Method_added hook in Ruby

Introduction

The method_added hook lets you run code automatically whenever a new instance method is created in a class. It helps you watch or change methods as they appear.

You want to log or track every new method added to a class.

You want to automatically wrap or modify methods right after they are defined.

You want to enforce rules or validations on methods as they are created.

You want to create debugging tools that react to new methods.

You want to build frameworks that extend method behavior dynamically.

Syntax

Ruby

def self.method_added(method_name)
  # code to run when a new instance method is added
end

This hook is a class method and is called with the new method's name as a symbol.

Be careful to avoid infinite loops if you define methods inside method_added.

Examples

This example prints the name of each instance method added to MyClass. When greet is defined, it prints "Added method: greet".

Ruby

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

  def greet
    puts 'Hello!'
  end
end

Here, each time an instance method is added (foo and bar), the hook prints its name.

Ruby

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

  def foo
    puts 'foo'
  end

  def bar
    puts 'bar'
  end
end

Sample Program

This program shows how method_added prints a message each time a new instance method is defined in the Tracker class. Then it calls the methods normally.

Ruby

class Tracker
  def self.method_added(method_name)
    puts "Method added: #{method_name}"
  end

  def hello
    puts 'Hi there!'
  end

  def bye
    puts 'Goodbye!'
  end
end

Tracker.new.hello
Tracker.new.bye

Important Notes

The method_added hook only works for instance methods, not class methods.

Defining methods inside method_added can cause infinite recursion, so use a guard variable if needed.

Summary

method_added runs automatically when a new instance method is defined.

It helps track or change methods as they appear in a class.

Use it carefully to avoid infinite loops.

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