PDG provides an event handling mechanism that can be used to run custom Python code when things happen during the cook. Event hooks are available for fine-grained changes in work items such as a state changes or dependency modifications, as well as more granular graph events like cook completion. The full list of events is documented in the pdg.EventType API reference.

To register an event handler you first need to have a reference to either a pdg.Node, pdg.WorkItem or a pdg.GraphContext. The Node and Graph Context can be accessed from a hou.TopNode once the TOP graph has been cooked. A work item can be found by looking it up using the workItemId on the pdg.Event with pdg.GraphContext.workItemById.

Prior to Houdini 18, an event emitted by a work item would also be emitted by the node holding it and by the graph context. That means that adding event handlers to the graph context would be enough to catch all work item, node and graph events, this is no longer the case, in order to catch a work item event, you need to add a handler to the work item. Some events will still be emitted by multiple objects, but that behavior is now specific to some events.

import hou
import pdg

topnode_path = '/tasks/topnet1/genericgenerator1'
top_node = hou.node(topnode_path)

# Ensure the TOP graph has been cooked so that we can
# access the underlying PDG graph
top_node.cookWorkItems(tops_only=True)

pdg_node = top_node.getPDGNode()
pdg_context = top_node.getPDGGraphContext()

The API for registering an event handler is the same on node and graph context objects. Each handler is create by defining a function that takes a pdg.Event as its first argument:

# Define a function to handle a work item event
def print_add(event):
    work_item = pdg_context.graph.workItemById(event.workItemId)
    print ("Work item '{}' was added to node {}".format(work_item.name, event.node.name))
    # Add a listener for state changes on this new work item

    def print_workitem_change(event):
        work_item = pdg_context.graph.workItemById(event.workItemId)
        print ('State Change for {}: {} -> {}'.format(work_item.name, event.lastState, event.currentState))
    work_item.addEventHandler(print_workitem_change, pdg.EventType.WorkItemStateChange)

# Define a function to handle a graph level event
def print_done(event):
    print ("Graph cook complete!")

# Register the handlers
node_handler = pdg_node.addEventHandler(print_add, pdg.EventType.WorkItemAdd)
context_handler = pdg_context.addEventHandler(print_done, pdg.EventType.CookComplete)

The best place to put event registration code depends on your set up. One option is to place the code in a shelf tool that registers the handlers when the tool is clicked.

Sometimes you may wish to remove event handlers after they have finished executing. One way to do this is to call the removeEventHandler method on the object that was initially used to add the handler:

pdg_node.removeEventHandler(node_handler)

It is also possible to remove a handler from within the event function itself. To do so you’ll need to include the extra pass_handler=True argument when registering the handler function. This is so that the event function knows which handler it is associated with it. Inside the event function itself you can then call the pdg.EventHandler.removeFromAllEmitters method on the handler:

def print_done_and_remove(handler, event):
    print ("Graph cook complete!")

    # Deregister the handler from all events its listening to
    handler.removeFromAllEmitters()

pdg_context.addEventHandler(print_done_and_remove, pdg.EventType.CookComplete, True)

PDG events are processed in the background on the same cook thread that emitted them. This means that generally speaking it isn’t safe to start another cook, change parameters or open UI dialogs from an event handler function. In some cases you can perform these actions by the using the hdefereval module to ensure that your code runs on the main thread instead in the background:

import hdefereval

# Define a function for displaying our message
def display_message(message):
    import hou
    hou.ui.displayMessage(message, title="Example Message from PDG")

# Define our event handler
def display_event(event):
    hdefereval.executeDeferred(display_message, "An example message created in"
        " a PDG Event but displayed from the main thread!")

pdg_context.addEventHandler(display_event, pdg.EventType.CookComplete)

All event sources are subclasses of the pdg.EventEmitter base class. You can check the list of event types supported by an instance of that class using the pdg.EventEmitter.supportedEventTypes property. The list of supported events for each type of event emitter is also listed below: