Graphics and Callbacks

It would be convenient to look at a graphical view of an account, so let’s create one. This allows you to illustrate another level of integration between Python and VB: callbacks. Within the framework you’ve just created, the VB COM client has been accessing the properties and methods of the Python COM server. But it doesn’t have to be that way; the Python code can access the properties and methods of any VB object, whether built-in or user-defined.

You need to pass a VB form to Python, which then draws directly on it. You could also instead write another view that grabs the same array of data as the last one and uses VB graphics code to do the drawing. That is a better design, since it keeps a strict separation between the engine and interface; once you add drawing code on the Python side, you presume that a VB form with a certain API is provided. But this example is fun and highly instructive.

Let’s start by creating a new form in our application called frmAccountChart, similar to the other views so far. Add a public method to the COMBookSet server called drawAccountChart, which expects a VB form as an argument. We’ll start with a simple test: our drawing routine sets the caption of the form with a timestamp. On the VB side, the UpdateView method for frmAccountChart asks Python to do the work:

'Method of frmAccountChart
Public Sub UpdateView()
    'ask Python to scribble on me
    frmMain.BookServer.drawAccountChart Me
End Sub

The form passes itself (using the keyword ...

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