# class IRB::Pager

The implementation of this class is borrowed from RDoc’s lib/rdoc/ri/driver.rb. Please do NOT use this class directly outside of `IRB`.

### Constants

PAGE_COMMANDS

### Public Class Methods

page(retain_content: false) { |pager| ... } click to toggle source

    # File lib/irb/pager.rb, line 20
    def page(retain_content: false)
      if should_page? && pager = setup_pager(retain_content: retain_content)
        begin
          pid = pager.pid
          yield pager
        ensure
          pager.close
        end
      else
        yield $stdout
      end
    # When user presses Ctrl-C, IRB would raise `IRB::Abort`
    # But since Pager is implemented by running paging commands like `less` in another process with `IO.popen`,
    # the `IRB::Abort` exception only interrupts IRB's execution but doesn't affect the pager
    # So to properly terminate the pager with Ctrl-C, we need to catch `IRB::Abort` and kill the pager process
    rescue IRB::Abort
      Process.kill("TERM", pid) if pid
      nil
    rescue Errno::EPIPE
    end

page_content(content, **options) click to toggle source

    # File lib/irb/pager.rb, line 10
    def page_content(content, **options)
      if content_exceeds_screen_height?(content)
        page(**options) do |io|
          io.puts content
        end
      else
        $stdout.puts content
      end
    end

### Private Class Methods

content_exceeds_screen_height?(content) click to toggle source

    # File lib/irb/pager.rb, line 47
    def content_exceeds_screen_height?(content)
      screen_height, screen_width = begin
        Reline.get_screen_size
      rescue Errno::EINVAL
        [24, 80]
      end

      pageable_height = screen_height - 3 # leave some space for previous and the current prompt

      # If the content has more lines than the pageable height
      content.lines.count > pageable_height ||
        # Or if the content is a few long lines
        pageable_height * screen_width < Reline::Unicode.calculate_width(content, true)
    end

setup_pager(retain_content:) click to toggle source

    # File lib/irb/pager.rb, line 62
    def setup_pager(retain_content:)
      require 'shellwords'

      PAGE_COMMANDS.each do |pager_cmd|
        cmd = Shellwords.split(pager_cmd)
        next if cmd.empty?

        if cmd.first == 'less'
          cmd << '-R' unless cmd.include?('-R')
          cmd << '-X' if retain_content && !cmd.include?('-X')
        end

        begin
          io = IO.popen(cmd, 'w')
        rescue
          next
        end

        if $? && $?.pid == io.pid && $?.exited? # pager didn't work
          next
        end

        return io
      end

      nil
    end

should_page?() click to toggle source

    # File lib/irb/pager.rb, line 43
    def should_page?
      IRB.conf[:USE_PAGER] && STDIN.tty? && (ENV.key?("TERM") && ENV["TERM"] != "dumb")
    end