Use document_ready for onload callback (#1562)

* Use document_ready for onload callback

* Require bokeh 2.2 for onload

panel/io/state.py

@@ -15,7 +15,7 @@
 from pyviz_comms import CommManager as _CommManager
 from tornado.web import decode_signed_value
 
-from ..util import base64url_decode
+from ..util import base64url_decode, bokeh_version
 
 
 class _state(param.Parameterized):
@@ -126,6 +126,10 @@ def link(*events):
                         _updating[id(obj)] = not_updated
         return link
 
+    def _on_load(self, event):
+        for cb in self._onload.pop(self.curdoc, []):
+            cb()
+
     #----------------------------------------------------------------
     # Public Methods
     #----------------------------------------------------------------
@@ -209,6 +213,8 @@ def onload(self, callback):
             return
         if self.curdoc not in self._onload:
             self._onload[self.curdoc] = []
+            if bokeh_version >= '2.2.0':
+                self.curdoc.on_event('document_ready', self._on_load)
         self._onload[self.curdoc].append(callback)
 
     def publish(self, endpoint, parameterized, parameters=None):