ui: reduce flicker in buttons, text, and pin dialog

src/trezor/ui/__init__.py

@@ -159,6 +159,11 @@ def grid(
 
 
 class Widget:
+    tainted = True
+
+    def taint(self):
+        self.tainted = True
+
     def render(self):
         pass
 
@@ -173,21 +178,3 @@ class Widget:
             event, *pos = yield touch
             result = self.touch(event, pos)
         return result
-
-
-class LazyWidget(Widget):
-    render_next_frame = True
-
-    def taint(self):
-        self.render_next_frame = True
-
-    def __iter__(self):
-        touch = loop.wait(io.TOUCH)
-        result = None
-        while result is None:
-            if self.render_next_frame:
-                self.render()
-                self.render_next_frame = False
-            event, *pos = yield touch
-            result = self.touch(event, pos)
-        return result

src/trezor/ui/button.py

@@ -1,7 +1,7 @@
 from micropython import const
 
 from trezor import io, ui
-from trezor.ui import LazyWidget, contains, display, rotate
+from trezor.ui import Widget, contains, display, rotate
 
 # button events
 BTN_CLICKED = const(1)
@@ -17,7 +17,7 @@ ICON = const(16)  # icon size in pixels
 BORDER = const(4)  # border size in pixels
 
 
-class Button(LazyWidget):
+class Button(Widget):
     def __init__(self, area: tuple, content: str, style: dict = ui.BTN_KEY):
         self.area = area
         self.content = content
@@ -29,14 +29,16 @@ class Button(LazyWidget):
     def enable(self):
         if self.state == BTN_DISABLED:
             self.state = BTN_INITIAL
-            self.render_next_frame = True
+            self.tainted = True
 
     def disable(self):
         if self.state != BTN_DISABLED:
             self.state = BTN_DISABLED
-            self.render_next_frame = True
+            self.tainted = True
 
     def render(self):
+        if not self.tainted:
+            return
         state = self.state
         if state == BTN_DISABLED:
             s = self.disabled_style
@@ -47,6 +49,7 @@ class Button(LazyWidget):
         ax, ay, aw, ah = self.area
         self.render_background(s, ax, ay, aw, ah)
         self.render_content(s, ax, ay, aw, ah)
+        self.tainted = False
 
     def render_background(self, s, ax, ay, aw, ah):
         radius = s["radius"]
@@ -89,20 +92,21 @@ class Button(LazyWidget):
         if event == io.TOUCH_START:
             if contains(self.area, pos):
                 self.state = BTN_ACTIVE
-                self.render_next_frame = True
+                self.tainted = True
 
         elif event == io.TOUCH_MOVE:
             if contains(self.area, pos):
                 if state == BTN_FOCUSED:
                     self.state = BTN_ACTIVE
-                    self.render_next_frame = True
+                    self.tainted = True
             else:
                 if state == BTN_ACTIVE:
                     self.state = BTN_FOCUSED
-                    self.render_next_frame = True
+                    self.tainted = True
 
         elif event == io.TOUCH_END:
-            self.state = BTN_INITIAL
+            if state != BTN_INITIAL:
-            self.render_next_frame = True
+                self.state = BTN_INITIAL
-            if state == BTN_ACTIVE and contains(self.area, pos):
+                self.tainted = True
-                return BTN_CLICKED
+                if state == BTN_ACTIVE and contains(self.area, pos):
+                    return BTN_CLICKED

src/trezor/ui/confirm.py

@@ -79,8 +79,7 @@ class HoldToConfirmDialog(Widget):
             self.loader.start()
             return _STARTED
         if was_active and not is_active:
-            if isinstance(self.content, ui.LazyWidget):
+            self.content.taint()
-                self.content.taint()
             if self.loader.stop():
                 return CONFIRMED
             else:

src/trezor/ui/pin.py

@@ -35,6 +35,13 @@ class PinMatrix(ui.Widget):
         self.onchange = None
 
     def render(self):
+        # pin matrix buttons
+        for btn in self.pin_buttons:
+            btn.render()
+
+        if not self.tainted:
+            return
+
         # clear canvas under input line
         display.bar(0, 0, ui.WIDTH, 45, ui.BG)
 
@@ -52,9 +59,7 @@ class PinMatrix(ui.Widget):
             # input line with header label
             display.text_center(ui.WIDTH // 2, 36, self.label, ui.BOLD, ui.GREY, ui.BG)
 
-        # pin matrix buttons
+        self.tainted = False
-        for btn in self.pin_buttons:
-            btn.render()
 
     def touch(self, event, pos):
         for btn in self.pin_buttons:
@@ -64,6 +69,7 @@ class PinMatrix(ui.Widget):
                 break
 
     def change(self, pin):
+        self.tainted = True
         self.pin = pin
         for btn in self.pin_buttons:
             if len(self.pin) == self.maxlength:

src/trezor/ui/text.py

@@ -106,7 +106,7 @@ def render_text(words: list, new_lines: bool, max_lines: int) -> None:
             offset_x += SPACE
 
 
-class Text(ui.LazyWidget):
+class Text(ui.Widget):
     def __init__(
         self,
         header_text: str,
@@ -121,6 +121,7 @@ class Text(ui.LazyWidget):
         self.max_lines = max_lines
         self.new_lines = new_lines
         self.content = []
+        self.tainted = True
 
     def normal(self, *content):
         self.content.append(ui.NORMAL)
@@ -142,7 +143,10 @@ class Text(ui.LazyWidget):
         self.content.append(BR)
 
     def render(self):
+        if not self.tainted:
+            return
         ui.header(
             self.header_text, self.header_icon, ui.TITLE_GREY, ui.BG, self.icon_color
         )
         render_text(self.content, self.new_lines, self.max_lines)
+        self.tainted = False