19: Building Custom Controls in Visual Studio .NET

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19: Building Custom Controls
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However, when data in the control is changed, the get_Value method won t be called unless you tell InfoPath that the data in the data source needs to be updated Fortunately, since we re using ATL, doing this is simple When we initially implemented the ATL control class, we speci ed in the wizard that we wanted our control to implement the IPropertyNotifySink interface Through this interface, an ActiveX control alerts InfoPath of property changes in the control When the data in your control changes, you simply need to call the FireOnChanged method of the CComControl base class to alert InfoPath that it needs to update the data When InfoPath receives the noti cation, it will call the control s get_Value method again to update the data For example, in the SimpleInfoPathControl class, when the focus leaves the control, we call the FireOnChanged method, passing the dispatch ID of the Value property like so:
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FireOnChanged(DISPID_EDIT_VALUE);
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Now that you ve implemented the Enabled and Value properties, your control is ready to be used You can install it into the Controls task pane and insert it into your form templates When users ll out forms that contain your control, the control will work like the built-in InfoPath controls
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ActiveX Controls Must Be Installed and Registered
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Before users can ll out forms that contain your ActiveX control, it must be installed and registered This is a basic ActiveX concept that we want to reiterate If a control is not installed and registered on a user s machine, the control simply won t run at all (If the control is installed by Microsoft Windows or some other application, this is not an issue However, when creating your own ActiveX control, you must implement this functionality yourself) You can create a separate installation application to install your control or create a CAB le that you then include with your form template
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Implementing the InfoPathControl Interface
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In addition to the basic support for control properties and property pages, InfoPath also includes two custom interfaces that your control can use to interact with the InfoPath form
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Building Custom Controls for InfoPath Using ActiveX
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The rst interface is the InfoPathControl interface This interface, which can be implemented by the ActiveX control, provides methods that enable your control to know when certain events occur in the form, such as when the control is being initialized or uninitialized by InfoPath, when the control should persist any internal state, and when the control should enable or disable itself Table 193 lists the methods included in the InfoPathControl interface The methods listed in Table 193 give you a little better integration with the InfoPath form Two methods are particularly interesting The Enable method is called by InfoPath to enable or disable your control You may wonder why this is needed since your control can just expose an Enabled property Besides the fact that there may be scenarios in which you don t want to expose this property, implementing this method instead actually makes it easier for other form template designers to install your control When an ActiveX control implements the InfoPathControl interface, the Specify an Enable or Disable Property page of the Add Custom Control Wizard will be skipped since InfoPath knows that your control should be enabled or disabled by calling the Enable method on the InfoPathControl interface
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Table 193: InfoPathControl Interface Methods Method Init Description
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Is called when the control is being initialized The control receives a pointer to the InfoPathControlSite interface implemented by InfoPath Alerts the control that it is about to be destroyed The control should perform any necessary cleanup when this method is called Is called to tell the control to persist any internal state Certain changes in the form will cause the ActiveX control to be destroyed and recreated In the SaveState method, the control should persist data that it wishes to save from one instance of the control to another When Init is called, the control can reload the saved data (Tip: Your control can use the SetNamedNodeProperty and GetNamedNodeProperty methods in the InfoPath object model to store state) Specifies whether the control should be enabled or disabled
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