Workflow: Setting default value for bpm:assignee - alfresco

Question

When I start my workflow, I want the option to assign it to somebody. If nobody is chosen, I want to default to assign it to the initiator.
Is this possible to do without creating a new model that extends bpm:assignee? If not, how would that extension be accomplished?
I believe this answer from Jeff Potts is relevant: https://stackoverflow.com/a/9418066/4542428
Note: I am using Community edition 4.2
EDIT: Stefan's answer got me the vast majority of the way to the answer, but it seems that I am somehow referencing the value of the association incorrectly. Context: I've never used associations, and this is likely just my failure to understand their difference from types and aspects.
From my model:
<type name="deliveryTicketWorkflow:start">
<parent>bpm:startTask</parent>
<properties>
</properties>
<associations />
<overrides />
<mandatory-aspects>
<aspect>deliveryTicketWorkflow:pmAspect</aspect>
<aspect>deliveryTicketWorkflow:requestDetailsAspect</aspect>
</mandatory-aspects>
</type>
<aspect name="deliveryTicketWorkflow:pmAspect">
<associations>
<association name="deliveryTicketWorkflow:assignedPM">
<source>
<mandatory>false</mandatory>
<many>false</many>
</source>
<target>
<class>cm:person</class>
<mandatory>false</mandatory>
<many>true</many>
</target>
</association>
</associations>
</aspect>
Which is used by my config as:
<config condition="activiti$deliveryTicketWorkflow" evaluator="string-compare">
<forms>
<form>
<field-visibility>
...
<show id="deliveryTicketWorkflow:assignedPM" />
...
</field-visibility>
<appearance>
...
<field id="deliveryTicketWorkflow:assignedPM" label-id="Project Manager" />
...
</appearance>
</form>
</forms>
</config>
My config for deliveryTicketworkflow:start is identical. This successfully displays the person selector, without making it mandatory, exactly 100% as Stefan said it would.
In my bpmn workflow definition, I then have these snippets in an execution listener for the start event:
if(!execution.getVariable("deliveryTicketWorkflow_assignedPM")){
execution.setVariable("deliveryTicketWorkflow_assignedPM", initiator);
}
...
deliveryTicket.properties["dtdlm:projectManager"] = execution.getVariable("deliveryTicketWorkflow_assignedPM").properties.firstName + " " + execution.getVariable("deliveryTicketWorkflow_assignedPM").properties.lastName;
When the workflow is run and I select somebody as the PM, that final line (where the first and last name of the PM is grabbed) returns a value of undefined for "deliveryTicketWorkflow_assignedPM". When it is left blank, everything works swimmingly but the General Info section of the Workflow description still lists the Project Manager as (None).

Answer 1

You could indeed customize the people picker, as jeff describes, but it requires quite some coding effort.
Alternatively you could use a workflow executionlistener for event start, and use it to set the bpm_assignee variable to the initiator in case it was empty on the form:
Add listener to you bpmn20:
<activiti:executionListener event="start" class="com.mycomp.Executionlistener"></activiti:executionListener>
In your first usertask, define the assignee to the association property in your workflow start form.
<userTask id="firsttask" name="firsttask" activiti:assignee="${mymodel.myassoc.properties.userName}" >
Also add this association to your start task content model.
Code in com.mycomp.Executionlistener will look like this:
public void notify(DelegateExecution execution) throws Exception {
if (execution.getVariable("mymodel_myassoc") == null ){
ActivitiScriptNode userScriptNode= (ActivitiScriptNode) execution.getVariable("initiator");
execution.setVariable("mymodel_myassoc",userScriptNode);
}
}