Ignore collapsed children when sizing box layouts

arcade/gui/widgets/layout.py

@@ -296,6 +296,7 @@ def add(self, child: W, **kwargs) -> W:
         bind(child, "size_hint", self._trigger_size_hint_update, weak=True)
         bind(child, "size_hint_min", self._trigger_size_hint_update, weak=True)
         bind(child, "size_hint_max", self._trigger_size_hint_update, weak=True)
+        bind(child, "visible", self._trigger_size_hint_update, weak=True)
 
         return super().add(child, **kwargs)
 
@@ -308,6 +309,7 @@ def remove(self, child: UIWidget):
         unbind(child, "size_hint", self._trigger_size_hint_update)
         unbind(child, "size_hint_min", self._trigger_size_hint_update)
         unbind(child, "size_hint_max", self._trigger_size_hint_update)
+        unbind(child, "visible", self._trigger_size_hint_update)
 
         return super().remove(child)
 
@@ -317,12 +319,12 @@ def _trigger_size_hint_update(self):
     def _update_size_hints(self):
         self._size_hint_requires_update = False
 
-        required_space_between = max(0, len(self.children) - 1) * self._space_between
         min_child_sizes = [
             UILayout.min_size_of(child) for child in self.children if child.visible is not None
         ]
+        required_space_between = max(0, len(min_child_sizes) - 1) * self._space_between
 
-        if len(self.children) == 0:
+        if not min_child_sizes:
             width = 0
             height = 0
         elif self.vertical:
@@ -362,12 +364,11 @@ def do_layout(self):
         start_y = self.content_rect.top
         start_x = self.content_rect.left
 
-        if not self.children:
-            return
-
         children_to_render = [
             (child, data) for child, data in self._children if child.visible is not None
         ]
+        if not children_to_render:
+            return
 
         # main axis
         constraints = [
@@ -377,7 +378,7 @@ def do_layout(self):
 
         available_space = (
             self.content_height if self.vertical else self.content_width
-        ) - self._space_between * (len(self.children) - 1)
+        ) - self._space_between * (len(children_to_render) - 1)
         main_sizes = _box_axis_algorithm(constraints, available_space)
 
         # orthogonal axis

tests/unit/gui/test_layouting_boxlayout_collapsed.py

@@ -0,0 +1,113 @@
+import pytest
+
+from arcade.gui import UIBoxLayout, UIWidget
+from arcade.types import LBWH
+
+
+@pytest.mark.parametrize("vertical", [False, True])
+@pytest.mark.parametrize("count", [0, 1, 3])
+def test_all_collapsed_children_preserve_only_padding_and_border(vertical, count):
+    children = [UIWidget(width=100, height=50) for _ in range(count)]
+    for child in children:
+        child.visible = None
+    original_rects = [child.rect for child in children]
+    box = UIBoxLayout(vertical=vertical, children=children, space_between=10)
+    box.with_padding(left=2, right=3, top=5, bottom=7).with_border(width=4)
+
+    box.fit_content()
+    box.do_layout()
+
+    assert box.size_hint_min == (13, 20)
+    assert box.size == (13, 20)
+    assert [child.rect for child in children] == original_rects
+
+
+@pytest.mark.parametrize("vertical", [False, True])
+@pytest.mark.parametrize("collapsed_index", [0, 1, 2])
+def test_collapsed_children_do_not_reserve_gaps(vertical, collapsed_index):
+    first = UIWidget(size_hint=(1, 1), size_hint_min=(20, 30))
+    second = UIWidget(size_hint=(1, 1), size_hint_min=(20, 30))
+    collapsed = UIWidget(width=100, height=100)
+    collapsed.visible = None
+    original_rect = collapsed.rect
+    children = [first, second]
+    children.insert(collapsed_index, collapsed)
+    box = UIBoxLayout(vertical=vertical, children=children, space_between=10)
+
+    assert box.size_hint_min == ((20, 70) if vertical else (50, 30))
+    box.rect = LBWH(0, 0, 120, 120)
+    box.do_layout()
+
+    if vertical:
+        assert first.rect == LBWH(0, 65, 120, 55)
+        assert second.rect == LBWH(0, 0, 120, 55)
+    else:
+        assert first.rect == LBWH(0, 0, 55, 120)
+        assert second.rect == LBWH(65, 0, 55, 120)
+    assert collapsed.rect == original_rect
+
+
+@pytest.mark.parametrize("vertical", [False, True])
+@pytest.mark.parametrize("visible", [False, True])
+def test_hidden_children_still_participate_in_layout(vertical, visible):
+    first = UIWidget(width=20, height=30)
+    first.visible = visible
+    second = UIWidget(width=40, height=50)
+    box = UIBoxLayout(vertical=vertical, children=[first, second], space_between=10)
+
+    box.fit_content()
+    box.do_layout()
+
+    assert box.size_hint_min == ((40, 90) if vertical else (70, 50))
+    if vertical:
+        assert first.top == box.top
+        assert first.bottom - second.top == 10
+        assert second.bottom == box.bottom
+    else:
+        assert first.left == box.left
+        assert second.left - first.right == 10
+        assert second.right == box.right
+
+
+@pytest.mark.parametrize("vertical", [False, True])
+def test_visibility_changes_update_the_minimum_size(vertical):
+    first = UIWidget(width=20, height=30)
+    second = UIWidget(width=40, height=50)
+    box = UIBoxLayout(vertical=vertical, children=[first, second], space_between=10)
+
+    first.visible = None
+    box._prepare_layout()
+    assert box.size_hint_min == (40, 50)
+
+    second.visible = None
+    box._prepare_layout()
+    assert box.size_hint_min == (0, 0)
+
+    first.visible = False
+    box._prepare_layout()
+    assert box.size_hint_min == (20, 30)
+
+    second.visible = True
+    box._prepare_layout()
+    assert box.size_hint_min == ((40, 90) if vertical else (70, 50))
+
+
+def test_collapse_and_restore_through_ui_manager(window, ui):
+    first = UIWidget(width=20, height=30)
+    second = UIWidget(width=40, height=50)
+    box = UIBoxLayout(children=[first, second], space_between=10)
+    ui.add(box)
+    ui.execute_layout()
+    assert box.size == (40, 90)
+
+    first.visible = None
+    ui.execute_layout()
+    assert box.size == (40, 50)
+
+    second.visible = None
+    ui.execute_layout()
+    assert box.size == (0, 0)
+
+    first.visible = False
+    ui.execute_layout()
+    assert box.size == (20, 30)