feat(ui): setText on text nodes

Assigning .label repainted nothing until the caller also remembered to invalidate, and
forgetting produced a screen that silently stopped updating with no error to follow.
setText makes the pair atomic and skips the repaint when the text is unchanged, so an
app can push a value on every tick without comparing first.

sdcard/apps/launcher/main.lua

@@ -40,11 +40,7 @@ function init()
 end
 
 function on_tick()
-  local label = sys.clockSynced() and os.date("%H:%M:%S") or PLACEHOLDER
-  if label ~= clock.label then
-    clock.label = label
-    clock:invalidate()
-  end
+  clock:setText(sys.clockSynced() and os.date("%H:%M:%S") or PLACEHOLDER)
 end
 
 function draw() screen:draw() end

sdcard/lib/ui.lua

@@ -260,6 +260,13 @@ function ui.text(label, spec)
   node.paint = function(self)
     gui.drawText(self.label, self.rect.x, self.rect.y, self.color, self.bg)
   end
+  -- Setting .label alone repaints nothing, which fails silently. Unchanged text also
+  -- costs nothing, so a caller can push a value every tick without thinking about it.
+  node.setText = function(self, text)
+    if text == self.label then return end
+    self.label = text
+    self:invalidate()
+  end
   return node
 end
 

test/ui_layout.lua

@@ -87,6 +87,17 @@ app:down(100, 20)
 app:up(100, 20)
 assert(fired == 1, "release inside must fire once")
 
+-- setText repaints only on a real change, so a caller may push a value every tick.
+local label = ui.text("12:00:00")
+local clockScreen = ui.screen(ui.box{label})
+clockScreen:draw()
+assert(not label.dirty, "a drawn node starts clean")
+label:setText("12:00:00")
+assert(not label.dirty, "unchanged text must not repaint")
+label:setText("12:00:01")
+assert(label.dirty, "changed text must repaint")
+assert(label.label == "12:00:01", label.label)
+
 -- The pressed look is held briefly, then cleared on a later draw.
 assert(button.pressed, "pressed look must outlast the release")
 device.now = device.now + 100