fix(renderer): clear child paths when callback returns None

Author: 0d00ciallo0721

dash/dash-renderer/src/observers/executedCallbacks.ts

@@ -321,7 +321,7 @@ const observer: IStoreObserverDefinition<IStoreState> = {
                                         childrenPropPath,
                                         appliedProps
                                     );
-                                    if (!children) {
+                                    if (children === undefined) {
                                         return;
                                     }
 

tests/integration/callbacks/test_children_none.py

@@ -0,0 +1,50 @@
+import time
+
+from dash import Dash, Input, Output, dcc, html
+from dash.testing.wait import until
+
+
+def test_callback_returning_none_removes_child_paths_and_can_remount(dash_duo):
+    view_calls = []
+
+    app = Dash(__name__, suppress_callback_exceptions=True)
+    app.layout = html.Div(
+        [
+            html.Button("Toggle", id="toggle"),
+            dcc.Interval(id="tick", interval=200),
+            html.Div(id="container"),
+        ]
+    )
+
+    @app.callback(
+        Output("container", "children"),
+        Input("toggle", "n_clicks"),
+        prevent_initial_call=True,
+    )
+    def toggle_view(n_clicks):
+        if n_clicks % 2:
+            return html.Div(html.Span(id="view-child"), id="view")
+        return None
+
+    @app.callback(Output("view-child", "children"), Input("tick", "n_intervals"))
+    def update_view(n_intervals):
+        view_calls.append(n_intervals)
+        return f"tick {n_intervals}"
+
+    dash_duo.start_server(app)
+
+    dash_duo.find_element("#toggle").click()
+    dash_duo.wait_for_element("#view-child")
+    until(lambda: "view-child" in dash_duo.redux_state_paths["strs"], timeout=3)
+    until(lambda: view_calls, timeout=3)
+
+    dash_duo.find_element("#toggle").click()
+    dash_duo.wait_for_no_elements("#view-child")
+    until(lambda: "view-child" not in dash_duo.redux_state_paths["strs"], timeout=3)
+    calls_after_unmount = len(view_calls)
+    time.sleep(0.5)
+    assert len(view_calls) == calls_after_unmount
+
+    dash_duo.find_element("#toggle").click()
+    dash_duo.wait_for_element("#view-child")
+    until(lambda: "view-child" in dash_duo.redux_state_paths["strs"], timeout=3)