Trigger updates when the todo.txt file changes

This is applicable for prompt mode and column mode. Changes are
immediately read whenever todo.txt is changed.

Current issues:

* Writes caused by the current topydo instance also trigger the file
  update: needless overhead.
* Column mode: items are not immediately redrawn on update

This mostly addresses issue #142.

topydo/lib/TodoFile.py

@@ -19,6 +19,7 @@ This module deals with todo.txt files.
 """
 
 import codecs
+import os.path
 
 
 class TodoFile(object):
@@ -27,8 +28,33 @@ class TodoFile(object):
     to.
     """
 
-    def __init__(self, p_path):
-        self.path = p_path
+    def __init__(self, p_path, p_on_update=None):
+        self.path = os.path.abspath(p_path)
+
+        if p_on_update:
+            from watchdog.observers import Observer
+            from watchdog.events import FileSystemEventHandler, FileModifiedEvent, FileCreatedEvent
+
+            class EventHandler(FileSystemEventHandler):
+                def _handle(_, p_event):
+                    right_type = isinstance(p_event, FileCreatedEvent) or isinstance(p_event, FileModifiedEvent)
+
+                    if right_type and p_event.src_path == self.path:
+                        p_on_update()
+
+                def on_created(self, p_event):
+                    """
+                    Because vim deletes and creates a file on buffer save, also
+                    catch a creation event.
+                    """
+                    self._handle(p_event)
+
+                def on_modified(self, p_event):
+                    self._handle(p_event)
+
+            observer = Observer()
+            observer.schedule(EventHandler(), os.path.dirname(self.path))
+            observer.start()
 
     def read(self):
         """ Reads the todo.txt file and returns a list of todo items. """

topydo/ui/columns/Main.py

@@ -108,8 +108,13 @@ class UIApplication(CLIApplicationBase):
             if opt == "-l":
                 self.alt_layout_path = value
 
+        def callback():
+            self.todolist.erase()
+            self.todolist.add_list(self.todofile.read())
+            self._update_all_columns()
+
         self.column_width = config().column_width()
-        self.todofile = TodoFile.TodoFile(config().todotxt())
+        self.todofile = TodoFile.TodoFile(config().todotxt(), callback)
         self.todolist = TodoList.TodoList(self.todofile.read())
 
         self.marked_todos = []

topydo/ui/prompt/Prompt.py

@@ -41,17 +41,6 @@ from topydo.lib import TodoFile
 from topydo.lib import TodoList
 
 
-def _todotxt_mtime():
-    """
-    Returns the mtime for the configured todo.txt file.
-    """
-    try:
-        return os.path.getmtime(config().todotxt())
-    except os.error:
-        # file not found
-        return None
-
-
 class PromptApplication(CLIApplicationBase):
     """
     This class implements a variant of topydo's CLI showing a shell and
@@ -62,33 +51,25 @@ class PromptApplication(CLIApplicationBase):
         super().__init__()
 
         self._process_flags()
-        self.mtime = None
         self.completer = None
+        self.todofile = TodoFile.TodoFile(config().todotxt(), self._load_file)
 
     def _load_file(self):
         """
         Reads the configured todo.txt file and loads it into the todo list
         instance.
-
-        If the modification time of the todo.txt file is equal to the last time
-        it was checked, nothing will be done.
         """
-        current_mtime = _todotxt_mtime()
-
-        if not self.todofile or self.mtime != current_mtime:
-            self.todofile = TodoFile.TodoFile(config().todotxt())
-            self.todolist = TodoList.TodoList(self.todofile.read())
-            self.mtime = current_mtime
-
-            self.completer = TopydoCompleter(self.todolist)
+        self.todolist.erase()
+        self.todolist.add_list(self.todofile.read())
+        self.completer = TopydoCompleter(self.todolist)
 
     def run(self):
         """ Main entry function. """
         history = InMemoryHistory()
+        self._load_file()
 
         while True:
             # (re)load the todo.txt file (only if it has been modified)
-            self._load_file()
 
             try:
                 user_input = prompt(u'topydo> ', history=history,
@@ -103,20 +84,12 @@ class PromptApplication(CLIApplicationBase):
                 error('Error: ' + str(verr))
                 continue
 
-            mtime_after = _todotxt_mtime()
-
             try:
                 (subcommand, args) = get_subcommand(user_input)
             except ConfigError as ce:
                 error('Error: ' + str(ce) + '. Check your aliases configuration')
                 continue
 
-            # refuse to perform operations such as 'del' and 'do' if the
-            # todo.txt file has been changed in the background.
-            if subcommand and not self.is_read_only(subcommand) and self.mtime != mtime_after:
-                error("WARNING: todo.txt file was modified by another application.\nTo prevent unintended changes, this operation was not executed.")
-                continue
-
             try:
                 if self._execute(subcommand, args) != False:
                     self._post_execute()