Trace method calls on objects

debug_proxy.rb

# ruby -rdebug_proxy -e "DebugProxy.new('foo', :string).gsub!(/o/, 'a')" 
# 
# DebugProxy (string) -------------------------
# |                                           |
# | Method called on proxy object:            |
# |   gsub!(/o/, "a")                         |
# |                                           |
# | Stack trace of method call:               |
# |   -e:1                                    |
# |                                           |
# | Value of proxy object before method call: |
# |   "foo"                                   |
# |                                           |
# | Value of proxy object after method call:  |
# |   "faa"                                   |
# |                                           |
# | Proxy was created at:                     |
# |   -e:1                                    |
# |                                           |
# ---------------------------------------------
class DebugProxy
  class Printer
    STDOUT = $stdout

    attr_reader :stream, :width, :maxlen, :title, :topbottom, :lpadding, :rpadding

    def self.print(stream, title, *lines)
      maxlen = 0
      lines = lines.flatten
      lines.each { |line| maxlen = [maxlen, line.size].max }

      new(stream, maxlen) do |printer|
        printer.header title
        lines.each { |line| printer.line line }
        printer.footer
      end
    end

    def initialize(stream, maxlen, topbottom="-", lpadding="| ", rpadding=" |")
      @stream, @maxlen, @width, @topbottom, @lpadding, @rpadding = stream, maxlen, maxlen + lpadding.size + rpadding.size, topbottom, lpadding, rpadding
      yield self if block_given?
    end

    def header(title)
      stream.puts
      stream.puts pad("#{title} ", width, topbottom)
    end

    def footer
      stream.puts pad('', width, topbottom)
    end

    def line(line)
      stream.puts lpadding + pad(crop(line, maxlen), maxlen) + rpadding
    end

    def self.undersized?(line, width)
      line.size < width
    end

    def undersized?(*args)
      self.class.undersized?(*args)
    end

    def self.oversized?(line, width)
      line.size > width
    end

    def oversized?(*args)
      self.class.oversized?(*args)
    end

    def self.pad(line, width, tail=' ')
      return line if line.size >= width
      line = line + tail * ((width - line.size) / tail.size)
      line + tail[0, width - line.size]
    end

    def pad(*args)
      self.class.pad(*args)
    end

    def self.crop(line, width, tail='...')
      return line if line.size <= width
      line[0, width-tail.size]+tail
    end

    def crop(*args)
      self.class.crop(*args)
    end
  end

  instance_methods.each { |m| undef_method m unless m =~ %r{^__|caller} }

  private
    def initialize(target, name=nil, &inspector)
      @name       = name
      @target     = target
      @inspector  = inspector || proc { |target| target.inspect }
      @birthplace = caller[1]
    end

    def method_missing(name, *args, &block)
      before = @inspector.call(@target)
      begin
        return @target.__send__(name, *args, &block)
      ensure
        after  = @inspector.call(@target)

        Printer.print(Printer::STDOUT, "DebugProxy#{" (#{@name})" if @name}",
          "",
          "Method called on proxy object:",
          "  #{name}(#{args.map {|a| a.inspect}.join(', ')}#{"#{", " unless args.empty?}&#{block.inspect}" if block})",
          "",
          "Stack trace of method call:",
          caller.map {|line| "  #{line}"},
          "",
          "Value of proxy object before method call:",
          before.gsub(/^/, '  '),
          "",
          "Value of proxy object after method call:",
          after.gsub(/^/, '  '),
          "",
          "Proxy was created at:",
          "  #{@birthplace}",
          ""
        )
      end
    end
end