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Enabling the Data-Binding Capabilities of an ActiveX Control

As you may recall from Chapter 8, many controls provide DataField and DataSource properties, so you can bind them to a particular field provided in the Recordset of a Data Control.

You can implement these fields and other data-bound fields as well with ActiveX custom controls. The following sections discuss how you can do this.

Providing DataSource and DataField Properties with the Procedure Attributes Dialog Box

A bound control must have a DataSource property to indicate the Data Control to bind to and a DataField property to indicate which field to bind from the DataSource's Recordset.

In addition, a moment's thought will tell you there is also a third property involved in this arrangement: a property that actually reflects the contents of the bound field. For a TextBox, this is the Text property, and for a Label, on the other hand, it's the Caption.

You can give your control DataField and DataSource properties and indicate a third property that will reflect the bound data by following these steps:

13.3 Giving Your Control DataField and DataSource Properties

  1. Change the UserControl's DataBindingBehavior property's value to 1-vbSimpleBound.

  2. Make sure your text cursor is in the code window for your UserControl, preferably in one of the Property procedures of the property you wish to reflect the bound data.

  3. Choose the Tools, Procedure Attributes option from the VB menu (see Figure 13.7).

    Specifying a property to bind to the DataField
    FIGURE 13.7 Specifying a property to bind to the DataField

  4. On the resulting dialog box, make sure the property that will reflect the bound data is chosen in the Names field.

  5. Click the Advanced button.

  6. In the Data Binding section at the bottom of the dialog, check that the box labeled Property is data bound, and then check that the box labeled This property binds to DataField.

If you follow these steps, then developers using your control will see the DataField and DataSource properties in the properties window of each instance of your control. If the developer points the DataSource and DataField properties to a valid Data Control and a valid field in the Data Control's Recordset, then the custom property you specified in step 3 above will reflect the contents of the underlying data field in the designated property.

The custom property's Property Let procedure will fire every time the underlying data in the specified field changes. To be completely safe in the Property Let procedure, you should call the CanPropertyChange method, as described in the following section.

Calling the CanPropertyChange Method Before Allowing a Property Value to Change

You may wonder what will happen if you implement a data-bound property as described in the previous section only to have a developer bind your property to a field in a read-only data source. Obviously, there will be a problem when there is an attempt to update the contents of the property and, therefore, the underlying data.

You therefore need to take precautions to ensure that an attempt to update your property won't cause some sort of runtime error if the underlying data it's bound to can't be updated.

The CanPropertyChange method is supposed to return a Boolean value indicating whether it's safe to attempt to update a property's value. If CanPropertyChange returns False, then it's not safe to update the data underlying the property. You can use CanPropertyChange in the Property Let procedure. Only if it returns True will you perform the actions needed to update the property's value, as illustrated in Listing 13.16.


Public Property Let LastName(ctrVal As String)
If CanPropertyChange("LastName") Then
m_LastName = ctrVal
PropertyChanged "LastName"
End If
End Property

NOTE - CanPropertyChange Doesn't Do Anything in Current Versions—Use Only for Future Compatability : Note that we said that CanPropertyChange is supposed to return a result indicating whether it's safe to update a property. As of Version 6 of VB, VB's CanPropertyChange method always returns True. Microsoft still recommends that you use it, however, as you may want it in place for future compatibility.

So how does VB currently handle an attempt to update a read-only data field if the CanPropertyChange method always says it's OK to update? VB raises no runtime errors and simply ignores the request to update the field.


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