fix #1223 - initialcall on new layout chunk edge case

CHANGELOG.md

@@ -8,6 +8,7 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 - [#1078](https://github.com/plotly/dash/pull/1078) Permit usage of arbitrary file extensions for assets within component libraries
 
 ### Fixed
+- [#1224](https://github.com/plotly/dash/pull/1224) Fixes [#1223](https://github.com/plotly/dash/issues/1223), a very specific situation in which initial callbacks will not fire.
 - [#1220](https://github.com/plotly/dash/pull/1220) Fixes [#1216](https://github.com/plotly/dash/issues/1216), a set of related issues about pattern-matching callbacks with `ALL` wildcards in their `Output` which would fail if no components matched the pattern.
 - [#1212](https://github.com/plotly/dash/pull/1212) Fixes [#1200](https://github.com/plotly/dash/issues/1200) - prior to Dash 1.11, if none of the inputs to a callback were on the page, it was not an error. This was, and is now again, treated as though the callback raised PreventUpdate. The one exception to this is with pattern-matching callbacks, when every Input uses a multi-value wildcard (ALL or ALLSMALLER), and every Output is on the page. In that case the callback fires as usual.
 - [#1201](https://github.com/plotly/dash/pull/1201) Fixes [#1193](https://github.com/plotly/dash/issues/1193) - prior to Dash 1.11, you could use `flask.has_request_context() == False` inside an `app.layout` function to provide a special layout containing all IDs for validation purposes in a multi-page app. Dash 1.11 broke this when we moved most of this validation into the renderer. This change makes it work again.

dash-renderer/src/actions/dependencies.js

@@ -1214,10 +1214,14 @@ export function getCallbacksInLayout(graphs, paths, layoutChunk, opts) {
         if (callback) {
             const foundIndex = foundCbIds[callback.resolvedId];
             if (foundIndex !== undefined) {
-                callbacks[foundIndex].changedPropIds = mergeMax(
-                    callbacks[foundIndex].changedPropIds,
+                const foundCb = callbacks[foundIndex];
+                foundCb.changedPropIds = mergeMax(
+                    foundCb.changedPropIds,
                     callback.changedPropIds
                 );
+                if (callback.initialCall) {
+                    foundCb.initialCall = true;
+                }
             } else {
                 foundCbIds[callback.resolvedId] = callbacks.length;
                 callbacks.push(callback);

tests/integration/callbacks/test_multiple_callbacks.py

@@ -262,3 +262,43 @@ def on_click(n_clicks):
     dash_duo.wait_for_text_to_equal("#output", "1")
     dash_duo.find_element("#btn").click()
     dash_duo.wait_for_text_to_equal("#output", "2")
+
+
+def test_cbmt007_early_preventupdate_inputs_above_below(dash_duo):
+    app = dash.Dash(__name__, suppress_callback_exceptions=True)
+    app.layout = html.Div(id="content")
+
+    @app.callback(Output("content", "children"), [Input("content", "style")])
+    def content(_):
+        return html.Div([
+            html.Div(42, id="above-in"),
+            html.Div(id="above-dummy"),
+            html.Hr(),
+            html.Div(0, id='above-out'),
+            html.Div(0, id='below-out'),
+            html.Hr(),
+            html.Div(id="below-dummy"),
+            html.Div(44, id="below-in"),
+        ])
+
+    # Create 4 callbacks - 2 above, 2 below.
+    for pos in ('above', 'below'):
+        @app.callback(
+            Output("{}-dummy".format(pos), "children"),
+            [Input("{}-dummy".format(pos), "style")]
+        )
+        def dummy(_):
+            raise PreventUpdate
+
+        @app.callback(
+            Output('{}-out'.format(pos), 'children'),
+            [Input('{}-in'.format(pos), 'children')]
+        )
+        def out(v):
+            return v
+
+    dash_duo.start_server(app)
+
+    # as of https://github.com/plotly/dash/issues/1223, above-out would be 0
+    dash_duo.wait_for_text_to_equal("#above-out", "42")
+    dash_duo.wait_for_text_to_equal("#below-out", "44")