DAVSL: Visio AddOn Framework for Delphi

	DAVSL: Visio AddOn Framework for Delphi
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Background (This page)
- Challenges
- Features
- Acknowledgements
Architecture
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Samples
Technical Notes

Background

To add really sophisticated functionality to Visio, it's necessary to build VB, C++, Delphi or other language modules ("AddOns") to control Visio and respond to events triggered in Visio. While it's possible to do quite a bit of this in VBA, one shortcoming is that VBA modules are housed in Visio documents, which leads to challenges about how to control when those documents are open and the projects accessible. In particular, it's obviously not possible for a VBA module to be modifying Visio's behavior when all documents are closed.

Hence in a lot of scenarios, it is more manageable to build the additional functionality as an external module.

One form which that external module can take is a stand-alone executable (xxx.exe). These can be built in a variety of languages including VB. For many applications this works fine. However, there are several reasons why you might prefer to build the external module as a DLL callable by Visio, which is refered to as a "VSL" (Visio Solution Library):

VSLs execute "in-process" and if involved in intensive communication with the Visio engine this makes for faster performance.
Visio's way of calling an external EXE (via RUNADDON or UI Items) is to run the EXE and pass arguments on the command line. This involves launching the EXE for each call, which is fine for addons that are called only infrequently. However, it is a rather clumsy proposition if calling frequently.
VSL lifetime is more parallel to that of the main Visio application, so it's easier to make initialization and cleanup happen at the right times.
No risk of having the AddOn attempting to run while Visio is closed.

Challenges

Of the two languages that Visio has provided development materials for, VB can't generate a suitable VSL-style DLL, so that means developers have been constrained to using C++ to develop VSLs. The special requirements of a VSL are few compared to a regular DLL, yet, despite examples in "Developing Visio Solutions" many developers have found the process daunting because it requires mastering a number of issues which are relatively advanced uses of C++, including building and debugging a DLL, and mastering use of COM Automation from C++.

For those so inclined, Borland Delphi represents a nice alternative:

Hides most of the messy details of Automation, including reference counting, interface navigation etc. Most Visio VB examples are almost directly applicable with minor syntax change.
Can readily produce VSL DLLs.
Easily produces nice Forms for use in the AddOn.
(And a whole list of reasons why Delphi programmers have affection for Delphi, but that's another story.

However, Delphi developers have been stalled for want of a suitable starting framework, and in trying to translate the various aspects of the C++ DVS material supplied with Visio.

Features of Framework

I hope the DAVSL framework presented here goes a long way to get Delphi-savvy developers unstuck. I have decided to go beyond simply translating Visio's C++ units. Instead it makes sense to provide a more complete foundation which supports a number of the features that VSLs customarily need, and organize it in an object-oriented fashion. A minimal VSL can be created with you providing a unit that is as brief as:

unit DAVSLBeep_AddOns;
interface
implementation
uses SysUtils, classes, DAVAO, DAVSL;

Type
TAddOnBeepBeep = class(TAddOn)
    constructor Create;
    function vlm_RUN(V2LRec: PVAOV2LRecord; Args: TStringList): TVAORC; override;
end;

Var
AddOnBeepBeep: TAddOnBeepBeep;

constructor TAddOnBeepBeep.Create;
begin
inherited Create(
    VAO_AOATTS_ISACTION OR VAO_AOATTS_HASABOUT , // atts
    VAO_ENABLEALWAYS , // enabMask
    0 , // invokeOnMask
    0 , // notifyOnMask
    'DAVSLBeep_Beep' , // lpName
    'Minimal demo AddOn, simply beeps twice when called.'
);
end;

function TAddOnBeepBeep.vlm_RUN(V2LRec: PVAOV2LRecord; Args: TStringList): TVAORC;
begin
Beep;
Beep;
end;

initialization
AddOnBeepBeep := TAddOnBeepBeep.Create;
end.

In more detail:

To create a VSL, you copy a generic DAVSL project, rename the Project file (which contains the DLL entry function as defined by Visio), and add one or more application-specific AddOn units to that project, following the simple samples provided.
As defined by Visio, a VSL can contain numerous separate AddOn functions. DAVSL provides a manager for multiple AddOn objects. To add or remove AddOns all you do is add or remove units from your VSL project.
The resulting VSL:
- Handles the ENUMADDONS process to register your AddOns with Visio.
- Directs Visio's messages to your AddOns, keeping track of AddOn Ids transparently.
- Automatically parses any arguments into a TStringList as Keyword-Value pairs.
- VSL forms use Visio as parent and thus behave as windows belonging to Visio.
- Can look up and translate argument values to integers if you wish.
- Provides an About box which lists each AddOn and its arguments and permissible values. The About box also shows Version and other information from the Project Version resource -- the same Version info you see in Explorer, so you can set the internally and externally visible version info in one place.
A provided example shows how to work with AddAdvise to register for and receive event notifications from Visio.

In short, a minimal VSL can be set up with only a few lines of code, and can be expanded upon without excessive mental distress.

Copyright

These DAVSL pages and the associated DAVSL framework source code may be used freely without charge. However, I would appreciate a mention in documentation or About box if you find them useful and deploy them in your own projects. Please also note the contributions of others (see below) when assigning credit. Thanks!

Acknowledgements

The first Delphi VSL-specific code I am aware of was a Delphi 2 VSL framework for Visio 4, by folks at The Diagrammatic Programming Corporation (involved in some interesting work quite apart from VSLs!) .

Recently Iwan C. van Ee of Ravelin contacted me enquiring about Delphi VSL code, and I relayed the by that time somewhat outdated Diagrammatic Programming VSL framework. A few weeks later Iwan returned to me their faithful Delphi translation of the Visio 2000 VAO library. This got me thinking about how to make the job a little easier and more Delphi-oriented, resulting in what we have here.

Article Created: 2000-09-20 Last edit: Last edit: 00-10-30 Graham Wideman
Go to: DiagramAntics.com

Contents

Background

Challenges

Features of Framework

Copyright

Acknowledgements