Implement Widget.render_delta_lines

The method `Widget.render_delta_lines` provides a mechanism for a widget to render itself and only issue strips that correspond to lines that actually changed.

src/textual/_compositor.py

@@ -899,12 +899,18 @@ class Compositor:
         return self._cuts
 
     def _get_renders(
-        self, crop: Region | None = None
+        self,
+        crop: Region | None = None,
+        allow_delta_updates: bool = False,
     ) -> Iterable[tuple[Region, Region, list[Strip]]]:
         """Get rendered widgets (lists of segments) in the composition.
 
         Args:
             crop: Region to crop to, or `None` for entire screen.
+            allow_delta_updates: Whether we can issue partial updates for widgets or
+                not. If `True`, widgets with `_enable_delta_updates` will produce only
+                updates for lines inside `crop` that have changed. If `False`, each
+                widget produces the full render for the region inside `crop`.
 
         Returns:
             An iterable of <region>, <clip region>, and <strips>
@@ -935,17 +941,16 @@ class Compositor:
 
         for widget, region, clip in widget_regions:
             if contains_region(clip, region):
-                yield region, clip, widget.render_lines(
-                    _Region(0, 0, region.width, region.height)
-                )
+                region_to_render = _Region(0, 0, region.width, region.height)
             else:
                 new_x, new_y, new_width, new_height = intersection(region, clip)
-                if new_width and new_height:
-                    yield region, clip, widget.render_lines(
-                        _Region(
-                            new_x - region.x, new_y - region.y, new_width, new_height
-                        )
-                    )
+                region_to_render = _Region(
+                    new_x - region.x, new_y - region.y, new_width, new_height
+                )
+            if allow_delta_updates and widget._enable_delta_updates:
+                yield from widget.render_delta_lines(region, clip, region_to_render)
+            else:
+                yield region, clip, widget.render_lines(region_to_render)
 
     def render_update(
         self, full: bool = False, screen_stack: list[Screen] | None = None
@@ -995,7 +1000,7 @@ class Compositor:
             is_rendered_line = {y for y, _, _ in spans}.__contains__
         else:
             return None
-        chops = self._render_chops(crop, is_rendered_line)
+        chops = self._render_chops(crop, is_rendered_line, allow_delta_updates=True)
         chop_ends = [cut_set[1:] for cut_set in self.cuts]
         return ChopsUpdate(chops, spans, chop_ends)
 
@@ -1013,12 +1018,17 @@ class Compositor:
         self,
         crop: Region,
         is_rendered_line: Callable[[int], bool],
+        allow_delta_updates: bool = False,
     ) -> Sequence[Mapping[int, Strip | None]]:
         """Render update 'chops'.
 
         Args:
             crop: Region to crop to.
             is_rendered_line: Callable to check if line should be rendered.
+            allow_delta_updates: Whether we can issue partial updates for widgets or
+                not. If `True`, widgets with `_enable_delta_updates` will produce only
+                updates for lines inside `crop` that have changed. If `False`, each
+                widget produces the full render for the region inside `crop`.
 
         Returns:
             Chops structure.
@@ -1031,7 +1041,7 @@ class Compositor:
         cut_strips: Iterable[Strip]
 
         # Go through all the renders in reverse order and fill buckets with no render
-        renders = self._get_renders(crop)
+        renders = self._get_renders(crop, allow_delta_updates=allow_delta_updates)
         intersection = Region.intersection
 
         for region, clip, strips in renders:

src/textual/scrollbar.py

@@ -234,6 +234,7 @@ class ScrollBar(Widget):
         self.grabbed_position: float = 0
         super().__init__(name=name)
         self.auto_links = False
+        self._enable_delta_updates = False
 
     window_virtual_size: Reactive[int] = Reactive(100)
     window_size: Reactive[int] = Reactive(0)
@@ -366,6 +367,7 @@ class ScrollBarCorner(Widget):
 
     def __init__(self, name: str | None = None):
         super().__init__(name=name)
+        self._enable_delta_updates = False
 
     def render(self) -> RenderableType:
         assert self.parent is not None

src/textual/widget.py

@@ -8,7 +8,7 @@ from asyncio import Lock, create_task, wait
 from collections import Counter
 from contextlib import asynccontextmanager
 from fractions import Fraction
-from itertools import islice
+from itertools import groupby, islice
 from types import TracebackType
 from typing import (
     TYPE_CHECKING,
@@ -361,6 +361,13 @@ class Widget(DOMNode):
 
         self._styles_cache = StylesCache()
         self._rich_style_cache: dict[str, tuple[Style, Style]] = {}
+        self._enable_delta_updates: bool = True
+        """Get the compositor to only rerender lines that changed between consecutive
+        updates.
+        """
+        self._delta_updates_cache: tuple[Region, Region, Region, list[Strip]] | None = (
+            None
+        )
 
         self._tooltip: RenderableType | None = None
         """The tooltip content."""
@@ -3265,9 +3272,58 @@ class Widget(DOMNode):
         Returns:
             A list of list of segments.
         """
+        self._delta_updates_cache = None
         strips = self._styles_cache.render_widget(self, crop)
         return strips
 
+    def render_delta_lines(
+        self,
+        region: Region,
+        clip: Region,
+        region_to_render: Region,
+    ) -> Generator[tuple[Region, Region, list[Strip]], None, None]:
+        """Render the lines of the widget that changed since the last render.
+
+        When a widget is rendered consecutively in the same region
+        """
+        print(self)
+        strips = self._styles_cache.render_widget(self, region_to_render)
+
+        if self._delta_updates_cache is None:
+            self._delta_updates_cache = (region, clip, region_to_render, strips)
+            yield region, clip, strips
+            return
+
+        cached_region, cached_clip, cached_region_to_render, cached_strips = (
+            self._delta_updates_cache
+        )
+        self._delta_updates_cache = (region, clip, region_to_render, strips)
+        if (
+            cached_region != region
+            or cached_clip != clip
+            or cached_region_to_render != region_to_render
+        ):
+            yield region, clip, strips
+            return
+
+        cached_strips_iter = iter(cached_strips)
+        y_offset = 0
+        for matches_cache, new_strips in groupby(
+            strips, key=lambda strip: strip == next(cached_strips_iter)
+        ):
+            new_strips = list(new_strips)
+            if matches_cache:
+                y_offset += len(new_strips)
+                continue
+            reg = Region(
+                region_to_render.x + region.x,
+                region_to_render.y + region.y + y_offset,
+                region.width,
+                len(new_strips),
+            )
+            y_offset += len(new_strips)
+            yield reg, clip, new_strips
+
     def get_style_at(self, x: int, y: int) -> Style:
         """Get the Rich style in a widget at a given relative offset.
 

src/textual/widgets/_tree.py

@@ -1008,6 +1008,15 @@ class Tree(Generic[TreeDataType], ScrollView, can_focus=True):
                 self.cursor_line = -1
         self.refresh()
 
+    def render_delta_lines(
+        self,
+        region: Region,
+        clip: Region,
+        region_to_render: Region,
+    ) -> tuple[Region, Region, list[Strip]]:
+        self._pseudo_class_state = self.get_pseudo_class_state()
+        return super().render_delta_lines(region, clip, region_to_render)
+
     def render_lines(self, crop: Region) -> list[Strip]:
         self._pseudo_class_state = self.get_pseudo_class_state()
         return super().render_lines(crop)