Layout root nodes once per loop

changes/2938.feature.rst

@@ -0,0 +1 @@
+Widget layout calculations are now deferred and performed once per event loop, instead of being done immediately when requested.

core/src/toga/widgets/base.py

@@ -17,6 +17,9 @@
 StyleT = TypeVar("StyleT", bound=BaseStyle)
 
 
+TAB = "    "
+
+
 class Widget(Node):
     _MIN_WIDTH = 100
     _MIN_HEIGHT = 100
@@ -319,19 +322,57 @@ def enabled(self) -> bool:
     def enabled(self, value: bool) -> None:
         self._impl.set_enabled(bool(value))
 
+    _layouts = 0
+    _level = 0
+
     def refresh(self) -> None:
+        name = type(self).__name__
+        # print(TAB * self._level + f"Refresh requested on {name}")
+
+        if not self.window:
+            # No need to do anything if the widget hasn't been added to a window.
+            return
+
         self._impl.refresh()
 
         # Refresh the layout
         if self._root:
             # We're not the root of the node hierarchy;
             # defer the refresh call to the root node.
             self._root.refresh()
+
         else:
-            # We can't compute a layout until we have a container
-            if self._impl.container:
-                super().refresh(self._impl.container)
-                self._impl.container.refreshed()
+            # Uncomment to always compute layout:
+
+            # self._refresh_layout()
+            # return
+
+            if self.window._currently_laying_out:
+                self._refresh_layout()
+                return
+
+            print(TAB * self._level + f"Adding {name} to dirty set")
+            self.window._dirty_root_widgets.add(self)
+
+            if self.window._pending_layout is None:
+                self.window._pending_layout = self.app.loop.call_soon(
+                    self.window._refresh_layouts
+                )
+
+    def _refresh_layout(self):
+        # print(self._level)
+        name = type(self).__name__
+
+        Widget._layouts += 1
+        print(TAB * self._level + f"#{self._layouts}. Laying out {name}")
+
+        Widget._level += 1
+
+        super().refresh(self._impl.container)
+        self._impl.container.refreshed()
+
+        Widget._level -= 1
+        # print(TAB * self._level + f"Done laying out {name}\n")
 
     def focus(self) -> None:
         """Give this widget the input focus.

core/src/toga/window.py

@@ -208,6 +208,10 @@ def __init__(
             size=Size(*size),
         )
 
+        self._pending_layout = None
+        self._dirty_root_widgets = set()
+        self._currently_laying_out = False
+
         # Add the window to the app
         App.app.windows.add(self)
 
@@ -369,6 +373,21 @@ def content(self, widget: Widget) -> None:
         # Update the geometry of the widget
         widget.refresh()
 
+    def _refresh_layouts(self):
+        self._currently_laying_out = True
+
+        toga.Widget._level += 1
+        print("\nLoop(")
+
+        while self._dirty_root_widgets:
+            self._dirty_root_widgets.pop()._refresh_layout()
+
+        print(")\n")
+        toga.Widget._level -= 1
+
+        self._currently_laying_out = False
+        self._pending_layout = None
+
     @property
     def widgets(self) -> FilteredWidgetRegistry:
         """The widgets contained in the window.