Tracer

The tracer gem provides helpful tracing utilities to help users observe their program's runtime behaviour.

The currently supported tracers are:

ObjectTracer
IvarTracer
CallTracer
ExceptionTracer
LineTracer

It also comes with experimental IRB integration to allow quick access from REPL.

Installation

$ bundle add tracer --group=development,test

Or directly add it to your Gemfile

group :development, :test do
  gem "tracer"
end

If bundler is not being used to manage dependencies, install the gem by executing:

$ gem install tracer

Usage

Tracer.trace(object) { ... } # trace object's activities in the given block
Tracer.trace_call { ... } # trace method calls in the given block
Tracer.trace_exception { ... } # trace exceptions in the given block

Example

require "tracer"

obj = Object.new

def obj.foo
  100
end

def bar(obj)
  obj.foo
end

Tracer.trace(obj) { bar(obj) }
 #depth:1  #<Object:0x000000010903c190> is used as a parameter obj of Object#bar at test.rb:13:in `block in <main>'
 #depth:2  #<Object:0x000000010903c190> receives .foo at test.rb:10:in `bar'

`tracer/helper`

If you want to avoid the Tracer namespace, you can do require "tracer/helper" instead:

require "tracer/helper"

trace(object) { ... } # trace object's activities in the given block
trace_call { ... } # trace method calls in the given block
trace_exception { ... } # trace exceptions in the given block

Tracer Classes

If you want to have more control over individual traces, you can use individual tracer classes:

ObjectTracer

class User
  def initialize(name) = (@name = name)

  def name() = @name
end

def authorized?(user)
  user.name == "John"
end

user = User.new("John")
tracer = ObjectTracer.new(user)
tracer.start do
  user.name
  authorized?(user)
end

 #depth:3  #<User:0x000000010696cad8 @name="John"> receives #name (User#name) at test.rb:14:in `block in <main>'
 #depth:3  #<User:0x000000010696cad8 @name="John"> is used as a parameter user of Object#authorized? at test.rb:15:in `block in <main>'
 #depth:4  #<User:0x000000010696cad8 @name="John"> receives #name (User#name) at test.rb:8:in `authorized?'

IvarTracer

Note Ruby 3.0 and below's accessor calls don't trigger TracePoint properly so the result may be inaccurate with those versions.

require "tracer"

class Cat
  attr_accessor :name
end

cat = Cat.new

tracer = IvarTracer.new(cat, :@name)
tracer.start do
  cat.name = "Kitty"
  cat.instance_variable_set(:@name, "Ketty")
end

#depth:3 Cat#name= sets @name = "Kitty" at test.rb:11
#depth:3 Kernel#instance_variable_set sets @name = "Ketty" at test.rb:12

ExceptionTracer

ExceptionTracer.new.start

begin
  raise "boom"
rescue StandardError
  nil
end

#depth:0  #<RuntimeError: boom> raised at test.rb:4
#depth:1  #<RuntimeError: boom> rescued at test.rb:6

CallTracer

class User
  def initialize(name) = (@name = name)

  def name() = @name
end

def authorized?(user)
  user.name == "John"
end

user = User.new("John")
tracer = CallTracer.new
tracer.start do
  user.name
  authorized?(user)
end

 #depth:4 >    block at test.rb:13
 #depth:5 >     User#name at test.rb:4
 #depth:5 <     User#name #=> "John" at test.rb:4
 #depth:5 >     Object#authorized? at test.rb:7
 #depth:6 >      User#name at test.rb:4
 #depth:6 <      User#name #=> "John" at test.rb:4
 #depth:6 >      String#== at test.rb:8
 #depth:6 <      String#== #=> true at test.rb:8
 #depth:5 <     Object#authorized? #=> true at test.rb:9
 #depth:4 <    block #=> true at test.rb:16

LineTracer

class User
  def initialize(name) = (@name = name)

  def name() = @name
end

def authorized?(user)
  user.name == "John"
end

user = User.new("John")
tracer = LineTracer.new
tracer.start do
  user.name
  authorized?(user)
end

 #depth:4  at test.rb:14
 #depth:4  at test.rb:15
 #depth:5  at test.rb:8

IRB-integration

Once required, tracer registers a few IRB commands to help you trace Ruby expressions:

trace              Trace the target object (or self) in the given expression. Usage: `trace [target,] <expression>`
trace_call         Trace method calls in the given expression. Usage: `trace_call <expression>`
trace_exception    Trace exceptions in the given expression. Usage: `trace_exception <expression>`

Example

# test.rb
require "tracer"

obj = Object.new

def obj.foo
  100
end

def bar(obj)
  obj.foo
end

binding.irb

irb(main):001:0> trace obj, bar(obj)
 #depth:23 #<Object:0x0000000107a86648> is used as a parameter obj of Object#bar at (eval):1:in `<main>'
 #depth:24 #<Object:0x0000000107a86648> receives .foo at test.rb:10:in `bar'
=> 100
irb(main):002:0> trace_call bar(obj)
 #depth:23>                            Object#bar at (eval):1:in `<main>'
 #depth:24>                             #<Object:0x0000000107a86648>.foo at test.rb:10:in `bar'
 #depth:24<                             #<Object:0x0000000107a86648>.foo #=> 100 at test.rb:10:in `bar'
 #depth:23<                            Object#bar #=> 100 at (eval):1:in `<main>'
=> 100

Customization

TBD

Acknowledgements

A big shout-out to @ko1 (Koichi Sasada) for his awesome work on ruby/debug. The cool tracers in ruby/debug were an inspiration and laid the groundwork for this project.

Development

After checking out the repo, run bin/setup to install dependencies. Then, run rake test-unit to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and the created tag, and push the .gem file to rubygems.org.

Contributing

Bug reports and pull requests are welcome on GitHub at https://github.com/ruby/tracer. This project is intended to be a safe, welcoming space for collaboration, and contributors are expected to adhere to the code of conduct.

License

The gem is available as open source under the terms of the 2-Clause BSD License.

Code of Conduct

Everyone interacting in the Ruby::Tracer project's codebases, issue trackers, chat rooms and mailing lists is expected to follow the code of conduct.