class Rack::Builder

# File lib/rack/builder.rb, line 136
def self.app(default_app = nil, &block)
  self.new(default_app, &block).to_app
end

Create a new Rack::Builder instance and return the Rack application generated from it.

# File lib/rack/builder.rb, line 87
def self.load_file(path, **options)
  config = ::File.read(path)
  config.slice!(/\A#{UTF_8_BOM}/) if config.encoding == Encoding::UTF_8

  if config[/^#\\(.*)/]
    fail "Parsing options from the first comment line is no longer supported: #{path}"
  end

  config.sub!(/^__END__\n.*\Z/m, '')

  return new_from_string(config, path, **options)
end

Load the given file as a rackup file, treating the contents as if specified inside a Rack::Builder block.

Ignores content in the file after __END__, so that use of __END__ will not result in a syntax error.

Example config.ru file:

$ cat config.ru

use Rack::ContentLength
require './app.rb'
run App

# File lib/rack/builder.rb, line 116
def initialize(default_app = nil, **options, &block)
  @use = []
  @map = nil
  @run = default_app
  @warmup = nil
  @freeze_app = false
  @options = options

  instance_eval(&block) if block_given?
end

Initialize a new Rack::Builder instance. default_app specifies the default application if run is not called later. If a block is given, it is evaluated in the context of the instance.

# File lib/rack/builder.rb, line 102
def self.new_from_string(builder_script, path = "(rackup)", **options)
  builder = self.new(**options)

  # We want to build a variant of TOPLEVEL_BINDING with self as a Rack::Builder instance.
  # We cannot use instance_eval(String) as that would resolve constants differently.
  binding = BUILDER_TOPLEVEL_BINDING.call(builder)
  eval(builder_script, binding, path)

  return builder.to_app
end

Evaluate the given builder_script string in the context of a Rack::Builder block, returning a Rack application.

# File lib/rack/builder.rb, line 65
def self.parse_file(path, **options)
  if path.end_with?('.ru')
    return self.load_file(path, **options)
  else
    require path
    return Object.const_get(::File.basename(path, '.rb').split('_').map(&:capitalize).join(''))
  end
end

Parse the given config file to get a Rack application.

If the config file ends in .ru, it is treated as a rackup file and the contents will be treated as if specified inside a Rack::Builder block.

If the config file does not end in .ru, it is required and Rack will use the basename of the file to guess which constant will be the Rack application to run.

Examples:

Rack::Builder.parse_file('config.ru')
# Rack application built using Rack::Builder.new

Rack::Builder.parse_file('app.rb')
# requires app.rb, which can be anywhere in Ruby's
# load path. After requiring, assumes App constant
# is a Rack application

Rack::Builder.parse_file('./my_app.rb')
# requires ./my_app.rb, which should be in the
# process's current directory.  After requiring,
# assumes MyApp constant is a Rack application

# File lib/rack/builder.rb, line 276
def call(env)
  to_app.call(env)
end

Call the Rack application generated by this builder instance. Note that this rebuilds the Rack application and runs the warmup code (if any) every time it is called, so it should not be used if performance is important.

# File lib/rack/builder.rb, line 259
def freeze_app
  @freeze_app = true
end

Freeze the app (set using run) and all middleware instances when building the application in to_app.

# File lib/rack/builder.rb, line 252
def map(path, &block)
  @map ||= {}
  @map[path] = block
end

Creates a route within the application. Routes under the mapped path will be sent to the Rack application specified by run inside the block. Other requests will be sent to the default application specified by run outside the block.

class App
  def call(env)
    [200, {'content-type' => 'text/plain'}, ["Hello World"]]
  end
end

class Heartbeat
  def call(env)
    [200, { "content-type" => "text/plain" }, ["OK"]]
  end
end

app = Rack::Builder.app do
  map '/heartbeat' do
    run Heartbeat.new
  end
  run App.new
end

run app

The use method can also be used inside the block to specify middleware to run under a specific path:

app = Rack::Builder.app do
  map '/heartbeat' do
    use Middleware
    run Heartbeat.new
  end
  run App.new
end

This example includes a piece of middleware which will run before /heartbeat requests hit Heartbeat.

Note that providing a path of / will ignore any default application given in a run statement outside the block.

# File lib/rack/builder.rb, line 193
def run(app = nil, &block)
  raise ArgumentError, "Both app and block given!" if app && block_given?

  @run = app || block
end

Takes a block or argument that is an object that responds to call and returns a Rack response.

You can use a block:

run do |env|
  [200, { "content-type" => "text/plain" }, ["Hello World!"]]
end

You can also provide a lambda:

run lambda { |env| [200, { "content-type" => "text/plain" }, ["OK"]] }

You can also provide a class instance:

class Heartbeat
  def call(env)
   [200, { "content-type" => "text/plain" }, ["OK"]]
  end
end

run Heartbeat.new

# File lib/rack/builder.rb, line 264
def to_app
  app = @map ? generate_map(@run, @map) : @run
  fail "missing run or map statement" unless app
  app.freeze if @freeze_app
  app = @use.reverse.inject(app) { |a, e| e[a].tap { |x| x.freeze if @freeze_app } }
  @warmup.call(app) if @warmup
  app
end

Return the Rack application generated by this instance.

# File lib/rack/builder.rb, line 159
def use(middleware, *args, &block)
  if @map
    mapping, @map = @map, nil
    @use << proc { |app| generate_map(app, mapping) }
  end
  @use << proc { |app| middleware.new(app, *args, &block) }
end

Specifies middleware to use in a stack.

class Middleware
  def initialize(app)
    @app = app
  end

  def call(env)
    env["rack.some_header"] = "setting an example"
    @app.call(env)
  end
end

use Middleware
run lambda { |env| [200, { "content-type" => "text/plain" }, ["OK"]] }

All requests through to this application will first be processed by the middleware class. The call method in this example sets an additional environment key which then can be referenced in the application if required.

# File lib/rack/builder.rb, line 209
def warmup(prc = nil, &block)
  @warmup = prc || block
end

Takes a lambda or block that is used to warm-up the application. This block is called before the Rack application is returned by to_app.

warmup do |app|
  client = Rack::MockRequest.new(app)
  client.get('/')
end

use SomeMiddleware
run MyApp