Term::Reader

A pure Crystal library that provides a set of methods for processing keyboard input in character, line and multiline modes. It maintains history of entered input with an ability to recall and re-edit those inputs. It lets you register to listen for keystroke events and trigger custom key events yourself.

Term::Reader provides an independant reader component for the crystal-term toolkit.

Compatibility

Term::Reader is not compatible with GNU Readline and doesn't aim to be. It is based completely on tty-reader and shares a very similar API, but that could change as things progress.

Features

• Pure Crystal
• Reading single keypresses
• Line editing
• Reading multiline input
• Ability to listen for keystroke events
• Track input history
• No global state
• Cross platform

Installation

1. Add the dependency to your shard.yml:

   dependencies:
     term-reader:
       github: crystal-term/reader
   

2. Run shards install

Usage

In just a few lines you can create a simple REPL.

require "term-reader"

Initialize the reader:

reader = Term::Reader.new

Then register to listen for key events, in this case for Ctrl-x. (Escape doesn't work right currently.)

reader.on_key(:ctrl_x) do
  puts "Exiting..."
  exit(0)
end

Finally show the user a prompt, and keep looping until we exit:

loop do
  reader.read_line(prompt: "=> ")
end

API

read_keypress

To read a single stroke from the user, use read_keypress:

reader.read_keypress
reader.read_keypress(nonblock: true)

read_line

By default read_line works in raw mode, which means it behaves like a line editor that allows you to read each character and respond to control characters such as Ctrl-a and Ctrl-b, or navigate through history.

For example, to read a single line terminated by a newline character:

reader.read_line

If you wish for keystrokes to be interpreted by the terminal instead, use cooked mode by setting raw to false:

reader.read_line(raw: false)

Any non-interpreted characters received are written back to the terminal, however you can stop this by setting echo to false:

reader.read_line(echo: false)

You can also provide a line prefix by specifying the prompt parameter:

reader.read_line(prompt: "=> ")
# =>

To pre-populate the line content for editing, specify the value parameter:

reader.read_line(prompt: "=>", value: "edit me")
# => edit me

read_multiline

By default read_multiline works in raw mode which means it behaves like a multiline editor that allows you to edit each character, respond to control characters such as Ctrl-a and Ctrl-b or navigate through history.

For example, to read more than one line terminated by Ctrl+d or Ctrl+z use read_multiline:

reader.read_multiline
# => [ "line1", "line2", ... ]

If you wish for the keystrokes to be interpreted by the terminal instead, use so called cooked mode by setting the raw option to false:

reader.read_line(raw: false)

You can also provide a line prefix (prompt) displayed before input by passing a string as a first argument:

reader.read_multiline("=> ")

on_key

You can register to listen to keypress events. This can be done by calling on_key with the name of a key or keys:

reader.on_key(:ctrl_x) { |key, event| ... }

Alternately, if you provide no keys all keypress events will be listened to:

reader.on_key { |key, event| ... }

Two things are yielded to the block whenever a on_key event is fired. The name of the key that was pressed, as a string, and a KeyEvent object which includes the fields:

• key - the key that was pressed
• value - the value of the key pressed
• line - the content of the line currently being edited

The value returns the actual key pressed rather than its name.

The key is an object with the following fields:

• name - the name of the key
• meta - true if is non-standard key associated
• shift - true if shift key was pressed with this key
• ctrl - true if ctrl was pressed with this key

For example, to listen to vim navigation keys:

reader.on_key do |key, event|
  case event.value
  when "j"
    # ...
  when "k"
    # ...
  end
end

Listeners are chainable, so you can subscribe to more than one event at once:

prompt.on_key          { |key, event| ... }
      .on_key(:ctrl_x) { |key, event| ... }

Configuration

interrupt

By default the InputInterrupt exception will be rased when the user hits the interrupt key (ctrl-c). However, you can customize this behavior by passing the interrupt option. The available options are:

• :error - raises IntputInterrupt exception
• :signal - sends interrupt signal
• :exit - exits with a non-zero status code
• :noop - skips the handler
• Proc - a custom handler (not available yet)

For example, to send an interrupt signal do:

reader = Term::Reader.new(interrupt: :signal)

track_history

read_line and read_multiline provide a history buffer that tracks all lines entered during Reader interactions. The history buffer provides previous or next lines when the user presses the up and down arrows respectively. However, you can disable this feature by setting track_history to false.

reader = Term::Reader.new(track_history: false)

history_cycle

This option determines whether the history buffer allows for infinite navigation. By default it is set to false. You can change this:

reader = Term::Reader.new(history_cycle: true)

history_duplicates

This option controls whether duplicate lines are stored in history. By default set to true. You can change this:

reader = term::reader.new(history_duplicates: false)

history_exclude

This option allows you to exclude specific lines from the history (think passwords). It accepts a Proc which takes in the current line and must return true to exclude the line, and false to keep it.

reader = term::reader.new(history_exclude = ->(line : String) { ... })

Contributing

1. Fork it (https://github.com/crystal-term/reader/fork)
2. Create your feature branch (git checkout -b my-new-feature)
3. Run the specs and make sure they pass (crystal spec)
4. Commit your changes (git commit -am 'Add some feature')
5. Push to the branch (git push origin my-new-feature)
6. Create a new Pull Request

Contributors

• Chris Watson - creator and maintainer