| VSFlexGrid Control > VSFlexGrid Properties, Events, and Methods > VSFlexGrid Properties > FlexDataSource Property |
Returns or sets a custom data source for the control.
[form!]VSFlexGrid.FlexDataSource[ = IVSFlexDataSource ]
The VSFlexGrid control can be bound to several types of data source, including ADO or DAO recordsets, Variant arrays, and other VSFlexGrid controls. The FlexDataSource property is yet another option, based on a custom COM interface that is easy to implement and very flexible.
The main advantages of data-binding through the FlexDataSource property are speed and flexibility. The main disadvantage is that you have to write more code than with the other options. You should consider using the FlexDataSource property when you have large amounts of data stored in custom structures or objects (other than database recordsets). By using the FlexDataSource property, you may display and edit the data in-place. There is no need to copy it to the grid and save it back later. In fact, the data may even be mostly virtual, consisting of dynamically calculated values rather than static information.
When you assign a new data source to the FlexDataSource property, the control will automatically set each column's header text and ColKey property based on the data source's field name. You may change these values after setting the FlexDataSource property, if you wish.
Note
When bound to a FlexDataSource, values on fixed rows are not regarded as bound data. You may set or retrieve them with code without affecting the data source. Fixed columns, on the other hand, are regarded as bound data. Their contents are read from and written to the data source.
To bind the VSFlexGrid control to a FlexDataSource property, you need to implement an object that exposes the IVSFlexDataSource COM interface. The interface is very simple. It consists of only two methods: GetData and SetData. GetData(Row, Col) returns a string to be displayed by the grid at the specified position. It is called by the grid when it needs to display a value. SetData(Row, Col, Data) updates the data source at the specified position with the new value. It is called by the grid when the user edits a cell.
The distribution CD includes sample code that show how to implement and use the FlexDataSource object in a fairly realistic scenario, both in Visual Basic and Visual C++. The following tutorial is a very simple example of how to use the FlexDataSource property.
Step 1: Create the project
Start a new Visual Basic project and add a VSFlexGrid control to the form (you may use any version: ADO, DAO, or Light). Chang the name of the grid control to fs.
Step 2: Create the FlexDataSource object
Add a new class module to your project by selecting the "Project|Add Class Module" command from the Visual Basic menu. On the new class code window, type the following statement:
Implements IVSFlexDataSource
This tells the world that the new class implements the IVSFlexDataSource methods and that they are available to anyone who cares to use them. Now select the IVSFlexDataSource item from the "Object Box" (the drop-down list on the top left of the code window). VB will immediately create a "stub" (empty function) for the GetFieldCount method. Now click on the "Procedures/Events Box" (the drop-down list on the top right of the code window) and select each method to create stubs for each one. Here is what the code window should look like by now:
Option Explicit
Implements IVSFlexDataSource
Private Function IVSFlexDataSource_GetFieldCount() As Long
End Function
Private Function IVSFlexDataSource_GetFieldName(ByVal Field As Long) As String
End Function
Private Function IVSFlexDataSource_GetRecordCount() As Long
End Function
Private Function IVSFlexDataSource_GetData(ByVal Field As Long, ByVal Record As Long) As String
End Function
Private Sub IVSFlexDataSource_SetData(ByVal Field As Long, ByVal Record As Long, ByVal newData As String)
End Sub
Step 3: Implement the Data Structure
In this example, the data will be completely virtual. We will simply display a table of angles in degrees and radians, their sines, and co-sines. The table will have four "fields" and 360 "records". Here is the code needed to implement this structure:
Private Function IVSFlexDataSource_GetFieldCount() As Long
IVSFlexDataSource_GetFieldCount = 4
End Function
Private Function IVSFlexDataSource_GetRecordCount() As Long
IVSFlexDataSource_GetRecordCount = 360
End Function
Private Function IVSFlexDataSource_GetFieldName(ByVal Field As Long) As String
Select Case Field
Case 0: IVSFlexDataSource_GetFieldName = "Angle (Deg)"
Case 1: IVSFlexDataSource_GetFieldName = "Angle (Rad)"
Case 2: IVSFlexDataSource_GetFieldName = "Sine"
Case 3: IVSFlexDataSource_GetFieldName = "Co-Sine"
End Select
End Function
Now that we have defined the data structure, we need to implement the functions that will supply the actual data.
Step 3: Implement the GetData and SetData methods
The GetData method is responsible for providing data to the consumer. In this example, there is no static data, only calculated fields:
Private Function IVSFlexDataSource_GetData(ByVal Field As Long, ByVal Record As Long) As String
Select Case Field
Case 0: IVSFlexDataSource_GetData = Record
Case 1: IVSFlexDataSource_GetData = Record / 180# * 3.1416
Case 2: IVSFlexDataSource_GetData = Sin(Record / 180# * 3.1416)
Case 3: IVSFlexDataSource_GetData = Cos(Record / 180# * 3.1416)
End Select
End Function
The SetData method is responsible for updating the data new information supplied by the user (e.g. by editing a grid cell). In this case, the data cannot be changed, so any attempt to do it will simply raise an error:
Private Sub IVSFlexDataSource_SetData(ByVal Field As Long, ByVal Record As Long, ByVal newData As String)
Err.Raise 666, "IVSFlexDataSource", "This data is read-only."
End Sub
The SetData method is trivial in this case, but in a more realistic application it could be used to perform data-validation and to allow editing of certain columns only.
Step 5: Hook up the VSFlexGrid and the Data Provider
Now that the FlexDataSource object is ready, all we need to do is hook it up to the grid. Double click on the main form (the one with the grid on it), and add the following code to the Form_Load event:
Private Sub Form_Load()
Dim fds As New Class1 ' create the data source object
fg.FlexDataSource = fds ' assign it to the grid
fg.ColFormat(-1) = "#.##" ' format grid columns
fg.ColFormat(0) = ""
End Sub
Run the project and you will see that it loads very quickly, and displays the information as expected. Just for fun, try changing the value returned by the GetRecordCount method to a really large value (say 500,000 or so) and run the project again. You will notice there's little or no speed degradation.
This is the end of the tutorial. For more details, refer to the samples on the distribution CD.
IVSFlexDataSource