Remove excessive calls to layout()

CHANGELOG.md

@@ -1,3 +1,7 @@
+0.18.1 - 2017-09-07
+## Fixed
+- 🐛 If `app.layout` was supplied a function, then it used to be called excessively. Now it is called just once on startup and just once on page load. https://github.com/plotly/dash/pull/128
+
 0.18.0 - 2017-09-07
 ## Changed
 - 🔒  Removes the `/static/` folder and endpoint that is implicitly initialized by flask. This is too implicit for my comfort level: I worry that users will not be aware that their files in their `static` folder are accessible
@@ -13,7 +17,7 @@
 - ✏️  Fix a typo in an exception
 - 🔧 Replaced all illegal characters in environment variable
 
-## Maintenance
+##🔧 Maintenance
 - 📝  Update README.md
 - ✅  Fix CircleCI tests. Note that the the [`dash-renderer`](https://github.com/plotly/dash-renderer) contains the bulk of the integration tests.
 - 💄 Flake8 fixes and tests (fixes #99 )

dash/dash.py

@@ -108,6 +108,7 @@ def __init__(
         self.server.before_first_request(self._setup_server)
 
         self._layout = None
+        self._cached_layout = None
         self.routes = []
 
     class config:
@@ -119,9 +120,10 @@ def layout(self):
 
     def _layout_value(self):
         if isinstance(self._layout, collections.Callable):
-            return self._layout()
+            self._cached_layout = self._layout()
         else:
-            return self._layout
+            self._cached_layout = self._layout
+        return self._cached_layout
 
     @layout.setter
     def layout(self, value):
@@ -134,8 +136,10 @@ def layout(self, value):
                 'a dash component.')
 
         self._layout = value
-        self.css._update_layout(value)
-        self.scripts._update_layout(value)
+
+        layout_value = self._layout_value()
+        self.css._update_layout(layout_value)
+        self.scripts._update_layout(layout_value)
         self._collect_and_register_resources(
             self.scripts.get_all_scripts()
         )
@@ -324,7 +328,8 @@ def react(self, *args, **kwargs):
             'so make sure to call `help(app.callback)` to learn more.')
 
     def _validate_callback(self, output, inputs, state, events):
-        layout = self._layout_value()
+        layout = self._cached_layout or self._layout_value()
+
         if (layout is None and
                 not self.config.supress_callback_exceptions):
             # Without a layout, we can't do validation on the IDs and

dash/resources.py

@@ -1,7 +1,6 @@
 from copy import copy
 import json
 import warnings
-import collections
 
 from .development.base_component import Component
 
@@ -64,10 +63,7 @@ def get_all_resources(self):
     def get_inferred_resources(self):
         namespaces = []
         resources = []
-        if isinstance(self.layout, collections.Callable):
-            layout = self.layout()
-        else:
-            layout = self.layout
+        layout = self.layout
 
         def extract_resource_from_component(component):
             if (isinstance(component, Component) and

dash/version.py

@@ -1 +1 @@
-__version__ = '0.18.0'
+__version__ = '0.18.1'