Development | Starting Out

Review of Odyssey Software's free DataViewer control for eVB
Written by Pete Vickers  [author's bio]  [read 47547 times]
Edited by Derek

         

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[Editor's note:] The day after the Pete's last article on the performance grid article was published, we were contacted by Odyssey Software asking if we wanted to test their virtual list view. What better person to review Odyssey's new (and free) control than the grid performance guru himself - Pete Vickers. Download this article's sample code and also check out Odyssey Software Developer Web Site. If you do contact Odyssey please mention this article. [Editor's note:]

Introduction

Odyssey are providing this control FREE to the developer community, and ROYALTY FREE distribution. As a software developer who buys controls, that is the kind of price I like. Then again, as a software developer who sells controls, I don't like the precedent. I think it is great of Odyssey to donate this control. This article will put it through its paces, and see if it is better than the grid, or normal listview.

I probably approached this from a jaundiced point of view, as I am a grizzled old developer, who generally has no truck with these newfangled 'bound' or ‘virtual’ controls. They make the job too easy - what is wrong with coding absolutely everything? Still, pushing aside my prejudices, away we go.

To quote from Odyssey...
"The DataViewer is very fast and much better than a regular ListView, Grid, or ComboBox. Especially the ComboBox because you can display multiple columns with column headings in the simulated combo (scrolling both vertically & horizontally), yet you can choose what column to return to the TextBox (used to hold the selection) when the user selects a record from the list. The main principal behind the DataViewer is that you are not loading up the control with a copy of your data, which the control then displays. Instead, you tell the control that you have data (and how much) and it will then ask you ONLY for the data it needs for the current viewport (displayable area of the DataViewer). So, it is really a virtual ListView control (report mode only). That is why the rendering speed of your data is always the same (typically < 1 sec.), regardless of the amount of data you have whether its 1 row or 100,000 rows.

This allows you to pull and display data directly from the data source (in most cases an ADOCE.Recordset) without loading a copy of the data in into the control (much more memory efficient and of course must faster)."

This fits in with what people have done with the grid control in the past, i.e. 'page' records.

I received a sample application from Odyssey, and used that as a test to see how easy the control was to use. I then developed an application (two as it turned out) from scratch, to gauge it from how easy it was to integrate into an application. Their sample used the northwind database. I decided to build a simple test database from scratch, and allow the developer to populate it with as many (or few) records as he liked, to test out the performance. This also allowed the sample application to demonstrate the use of Menus, and a simple progress bar. I also decided to add a simple edit facility.

The design of the form finished up as this:

The application would therefore allow us to:

• Browse Records
• Show a Tap and Hold
• Respond to Tap and Hold
• Retrieve records using 'movenext'
• Retrieve records using getrows

Our menu has 4 items under the database menu:

1. Drop the database
2. Create the database
3. Create the table
4. Populate the table

In addition, we have an 'options' menu that will allow us to choose whether to use 'getrows' or ‘movenext’.

The real engine of the DataViewer is in the 'GetItemText', and is the one that is probably the most difficult to get your head around (it was for me, but hey, the brain cells are now dying rapidly!). The 'GetItemText' is much dependent on the rowcount property.

Effectively, we set the .rowcount to the number of the records in the recordset. We then give the dataview control the focus. This fires the 'GetItemText' event, passing in the row and column to popuplate...

Private Sub dvRecs_GetItemText(ByVal Row As Long _
, ByVal Column As Long, Text As Variant)

So, the very first time in, we are sent row = 1, column = 1. We then need to populate the 'text'. This is done here by a 'general' subroutine...

Text = CStr(GetRecordsetFieldValue _
(rs, Row, Column))

Private Function GetRecordsetFieldValue _
(rsSet As ADOCE.Recordset, _
lngRow As Long, lngColumn As Long) _
As Variant
If Not (rsSet.BOF And rsRecordset.EOF) Then
If Not rsSet.AbsolutePosition = lngRow Then
rsSet.Move (lngRow - rsSet.AbsolutePosition)
End If
If IsNull(rsSet(lngColumn - 1).Value) Then
GetRecordsetFieldValue = "(null)"
Else
GetRecordsetFieldValue = _
rsSet(lngColumn - 1).Value
End If
Else
GetRecordsetFieldValue = ""
End If
End Function

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