feat(command): open external editor command in composing message

OPEN_EXTERNAL_EDITOR command paste the composing message in a python tempfile,
run external editor over it and wait the exit before update the message.

$ZULIP_EDITOR_COMMAND and fallback $EDITOR are use for the external editor
command.

docs/FAQ.md

@@ -428,6 +428,31 @@ If any of the hotkeys don't work for you, feel free to open an issue or discuss
 it on
 [#zulip-terminal](https://chat.zulip.org/#narrow/stream/206-zulip-terminal).
 
+## Can I compose messages in another editor ?
+
+In the main branch of zulip-terminal, you can now use an external editor to 
+compose your message using `ctrl o` shortcut. If `ZULIP_EDITOR_COMMAND` or 
+`EDITOR` environment variable is set, this command or program would be used 
+to open the message by appending a temporary file filepath of the current message.
+
+It will work directly for most terminal editors with only the program name `vim`, 
+`nano`, `helix`, `kakoune`, `nvim`...
+
+It can also be used for desktop editor with some constraint which needs to be 
+address using `ZULIP_EDITOR_COMMAND` custom command. The program must not fork 
+or detach from the running terminal and should open in a new window, some 
+examples:
+
+- [lapce](https://github.com/lapce/lapce) with `lapce -n -w`
+- [sublime-text](https://www.sublimetext.com/) with `subl -n -w`
+- [marker](https://github.com/fabiocolacio/Marker) with `marker`
+- [vim](https://github.com/vim/vim) with `vim -g -f` or `gvim -f`
+- [vscode](https://github.com/microsoft/vscode) with `code -n -w`
+
+When the external editor process ends (closing the window or quitting terminal 
+editor), the composing box will be updated with the new message content from 
+the temporary file.
+
 ## Zulip-term crashed!
 
 We hope this doesn't happen, but would love to hear about this in order to fix

docs/hotkeys.md

@@ -81,6 +81,7 @@
 |Autocomplete @mentions, #stream_names, :emoji: and topics|<kbd>ctrl</kbd> + <kbd>f</kbd>|
 |Cycle through autocomplete suggestions in reverse|<kbd>ctrl</kbd> + <kbd>r</kbd>|
 |Narrow to compose box message recipient|<kbd>meta</kbd> + <kbd>.</kbd>|
+|Open the message in external editor|<kbd>ctrl</kbd> + <kbd>o</kbd>|
 |Jump to the beginning of line|<kbd>ctrl</kbd> + <kbd>a</kbd>|
 |Jump to the end of line|<kbd>ctrl</kbd> + <kbd>e</kbd>|
 |Jump backward one word|<kbd>meta</kbd> + <kbd>b</kbd>|

zulipterminal/config/keys.py

@@ -310,6 +310,11 @@ class KeyBinding(TypedDict):
         'help_text': 'View user information (From Users list)',
         'key_category': 'general',
     },
+    'OPEN_EXTERNAL_EDITOR': {
+        'keys': ['ctrl o'],
+        'help_text': 'Open the message in external editor',
+        'key_category': 'msg_compose',
+    },
     'BEGINNING_OF_LINE': {
         'keys': ['ctrl a'],
         'help_text': 'Jump to the beginning of line',

zulipterminal/ui_tools/boxes.py

@@ -2,10 +2,14 @@
 UI boxes for entering text: WriteBox, MessageSearchBox, PanelSearchBox
 """
 
+import os
 import re
+import shutil
+import subprocess
 import unicodedata
 from collections import Counter
 from datetime import datetime, timedelta
+from tempfile import NamedTemporaryFile
 from time import sleep
 from typing import Any, Callable, Dict, List, NamedTuple, Optional, Tuple
 
@@ -808,6 +812,32 @@ def keypress(self, size: urwid_Size, key: str) -> Optional[str]:
         elif is_command_key("MARKDOWN_HELP", key):
             self.view.controller.show_markdown_help()
             return key
+        elif is_command_key("OPEN_EXTERNAL_EDITOR", key):
+            editor = os.environ.get("ZULIP_EDITOR_COMMAND", os.environ.get("EDITOR"))
+            if editor is None:
+                self.view.controller.report_error(
+                    "Configure $EDITOR or $ZULIP_EDITOR_COMMAND shell environment."
+                )
+                return key
+            editor_splits = editor.split(" ")
+            fullpath_program = shutil.which(editor_splits[0])
+            if fullpath_program is None:
+                self.view.controller.report_error(
+                    "Editor program not found, check $EDITOR "
+                    "or $ZULIP_EDITOR_COMMAND."
+                )
+                return key
+            editor_splits[0] = fullpath_program
+            with NamedTemporaryFile(suffix=".md") as edit_tempfile:
+                with open(edit_tempfile.name, mode="w") as edit_writer:
+                    edit_writer.write(self.msg_write_box.edit_text)
+                self.view.controller.loop.screen.stop()
+                editor_splits.append(edit_tempfile.name)
+                subprocess.call(editor_splits)
+                with open(edit_tempfile.name, mode="r") as edit_reader:
+                    self.msg_write_box.edit_text = edit_reader.read().rstrip()
+            self.view.controller.loop.screen.start()
+            return key
         elif is_command_key("SAVE_AS_DRAFT", key):
             if self.msg_edit_state is None:
                 if self.compose_box_status == "open_with_private":