fix(core): prevent flickering when homescreen does not need to redraw

core/src/apps/homescreen/__init__.py

@@ -1,11 +1,32 @@
+import storage.cache
 import storage.device
 from trezor import res, ui
 
 
 class HomescreenBase(ui.Layout):
+    RENDER_INDICATOR: object | None = None
+
     def __init__(self) -> None:
         super().__init__()
         self.label = storage.device.get_label() or "My Trezor"
         self.image = storage.device.get_homescreen() or res.load(
             "apps/homescreen/res/bg.toif"
         )
+        self.repaint = storage.cache.homescreen_shown is not self.RENDER_INDICATOR
+
+    def on_render(self) -> None:
+        if not self.repaint:
+            return
+        self.do_render()
+        self.set_repaint(False)
+
+    def do_render(self) -> None:
+        raise NotImplementedError
+
+    def set_repaint(self, value: bool) -> None:
+        self.repaint = value
+        storage.cache.homescreen_shown = None if value else self.RENDER_INDICATOR
+
+    def _before_render(self) -> None:
+        if storage.cache.homescreen_shown is not self.RENDER_INDICATOR:
+            return super()._before_render()

core/src/apps/homescreen/homescreen.py

@@ -2,6 +2,7 @@ import utime
 from micropython import const
 
 import storage
+import storage.cache
 import storage.device
 from trezor import config, ui
 from trezor.ui.loader import Loader, LoaderNeutral
@@ -20,6 +21,8 @@ async def homescreen() -> None:
 
 
 class Homescreen(HomescreenBase):
+    RENDER_INDICATOR = storage.cache.HOMESCREEN_ON
+
     def __init__(self) -> None:
         super().__init__()
         if not storage.device.is_initialized():
@@ -33,10 +36,7 @@ class Homescreen(HomescreenBase):
         )
         self.touch_ms: int | None = None
 
-    def on_render(self) -> None:
-        if not self.repaint:
-            return
-
+    def do_render(self) -> None:
         # warning bar on top
         if storage.device.is_initialized() and storage.device.no_backup():
             ui.header_error("SEEDLESS")
@@ -55,8 +55,6 @@ class Homescreen(HomescreenBase):
         ui.display.avatar(48, 48 - 10, self.image, ui.WHITE, ui.BLACK)
         ui.display.text_center(ui.WIDTH // 2, 220, self.label, ui.BOLD, ui.FG, ui.BG)
 
-        self.repaint = False
-
     def on_touch_start(self, _x: int, _y: int) -> None:
         if self.loader.start_ms is not None:
             self.loader.start()
@@ -65,7 +63,7 @@ class Homescreen(HomescreenBase):
 
     def on_touch_end(self, _x: int, _y: int) -> None:
         if self.loader.start_ms is not None:
-            self.repaint = True
+            self.set_repaint(True)
         self.loader.stop()
         self.touch_ms = None
 

core/src/apps/homescreen/lockscreen.py

@@ -1,3 +1,4 @@
+import storage.cache
 from trezor import loop, res, ui, wire
 
 from . import HomescreenBase
@@ -22,6 +23,7 @@ async def lockscreen() -> None:
 class Lockscreen(HomescreenBase):
     BACKLIGHT_LEVEL = ui.BACKLIGHT_LOW
     RENDER_SLEEP = loop.SLEEP_FOREVER
+    RENDER_INDICATOR = storage.cache.LOCKSCREEN_ON
 
     def __init__(self, bootscreen: bool = False) -> None:
         if bootscreen:
@@ -34,10 +36,7 @@ class Lockscreen(HomescreenBase):
 
         super().__init__()
 
-    def on_render(self) -> None:
-        if not self.repaint:
-            return
-
+    def do_render(self) -> None:
         # homescreen with label text on top
         ui.display.text_center(
             ui.WIDTH // 2, 35, self.label, ui.BOLD, ui.TITLE_GREY, ui.BG
@@ -57,7 +56,5 @@ class Lockscreen(HomescreenBase):
         )
         ui.display.icon(45, 202, res.load(ui.ICON_CLICK), ui.TITLE_GREY, ui.BG)
 
-        self.repaint = False
-
     def on_touch_end(self, _x: int, _y: int) -> None:
         raise ui.Result(None)

core/src/storage/cache.py

@@ -23,6 +23,17 @@ APP_COMMON_SAFETY_CHECKS_TEMPORARY = 1 | _SESSIONLESS_FLAG
 STORAGE_DEVICE_EXPERIMENTAL_FEATURES = 2 | _SESSIONLESS_FLAG
 
 
+# === Homescreen storage ===
+# This does not logically belong to the "cache" functionality, but the cache module is
+# a convenient place to put this.
+# When a Homescreen layout is instantiated, it checks the value of `homescreen_shown`
+# to know whether it should render itself or whether the result of a previous instance
+# is still on. This way we can avoid unnecessary fadeins/fadeouts when a workflow ends.
+HOMESCREEN_ON = object()
+LOCKSCREEN_ON = object()
+homescreen_shown: object | None = None
+
+
 class InvalidSessionError(Exception):
     pass
 

core/src/trezor/ui/__init__.py

@@ -362,8 +362,7 @@ class Layout(Component):
             # way to get the lowest input-to-render latency.
             self.dispatch(RENDER, 0, 0)
 
-    def handle_rendering(self) -> loop.Task:  # type: ignore
-        """Task that is rendering the layout in a busy loop."""
+    def _before_render(self) -> None:
         # Before the first render, we dim the display.
         backlight_fade(style.BACKLIGHT_DIM)
         # Clear the screen of any leftovers, make sure everything is marked for
@@ -387,6 +386,10 @@ class Layout(Component):
         # the brightness on again.
         refresh()
         backlight_fade(self.BACKLIGHT_LEVEL)
+
+    def handle_rendering(self) -> loop.Task:  # type: ignore
+        """Task that is rendering the layout in a busy loop."""
+        self._before_render()
         sleep = self.RENDER_SLEEP
         while True:
             # Wait for a couple of ms and render the layout again.  Because

core/src/trezor/workflow.py

@@ -1,5 +1,6 @@
 import utime
 
+import storage.cache
 from trezor import log, loop
 
 if False:
@@ -128,6 +129,8 @@ def close_others() -> None:
         if not task.is_running():
             task.close()
 
+    storage.cache.homescreen_shown = None
+
     # if tasks were running, closing the last of them will run start_default