Cache TodoWidgets to improve performance

Having hundreds of todo items slows down the column UI quite a lot when doing
an update. An update basically throws away all widgets and creates them from
scratch. That is quite a waste when only one item has changed.

Instead, keep all widgets in a cache, retrievable by the todo's raw text. When
the raw text changes, a new widget is constructed.

topydo/ui/columns/TodoListWidget.py

@@ -93,7 +93,8 @@ class TodoListWidget(urwid.LineBox):
         del self.todolist[:]
 
         for todo in self.view.todos:
-            todowidget = TodoWidget(todo, self.view.todolist.number(todo))
+            todowidget = TodoWidget.create(todo)
+            todowidget.number = self.view.todolist.number(todo)
             self.todolist.append(todowidget)
             self.todolist.append(urwid.Divider('-'))
 
@@ -105,6 +106,10 @@ class TodoListWidget(urwid.LineBox):
                 # -2 for the same reason as in self._scroll_to_bottom()
                 self.todolist.set_focus(len(self.todolist) - 2)
 
+        # after the update there might be old stuff left in the widget cache,
+        # clean it
+        TodoWidget.clean_cache(self.view.todolist)
+
     def _scroll_to_top(self, p_size):
         self.listbox.set_focus(0)
 

topydo/ui/columns/TodoWidget.py

@@ -53,7 +53,7 @@ def _markup(p_todo, p_focus):
 
 
 class TodoWidget(urwid.WidgetWrap):
-    def __init__(self, p_todo, p_number):
+    def __init__(self, p_todo):
         # clients use this to associate this widget with the given todo item
         self.todo = p_todo
 
@@ -83,21 +83,21 @@ class TodoWidget(urwid.WidgetWrap):
             else:
                 txt_markup.append(substring)
 
-        id_widget = urwid.Text(str(p_number), align='right')
+        self.id_widget = urwid.Text('', align='right')
         priority_widget = urwid.Text(priority_text)
         self.text_widget = urwid.Text(txt_markup)
 
         progress = to_urwid_color(progress_color(p_todo)) if config().colors() else PaletteItem.DEFAULT
-        progress_bar = urwid.AttrMap(
+        self.progress_bar = urwid.AttrMap(
                 urwid.SolidFill(' '),
-                urwid.AttrSpec(PaletteItem.DEFAULT, progress, 256),
-                urwid.AttrSpec(PaletteItem.DEFAULT, progress, 256),
+                {},
         )
+        self.update_progress()
 
         self.columns = urwid.Columns(
             [
-                (1, progress_bar),
-                (4, id_widget),
+                (1, self.progress_bar),
+                (4, self.id_widget),
                 (3, priority_widget),
                 ('weight', 1, self.text_widget),
             ],
@@ -124,6 +124,21 @@ class TodoWidget(urwid.WidgetWrap):
         # make sure that ListBox will highlight this widget
         return True
 
+    @property
+    def number(self):
+        pass
+
+    @number.setter
+    def number(self, p_number):
+        self.id_widget.set_text(str(p_number))
+
+    def update_progress(self):
+        color = to_urwid_color(progress_color(self.todo)) if config().colors() else PaletteItem.DEFAULT
+
+        self.progress_bar.set_attr_map(
+            {None: urwid.AttrSpec(PaletteItem.DEFAULT, color, 256)}
+        )
+
     def mark(self):
         attr_map = {
             None:                 PaletteItem.MARKED,
@@ -136,3 +151,49 @@ class TodoWidget(urwid.WidgetWrap):
 
     def unmark(self):
         self.widget.set_attr_map(_markup(self.todo, False))
+
+    cache = {}
+
+    @classmethod
+    def create(p_class, p_todo):
+        """
+        Creates a TodoWidget instance for the given todo. Widgets are
+        cached, the same object is returned for the same todo item.
+        """
+
+        def parent_progress_may_have_changed(p_todo):
+            """
+            Returns True when a todo's progress should be updated because it is
+            dependent on the parent's progress.
+            """
+            return p_todo.has_tag('p') and not p_todo.has_tag('due')
+
+        source = p_todo.source()
+
+        if source in p_class.cache:
+            widget = p_class.cache[source]
+
+            if parent_progress_may_have_changed(p_todo):
+                widget.update_progress()
+        else:
+            widget = p_class(p_todo)
+            p_class.cache[source] = widget
+
+        return widget
+
+    cache_clean_counter = 0
+
+    @classmethod
+    def clean_cache(p_class, p_todolist):
+        """ Cleans the widget cache once every 500 column updates. """
+
+        if p_class.cache_clean_counter % 500 != 0:
+            # wait
+            p_class.cache_clean_counter += 1
+            return
+
+        sources_in_list = set([todo.source() for todo in p_todolist])
+        sources_in_cache = set(p_class.cache.keys())
+
+        for stale in sources_in_cache - sources_in_list:
+            del p_class.cache[stale]