add extra wait for callbacks in queue

CHANGELOG.md

@@ -11,6 +11,11 @@ updates in clientside functions.
 - [#899](https://github.com/plotly/dash/pull/899) Add support for async dependencies and components
 - [#973](https://github.com/plotly/dash/pull/973) Adds support for resource caching and adds a fallback caching mechanism through etag
 
+
+### Fixed
+- [#974](https://github.com/plotly/dash/pull/974) Fix and improve a percy snapshot behavior issue we found in dash-docs testing. It adds a flag `wait_for_callbacks`
+to ensure that, in the context of a dash app testing, the percy snapshot action will happen only after all callbacks get fired.
+
 ## [1.4.1] - 2019-10-17
 ### Fixed
 - [#969](https://github.com/plotly/dash/pull/969) Fix warnings emitted by react devtools coming from our own devtools components.

dash/testing/browser.py

@@ -100,14 +100,16 @@ def __exit__(self, exc_type, exc_val, traceback):
         except percy.errors.Error:
             logger.exception("percy runner failed to finalize properly")
 
-    def percy_snapshot(self, name=""):
+    def percy_snapshot(self, name="", wait_for_callbacks=False):
         """percy_snapshot - visual test api shortcut to `percy_runner.snapshot`.
         It also combines the snapshot `name` with the python version.
         """
         snapshot_name = "{} - py{}.{}".format(
             name, sys.version_info.major, sys.version_info.minor
         )
         logger.info("taking snapshot name => %s", snapshot_name)
+        if wait_for_callbacks:
+            until(self._wait_for_callbacks, timeout=10)
         self.percy_runner.snapshot(name=snapshot_name)
 
     def take_snapshot(self, name):
@@ -445,7 +447,7 @@ def zoom_in_graph_by_ratio(
         elem_or_selector,
         start_fraction=0.5,
         zoom_box_fraction=0.2,
-        compare=True
+        compare=True,
     ):
         """Zoom out a graph with a zoom box fraction of component dimension
         default start at middle with a rectangle of 1/5 of the dimension use
@@ -498,14 +500,18 @@ def reset_log_timestamp(self):
             if entries:
                 self._last_ts = entries[-1]["timestamp"]
 
-    def visit_and_snapshot(self, resource_path, hook_id, assert_check=True):
+    def visit_and_snapshot(
+        self, resource_path, hook_id, wait_for_callbacks=True, assert_check=True
+    ):
         try:
             path = resource_path.lstrip("/")
             if path != resource_path:
                 logger.warning("we stripped the left '/' in resource_path")
             self.driver.get("{}/{}".format(self.server_url.rstrip("/"), path))
+
+            # wait for the hook_id to present and all callbacks get fired
             self.wait_for_element_by_id(hook_id)
-            self.percy_snapshot(path)
+            self.percy_snapshot(path, wait_for_callbacks=wait_for_callbacks)
             if assert_check:
                 assert not self.driver.find_elements_by_css_selector(
                     "div.dash-debug-alert"

dash/testing/dash_page.py

@@ -36,6 +36,17 @@ def redux_state_rqs(self):
             "return window.store.getState().requestQueue"
         )
 
+    @property
+    def window_store(self):
+        return self.driver.execute_script("return window.store")
+
+    def _wait_for_callbacks(self):
+        if self.window_store:
+            return self.redux_state_rqs and all(
+                (_.get("responseTime") for _ in self.redux_state_rqs)
+            )
+        return True
+
     def get_local_storage(self, store_id="local"):
         return self.driver.execute_script(
             "return JSON.parse(window.localStorage.getItem('{}'));".format(