called	Output
1	Start class definition	-	No	-
2	Define method_added hook	-	No	-
3	Define method greet	greet	Yes	Added method: greet
4	Class definition ends	-	No	-

💡 Class definition ends, no more methods added, hook no longer triggered

Variable Tracker

Variable	Start	After Step 3	Final
name	-	:greet	:greet

Key Moments - 3 Insights

Why does method_added receive the method name as a symbol?

Because Ruby passes the newly defined method's name as a symbol to method_added, as shown in execution_table step 3 where 'greet' is passed as :greet.

Does method_added get called when defining the hook itself?

No, method_added is not called when defining itself, only when other methods are added, as seen in execution_table steps 1 and 2.

Can method_added be used to track all methods added to a class?

Yes, method_added runs every time a new method is defined, so it can track all methods, demonstrated by the hook printing method names in step 3.

Visual Quiz - 3 Questions

Look at the execution_table, what is the output when the greet method is defined?

A method_added hook called

B Added method: greet

C Hello

D No output

💡 Hint

Check the Output column at Step 3 in the execution_table.

At which step does Ruby call the method_added hook?

A Step 3

B Step 2

C Step 1

D Step 4

💡 Hint

Look at the Hook Called column in the execution_table.

If you add another method after greet, what will happen?

A method_added hook will not run

B The class will crash

C method_added hook will run again with new method name

D The greet method will be removed

💡 Hint

method_added runs every time a new method is defined, as shown in the concept_flow.

Concept Snapshot

method_added is a Ruby hook method called automatically when a new method is defined in a class or module.
It receives the new method's name as a symbol.
Use it to run custom code right after method creation.
It does not trigger when defining itself.
Useful for tracking or modifying methods dynamically.

Full Transcript

In Ruby, when you define a new method inside a class or module, Ruby automatically calls a special hook method called method_added if it exists. This hook receives the name of the new method as a symbol. For example, if you define a method named greet, Ruby calls method_added(:greet). This lets you run code right after a method is created, such as printing the method name. The method_added hook itself is not called when you define it, only for other methods added later. This feature is useful to track or modify methods dynamically as they are defined.

Practice

1. What does the method_added hook do in Ruby?

easy

A. It runs when a class is inherited by another class.

B. It runs automatically whenever a new instance method is defined in a class.

C. It runs when a method is called on an object.

D. It runs when a module is included in a class.

Solution

1. Step 1: Understand the purpose of method_added

   The method_added hook is a special method in Ruby that triggers automatically when a new instance method is defined inside a class.

2. Step 2: Compare with other hooks

   Other hooks like inherited or included run on inheritance or module inclusion, not on method definition.

3. Final Answer:

   It runs automatically whenever a new instance method is defined in a class. -> Option B

4. Quick Check:

   method_added triggers on method definition [OK]

Hint: Remember: method_added triggers on new instance methods [OK]

Common Mistakes:

• Confusing method_added with inherited or included hooks
• Thinking it runs when methods are called
• Assuming it triggers on class creation

2. Which of the following is the correct way to define a method_added hook inside a Ruby class?

easy

A. def self.method_added(method_name); puts "Added: #{method_name}"; end

B. def self.method_added; puts "Added method"; end

C. def method_added(method_name); puts "Added: #{method_name}"; end

D. def method_added; puts "Added method"; end

Solution

1. Step 1: Identify the correct method signature

   The method_added hook is an instance method of the class's singleton class, so it should be defined as an instance method inside the class (not a class method). It receives the method name as a symbol.

2. Step 2: Check the options

   def method_added(method_name); puts "Added: #{method_name}"; end defines def method_added(method_name) which is correct. def self.method_added(method_name); puts "Added: #{method_name}"; end defines it as a class method (self.method_added), which is incorrect. Options B and C miss the required parameter.

3. Final Answer:

   def method_added(method_name); puts "Added: #{method_name}"; end -> Option C

4. Quick Check:

   method_added is an instance method with one parameter [OK]

Hint: Define method_added as instance method with one parameter [OK]

Common Mistakes:

• Defining method_added as a class method
• Omitting the method_name parameter
• Using wrong method signature without parameters

3. What will be the output of the following Ruby code?

class Test
  def method_added(name)
    puts "Method added: #{name}"
  end

  def hello
    puts 'Hello world'
  end
end

Test.new.hello

medium

A. Method added: hello
Hello world

B. Hello world

C. Method added: hello

D. No output

Solution

1. Step 1: Trace the class definition

   During class definition, def method_added is processed first; the hook for defining method_added itself does not print as it is not yet active. Then def hello triggers Test.method_added(:hello), printing "Method added: hello".

2. Step 2: Runtime execution

   Test.new.hello prints "Hello world". Combined output matches Method added: hello Hello world.

3. Final Answer:

   Method added: hello Hello world -> Option A

4. Quick Check:

   Hook triggers for methods defined after method_added [OK]

Hint: method_added hook triggers during class def for subsequent methods [OK]

Common Mistakes:

• Thinking method_added as instance method does not hook
• Forgetting hook triggers at class definition time
• Expecting only "Hello world"

4. Identify the error in this Ruby code using method_added hook:

class Demo
  def method_added(name)
    puts "Added: #{name}"
    def new_method
      puts 'New method called'
    end
  end

  def test
    puts 'Test method'
  end
end

Demo.new.new_method

medium

A. Infinite recursion caused by defining a method inside method_added

B. Syntax error due to missing end keyword

C. No error; code runs and prints 'New method called'

D. new_method is private and cannot be called

Solution

1. Step 1: Analyze method_added behavior

   Inside method_added, a new method new_method is defined. Defining a method triggers method_added again, causing infinite recursion.

2. Step 2: Understand the recursion effect

   This recursion never ends, leading to a SystemStackError (stack overflow) at runtime.

3. Final Answer:

   Infinite recursion caused by defining a method inside method_added -> Option A

4. Quick Check:

   Defining methods inside method_added causes recursion [OK]

Hint: Avoid defining methods inside method_added to prevent recursion [OK]

Common Mistakes:

• Ignoring infinite recursion risk
• Thinking code runs normally
• Confusing syntax error with runtime error

5. How can you safely use method_added to track new methods without causing infinite recursion when defining methods inside the hook?

hard

A. Use alias_method to rename methods inside method_added

B. Define all methods outside the class to avoid triggering method_added

C. Remove method_added after the first method is added

D. Use a guard variable to skip method_added logic when defining methods inside the hook

Solution

1. Step 1: Understand the recursion problem

   Defining methods inside method_added triggers the hook again, causing infinite recursion.

2. Step 2: Use a guard variable

   By setting a flag (e.g., a class variable) before defining methods inside the hook and checking it at the start of method_added, you can skip the hook logic during internal method definitions.

3. Step 3: Implement safe tracking

   This approach allows tracking new methods safely without recursion.

4. Final Answer:

   Use a guard variable to skip method_added logic when defining methods inside the hook -> Option D

5. Quick Check:

   Guard variable prevents recursion in method_added [OK]

Hint: Use a flag to skip method_added during internal method definitions [OK]

Common Mistakes:

• Not using a guard variable and causing recursion
• Trying to remove method_added dynamically
• Misusing alias_method to fix recursion