buffered_json_array.rb

class BufferedJsonArray
  ClosedError = Class.new(StandardError)

  def initialize(target_buffer)
    @target_buffer = target_buffer
    @internal_buffer = "["
    @closed = false
    @first_entry = true
  end

  def <<(value)
    append "," unless first_entry?
    append value.to_json
    flush
  end

  def close
    append "]"
    flush
    closed!
  end

  private

  def append(string)
    fail ClosedError, "Buffer has already been closed" if closed?

    @first_entry = false
    @internal_buffer << string
  end

  def flush
    @target_buffer << @internal_buffer
    @internal_buffer = ""
  end

  def first_entry?
    @first_entry
  end

  def closed!
    @closed = true
  end

  def closed?
    @closed
  end
end

buffered_json_array_spec.rb

require 'rails_helper'

RSpec.describe BufferedJsonArray do
  let(:stream) { "" }
  let(:buffered) { BufferedJsonArray.new(stream) }

  it "can successively yields JSON in an array" do
    expect { buffered << 'A' }.to change { stream }.from('').to(%{["A"})
    expect { buffered << 'B' }.to change { stream }.to(%{["A","B"})
    expect { buffered << 'C' }.to change { stream }.to(%{["A","B","C"})
    expect { buffered.close }.to change { stream }.to(%{["A","B","C"]})
  end

  it "can yield a valid empty array" do
    buffered.close
    expect(stream).to eq "[]"
  end

  it "cannot be written after closing" do
    buffered.close
    expect { buffered << "value" }.to raise_error(BufferedJsonArray::ClosedError)
  end
end