core/homescreen: render homescreen in a layout

FIxes a bug introduced in bb25562

core/src/apps/homescreen/homescreen.py

@@ -1,40 +1,41 @@
-from trezor import config, io, loop, res, ui
+from trezor import config, res, ui
 
 from apps.common import storage
 
 
 async def homescreen() -> None:
-    # render homescreen in dimmed mode and fade back in
-    ui.backlight_fade(ui.BACKLIGHT_DIM)
-    display_homescreen()
-    ui.backlight_fade(ui.BACKLIGHT_NORMAL)
-
-    # loop forever, never return
-    touch = loop.wait(io.TOUCH)
-    while True:
-        await touch
-
-
-def display_homescreen() -> None:
-    image = None
-    if not storage.is_initialized():
-        label = "Go to trezor.io/start"
-    else:
-        label = storage.device.get_label() or "My Trezor"
-        image = storage.device.get_homescreen()
-
-    if not image:
-        image = res.load("apps/homescreen/res/bg.toif")
-
-    if storage.is_initialized() and storage.device.no_backup():
-        ui.header_error("SEEDLESS")
-    elif storage.is_initialized() and storage.device.unfinished_backup():
-        ui.header_error("BACKUP FAILED!")
-    elif storage.is_initialized() and storage.device.needs_backup():
-        ui.header_warning("NEEDS BACKUP!")
-    elif storage.is_initialized() and not config.has_pin():
-        ui.header_warning("PIN NOT SET!")
-    else:
-        ui.display.bar(0, 0, ui.WIDTH, ui.HEIGHT, ui.BG)
-    ui.display.avatar(48, 48 - 10, image, ui.WHITE, ui.BLACK)
-    ui.display.text_center(ui.WIDTH // 2, 220, label, ui.BOLD, ui.FG, ui.BG)
+    await Homescreen()
+
+
+class Homescreen(ui.Layout):
+    def __init__(self):
+        self.repaint = True
+
+    def on_render(self):
+        if not self.repaint:
+            return
+
+        image = None
+        if not storage.is_initialized():
+            label = "Go to trezor.io/start"
+        else:
+            label = storage.device.get_label() or "My Trezor"
+            image = storage.device.get_homescreen()
+
+        if not image:
+            image = res.load("apps/homescreen/res/bg.toif")
+
+        if storage.is_initialized() and storage.device.no_backup():
+            ui.header_error("SEEDLESS")
+        elif storage.is_initialized() and storage.device.unfinished_backup():
+            ui.header_error("BACKUP FAILED!")
+        elif storage.is_initialized() and storage.device.needs_backup():
+            ui.header_warning("NEEDS BACKUP!")
+        elif storage.is_initialized() and not config.has_pin():
+            ui.header_warning("PIN NOT SET!")
+        else:
+            ui.display.bar(0, 0, ui.WIDTH, ui.HEIGHT, ui.BG)
+        ui.display.avatar(48, 48 - 10, image, ui.WHITE, ui.BLACK)
+        ui.display.text_center(ui.WIDTH // 2, 220, label, ui.BOLD, ui.FG, ui.BG)
+
+        self.repaint = False