Ruby’s inherited Hook: The Timing Problem and a TracePoint Solution

When building Ruby frameworks or libraries that need to inspect subclasses, you’ll likely encounter the inherited hook. This powerful callback allows parent classes to respond when they’re subclassed. However, there’s a subtle timing issue that can trip up even experienced Ruby developers: the inherited hook fires before the subclass is fully defined.

The Problem: Early Execution of inherited

Let’s say you’re building a framework where you want to automatically register all methods defined in subclasses. Your first instinct might be to use the inherited hook:

class BaseModel
  @@registered_methods = {}
  
  def self.inherited(subclass)
    puts "Registering methods for #{subclass}..."
    @@registered_methods[subclass] = subclass.instance_methods(false)
    puts "Found methods: #{@@registered_methods[subclass]}"
  end
end

class User < BaseModel
  def name
    @name
  end
  
  def email
    @email
  end
end

Running this code produces surprising output:

Registering methods for User...
Found methods: []

The methods array is empty! But why?

Understanding the Timing Issue

The inherited hook is called immediately when Ruby encounters the class declaration line (class User < BaseModel), but before any of the method definitions in the class body are processed. Here’s the execution order:

  1. Ruby sees class User < BaseModel
  2. inherited(User) is called immediately
  3. The class body is processed (method definitions, etc.)
  4. Class definition completes

This timing makes inherited perfect for setting up class-level configuration or establishing inheritance hierarchies, but useless for inspecting the fully-defined subclass.

Let’s prove this with a more detailed example:

class BaseModel
  def self.inherited(subclass)
    puts "inherited called for #{subclass}"
    puts "Methods at this point: #{subclass.instance_methods(false)}"
  end
end

class User < BaseModel
  puts "About to define name method"
  
  def name
    @name
  end
  
  puts "About to define email method"
  
  def email
    @email
  end
  
  puts "Finished defining methods"
end

puts "Final methods in User: #{User.instance_methods(false)}"

Output:

inherited called for User
Methods at this point: []
About to define name method
About to define email method
Finished defining methods
Final methods in User: [:name, :email]

The inherited hook fires before any method definitions are processed.

Alternative Approaches (And Why They Fall Short)

Method 1: method_added Hook

You might try using method_added to track each method as it’s defined:

class BaseModel
  def self.inherited(subclass)
    subclass.define_singleton_method(:method_added) do |method_name|
      puts "Method #{method_name} added to #{self}"
    end
  end
end

This works for tracking individual methods, but it needs to run several times and doesn’t give you a clean “class is complete” callback. You’d need additional logic to determine when all methods have been defined.

Method 2: Manual Registration

You could require subclasses to manually call a registration method:

class BaseModel
  def self.finalize!
    puts "#{self} finalized with methods: #{instance_methods(false)}"
  end
end

class User < BaseModel
  def name; @name; end
  def email; @email; end
  
  finalize! # Manual call required
end

This works but defeats the purpose of guaranteeing that all subclasses perform the registration, since it relies on the developer remembering to call the registration method in the new subclass.

The Solution: TracePoint to the Rescue

Ruby’s TracePoint API allows us to trace various execution events, including when class definitions end. We can combine this with the inherited hook to get the best of both worlds:

class BaseModel
  @@registered_methods = {}
  
  def self.inherited(subclass)
    # Set up a trace to detect when the subclass definition ends
    trace = TracePoint.new(:end) do |trace_point|
      # Check if we're ending the definition of our subclass
      if trace_point.self == subclass
        puts "Class #{subclass} finished loading!"
        
        # Now we can safely inspect the fully-defined class
        methods = subclass.instance_methods(false)
        @@registered_methods[subclass] = methods
        puts "Registered methods: #{methods}"
        
        # Clean up - disable the trace since we only need it once
        trace.disable
      end
    end
    
    # Enable the trace
    trace.enable
  end
  
  def self.registered_methods
    @@registered_methods
  end
end

class User < BaseModel
  def name
    @name
  end
  
  def email
    @email
  end
  
  def age
    @age
  end
end

class Product < BaseModel
  def title
    @title
  end
  
  def price
    @price
  end
end

Output:

Class User finished loading!
Registered methods: [:name, :email, :age]
Class Product finished loading!
Registered methods: [:title, :price]

Perfect! Now we can inspect the fully-defined subclasses.

How the TracePoint Solution Works

  1. Inherited hook fires: When class User < BaseModel is encountered, inherited(User) is called
  2. TracePoint setup: We create a TracePoint that listens for :end events (when end keywords are processed)
  3. Class body processing: Ruby processes the method definitions inside the class
  4. End event fires: When Ruby hits the final end of the class definition, our TracePoint callback executes
  5. Safe inspection: At this point, the class is fully defined and we can safely inspect its methods
  6. Cleanup: We disable the TracePoint since we only need it once per class

TracePoint Events

The :end event fires whenever Ruby processes an end keyword, which includes:

  • End of class definitions
  • End of method definitions
  • End of module definitions
  • End of blocks

That’s why we check trace_point.self == subclass to ensure we’re responding to the end of the specific class we care about.

Considerations and Limitations

Performance

TracePoint has some performance overhead since it hooks into Ruby’s execution. For production systems, consider:

  • Only enabling traces when needed
  • Disabling traces promptly after use
  • Measuring impact in performance-critical applications

Complexity

This solution is more complex than simple hooks. Document it well and consider whether the automatic behavior is worth the added complexity.

Alternative Events

Depending on your needs, you might want to trace different events:

  • :class - when a class is opened
  • :call - when methods are called
  • :return - when methods return

Conclusion

Ruby’s inherited hook is powerful but has a crucial timing limitation: it fires before the subclass is fully defined. When you need to inspect or work with fully-defined subclasses, combining inherited with TracePoint’s :end event provides an elegant solution.

This pattern gives you the automatic behavior of inheritance hooks while ensuring you have access to the complete class definition. Just remember to balance the convenience against the added complexity and performance considerations.

The next time you’re building a Ruby framework and find yourself frustrated that inherited doesn’t see your subclass methods, reach for TracePoint – it might be exactly what you need.

[This article was originally drafted by Claude (Anthropic) AI and subsequently edited and refined.]