Smart Line Labeller

The line labeller described here demonstrates how to pick a line element, then compose a TextBlock that includes a TextField to display a DGN element property. The TextField in this example displays the line's length …

You can display several properties by creating a TextField for each, then appending that TextField to the TextBlock.

Text Background

By default, MicroStation doesn't apply the text style background of the text element to the text field. You can see that in my screenshot above: text Length: is plain text with a brown background; text 3.15m is the TextField without that background.

You can change a text field's rendering in MicroStation preferences. Navigate to Preferences Dialog, Text Category and toggle option Use Test Style Background Color for Field Background. MicroStation help tells us: If on, the TextFields will display the Field background color defined in the text style. You can switch from default grey background color to the Field background from the TextStyle, if defined.

Here's the same line and label with that option enabled …

Association

The label is a DGN text node element. It is stand-alone text and is not associated with the picked line. If you move the line, the text position does not change. If you edit the line and change its length, the text label updates automatically to reflect the new length.

If the label were associated with the line, then moving the line would cause the label also to move. It's possible to associate the label with its host. But, to keep things simple, I don't do that in this example.

Dependency

The TextField has a dependency on the host line element. That is, the TextField is dependent on that line. That's how the label knows to update if the host element is changed. Use the ANALYZE ELEMENT tool to see those dependency data on the label text node.

Introduction to the MicroStation TextField Class

A MicroStation user can create a TextField using the text editor.

Here's an overview of the TextField object. It provides detailed information that will help you to program a TextField using one of the supported languages: Python, C++ and C#. VBA does not support TextFields.

TextField Documentation

Source	Link
MicroStation Help	Fields - Derived Content in Text
MicroStation Python API Help	TextField class

The class connects to a DGN element, model or file …

• TextField.CreateForElement()
• TextField.CreateForModel()
• TextField.CreateForFile()

EC Classes

EC data are linked to a DGN object by that object's Element ID. An Element ID is a 64-bit integer. It is unique within the context of a DGN file. MicroStation provides formatter classes that link to an element using the same method. A formatter understands the context of a DGN element: whether it's 2D or 3D; whether it's open or closed (i.e. a line vs. a shape); how it should be measured.

Using the TextField Class

We need code that makes an EC formatter from the EC data belonging to an element. We'll assign that formatter to the TextField. The TextField that we create needs to know about its target DGN element.

The smart label line picker is implemented in cmdPickLineElement, a line picker class that inherits from DgnElementSetTool. There are several examples that show how to use DgnElementSetTool and I won't discuss it further here.

class cmdPickLineElement(DgnElementSetTool):
    '''
    Inherits from DgnElementSetTool.  Prompts user to pick a line element.
    'line element' means anything that converts to an open CurveVector.
    '''

What I will discuss is how to connect the picked element with its corresponding TextField. That's implemented by a LabelHandler class.

_LabelHandler Class

The _LabelHandler class is common to several examples and is described here.

CreateDistanceTextField

For a distance text field we need to compute the class name and property name for the formatter. The element picker accepts several DGN element types, which we use like this with class CreateDistanceTextField.

from EC.EC_Helpers.label_handler import (DistanceLabelHandler, )

CreateDistanceTextField is implemented in the _LabelHandler class file label_handler.py …

  def CreateDistanceTextField(self)->TextField:
    '''
    Use this for a linear version of Annotator.
    We calculate the class and property name according to the DGN element type.
    For line and line-string elements, the class & property "MstnLineSegments", "TotalLength" work OK.
    For a complex string, use "MstnComplex", "TotalLength".
    For a B-Spline element, "MstnBSplineCurve" and "Length"
    For an arc element, "MstnArc" and "Length"
    '''
    match (self._host_element.GetElementType ()):
      case MSElementTypes.eLINE_ELM | MSElementTypes.eLINE_STRING_ELM:
        ec_class = "MstnLineSegments"
        ec_prop  = "TotalLength"
      case MSElementTypes.eCMPLX_STRING_ELM:
        ec_class = "MstnComplex"
        ec_prop  = "TotalLength"
      case MSElementTypes.eBSPLINE_CURVE_ELM:
        ec_class = "MstnBSplineCurve"
        ec_prop  = "Length"
      case MSElementTypes.eARC_ELM:
        ec_class = "MstnArc"
        ec_prop  = "Length"
      case _:
        warn = f"DistanceLabelHandler.CreateDistanceTextField: attempt to use unsupported element type {eh.GetElementType ()}"
        print (warn)
        return None
    #  Call the base class method CreateTextField()
    return self.CreateTextField (ec_class, ec_prop)

Questions

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