How to iterate over multiple workspace inputs in tool validation

[MattHowe]

I have a tool that has a workspace parameter and accepts multiple values. In the tool validation script, I would like to list all feature classes found within all chosen workspaces in the second parameter. 

How do I iterate over the workspaces in the first parameter?

def updateParameters(self):
  # Modify parameter values and properties.
  # This gets called each time a parameter is modified, before 
  # standard validation.
  if self.params[0].altered:
    # something like below ?
    for ws in self.params[0].split(";"):

[DanPatterson]

Walk—ArcGIS Pro | Documentation

Listing things using arcpy 

should pretty well cover the things you are looking for.  ListWorkspaces, ListFeatureclasses in conjunction with Walk 

[MattHowe · Accepted solution]

Cheers Dan. Figured it out.

def updateParameters(self):
    # Modify parameter values and properties.
    # This gets called each time a parameter is modified, before 
    # standard validation.
    self.params[1].filter.list = []
    if self.params[0].altered:
        if self.params[0].value:
            fcs = []
            ws = self.params[0].values
            for item in ws:
                arcpy.env.workspace = item
                data = arcpy.da.Walk(item, datatype="FeatureClass")
                for dirpath, dirnames, filenames in data:
                    for filename in filenames:
                        fcs.append(filename)
        self.params[1].filter.list = fcs
    return

 It was the '.values' property of the multiple value parameter I was missing.