最后找到了如何做到这一点。使用write_nonblock和readpartial。你必须注意的是，stdout.readpartial完全按照它所说的那样做，这意味着你将不得不聚集数据并通过查找换行符来执行gets。

require 'open3'
env = {"FOO"=>"BAR", "BAZ"=>nil}
options = {}
Open3.popen3(env, "cat", **options) {|stdin, stdout, stderr, wait_thr|
    stdin.write_nonblock("hello")

    puts stdout.readpartial(4096)
    # the magic 4096 is just a size of memory from this example:
    # https://apidock.com/ruby/IO/readpartial

    stdin.close
    stdout.close
    stderr.close
    wait_thr.join
}

对于寻求更通用的交互性（例如ssh交互）的用户，您可能希望创建单独的线程来聚合stdout和触发stdin。

require 'open3'
env = {"FOO"=>"BAR", "BAZ"=>nil}
options = {}
unprocessed_output = ""
Open3.popen3(env, "cat", **options) {|stdin, stdout, stderr, wait_thr|

    on_newline = ->(new_line) do
        puts "process said: #{new_line}"
        # close after a particular line
        stdin.close
        stdout.close
        stderr.close
    end

    Thread.new do
        while not stdout.closed? # FYI this check is probably close to useless/bad
            unprocessed_output += stdout.readpartial(4096)
            if unprocessed_output =~ /(.+)\n/
                # extract the line
                new_line = $1
                # remove the line from unprocessed_output
                unprocessed_output.sub!(/(.+)\n/,"")
                # run the on_newline
                on_newline[new_line]
            end

            # in theres no newline, this process will hang forever
            # (e.g. probably want to add a timeout)
        end
    end

    stdin.write_nonblock("hello\n")

    wait_thr.join
}

顺便说一句，这不是非常线程安全的。这只是一个未优化的，但功能的解决方案，我发现，希望在未来得到改善。

我尝试了一下@jeff-hykin的答案。所以，主要的一点是以非阻塞模式发送来自sum.rb的数据，即使用STDOUT.write_nonblock：

# sum.rb
STDOUT.write_nonblock "Enter number A\n"
a = gets.chomp
STDOUT.write_nonblock "Enter number B\n"
b = gets.chomp
STDOUT.write_nonblock "sum is #{a.to_i + b.to_i}"

--注意STDOUT.write_nonblock调用中的\n。它分隔字符串/行，这些字符串/行是用robot.rb中的gets“get string”读取的。然后robot.rb可以保持不变。我只会在条件中添加strip：

# robot.rb
require 'open3'

Open3.popen3('ruby sum.rb') do |stdin, stdout, stderr, wait_thr|
  while line = stdout.gets
    puts "line: #{line}" # for debugging
    if line.strip == "Enter number A"
      stdin.write("10\n")
    elsif line.strip == "Enter number B"
      stdin.write("30\n")
    else
      puts line
    end
  end
end

Ruby版本是3.0.2。