[Revit API] Simple Modeless Form (External Event Handler) in pyRevit

I struggled for a while to make a modeless form. Why did I need it ? Because each time I was trying to get user to select object after WPF appear I was going out of Revit API thread and got this very common exception «Autodesk.Revit.Exceptions.InvalidOperationException: Attempting to create an ExternalEvent outside of a standard API execution». As Jeremy Tammik says :

One of the most frequently raised questions around the Revit API is how to drive Revit from outside, e.g., from a separate thread, a modeless dialogue, or a stand-alone executable.

I have read many examples on the subject. Most on them were in C#.

So I made a very simple form to make a very simple ExternalEventHandler sample as pyRevit script. It will help me and I hope it will help some hackers to struggle less than I did.

Let’s start with common import statement using built-in pyRevit utils :

# noinspection PyUnresolvedReferences
from Autodesk.Revit.UI import IExternalEventHandler, ExternalEvent
# noinspection PyUnresolvedReferences
from Autodesk.Revit.DB import Transaction
# noinspection PyUnresolvedReferences
from Autodesk.Revit.Exceptions import InvalidOperationException
from revitutils import selection, uidoc, doc
from scriptutils.userinput import WPFWindow

__doc__ = "A simple modeless form sample"
__title__ = "Modeless Form"
__author__ = "Cyril Waechter"

Then let’s write a simple function we want to execute modeless  (here it just delete selected elements) :

# Simple function we want to run
def delete_elements():
    t = Transaction(doc, "Failing script")
    for elid in uidoc.Selection.GetElementIds():
        print elid

And now come the new magic thing that let you enter in a valid Revit API context. The «ExternalEvent» class with his «IExternalEventHandler» class :

# Create a subclass of IExternalEventHandler
class SimpleEventHandler(IExternalEventHandler):
    Simple IExternalEventHandler sample

    # __init__ is used to make function from outside of the class to be executed by the handler. \
    # Instructions could be simply written under Execute method only
    def __init__(self, do_this):
        self.do_this = do_this

    # Execute method run in Revit API environment.
    def Execute(self, uiapp):
        except InvalidOperationException:
            # If you don't catch this exeption Revit may crash.
            print "InvalidOperationException catched"

    def GetName(self):
        return "simple function executed by an IExternalEventHandler in a Form"

# Now we need to make an instance of this handler. Moreover, it shows that the same class could be used to for
# different functions using different handler class instances
simple_event_handler = SimpleEventHandler(delete_elements)
# We now need to create the ExternalEvent
ext_event = ExternalEvent.Create(simple_event_handler)

Let’s do a simple form so easily created thanks to pyRevit in order to use our new toy :

# A simple WPF form used to call the ExternalEvent
class ModelessForm(WPFWindow):
    Simple modeless form sample

    def __init__(self, xaml_file_name):
        WPFWindow.__init__(self, xaml_file_name)
        self.simple_text.Text = "Hello World"

    def delete_click(self, sender, e):
        # This Raise() method launch a signal to Revit to tell him you want to do something in the API context

# Let's launch our beautiful and useful form !
modeless_form = ModelessForm("ModelessForm.xaml")

Here is the xaml code :

<Window xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
 Title="Delete things:" Height="150" Width="300" ShowInTaskbar="False" Topmost="True"
 WindowStartupLocation="CenterScreen" ScrollViewer.VerticalScrollBarVisibility="Disabled" HorizontalContentAlignment="Center">
 <StackPanel Margin="20" HorizontalAlignment="Stretch">
 <TextBlock x:Name="simple_text" Text="" Grid.Column="0" HorizontalAlignment="Center" FontWeight="Bold"/>
 <Button Content="Delete selected elements" Height="30" Margin="10,10" Click="delete_click"/>

Thanks a lot to all people mentioned in this article and linked article and stuffs.


  1. where should i input those scripts ? The RevitPythonShell can not import from revitutils import selection, uidoc, doc and from scriptutils.userinput import WPFWindow ?

Leave a Reply

Your email address will not be published. Required fields are marked *