Add Widget.move_child

Adds a method to Widget that allows moving a child of that widget within its
list of children. Options are to move before or after a specific location,
or a sibling widget.

Seeks to implement #1121.

tests/test_widget_child_moving.py

@@ -0,0 +1,100 @@
+import pytest
+
+from textual.app import App
+from textual.widget import Widget, WidgetError
+
+async def test_widget_move_child() -> None:
+    """Test moving a widget in a child list."""
+
+    # Test calling move_child with no direction.
+    async with App().run_test() as pilot:
+        child = Widget(Widget())
+        await pilot.app.mount(child)
+        with pytest.raises(WidgetError):
+            pilot.app.screen.move_child(child)
+
+    # Test calling move_child with more than one direction.
+    async with App().run_test() as pilot:
+        child = Widget(Widget())
+        await pilot.app.mount(child)
+        with pytest.raises(WidgetError):
+            pilot.app.screen.move_child(child, before=1, after=2)
+
+    # Test attempting to move a child that isn't ours.
+    async with App().run_test() as pilot:
+        child = Widget(Widget())
+        await pilot.app.mount(child)
+        with pytest.raises(WidgetError):
+            pilot.app.screen.move_child(Widget(), before=child)
+
+    # Test attempting to move relative to a widget that isn't a child.
+    async with App().run_test() as pilot:
+        child = Widget(Widget())
+        await pilot.app.mount(child)
+        with pytest.raises(WidgetError):
+            pilot.app.screen.move_child(child, before=Widget())
+
+    # Make a background set of widgets.
+    widgets = [Widget(id=f"widget-{n}") for n in range( 10 )]
+
+    # Test attempting to move past the end of the child list.
+    async with App().run_test() as pilot:
+        container = Widget(*widgets)
+        await pilot.app.mount(container)
+        with pytest.raises(WidgetError):
+            container.move_child(widgets[0], before=len(widgets)+10)
+
+    # Test attempting to move before the end of the child list.
+    async with App().run_test() as pilot:
+        container = Widget(*widgets)
+        await pilot.app.mount(container)
+        with pytest.raises(WidgetError):
+            container.move_child(widgets[0], before=-(len(widgets)+10))
+
+    # Test the different permutations of moving one widget before another.
+    perms = (
+        ( 1, 0 ),
+        ( widgets[1], 0 ),
+        ( 1, widgets[ 0 ] ),
+        ( widgets[ 1 ], widgets[ 0 ])
+    )
+    for child, target in perms:
+        async with App().run_test() as pilot:
+            container = Widget(*widgets)
+            await pilot.app.mount(container)
+            container.move_child(child, before=target)
+            assert container.children[0].id == "widget-1"
+            assert container.children[1].id == "widget-0"
+            assert container.children[2].id == "widget-2"
+
+    # Test the different permutations of moving one widget after another.
+    perms = (
+        ( 0, 1 ),
+        ( widgets[0], 1 ),
+        ( 0, widgets[ 1 ] ),
+        ( widgets[ 0 ], widgets[ 1 ])
+    )
+    for child, target in perms:
+        async with App().run_test() as pilot:
+            container = Widget(*widgets)
+            await pilot.app.mount(container)
+            container.move_child(child, after=target)
+            assert container.children[0].id == "widget-1"
+            assert container.children[1].id == "widget-0"
+            assert container.children[2].id == "widget-2"
+
+    # Test moving after a child after the last child.
+    async with App().run_test() as pilot:
+        container = Widget(*widgets)
+        await pilot.app.mount(container)
+        container.move_child(widgets[0], after=widgets[-1])
+        assert container.children[0].id == "widget-1"
+        assert container.children[-1].id == "widget-0"
+
+    # Test moving after a child after the last child's numeric position.
+    async with App().run_test() as pilot:
+        container = Widget(*widgets)
+        await pilot.app.mount(container)
+        container.move_child(widgets[0], after=widgets[9])
+        assert container.children[0].id == "widget-1"
+        assert container.children[-1].id == "widget-0"