Bewerken

Delen via

Writing WIA Item Properties by an Application

  • Artikel

When a WIA application writes to a WIA property (and updates the value stored in the property), the WIA service gives the WIA minidriver the opportunity to validate incoming values by calling the IWiaMiniDrv::drvValidateItemProperties method. The WIA minidriver compares the incoming values to any current values by reading the properties in its own driver item tree. The WIA service library provides functions for accessing these values.

The IWiaMiniDrv::drvValidateItemProperties method should perform the following tasks:

  1. Determine the item type.

  2. Determine whether any special validation should be performed on the incoming WIA properties. To determine which WIA properties are being written, the WIA minidriver can use an array of PROPSPEC structures (the PROPSPEC structure is described in the Microsoft Windows SDK documentation). It is recommended that the WIA minidriver determine the item type before it processes the PROPSPEC array to reduce the need to traverse the array on every IWiaMiniDrv::drvValidateItemProperties call. If there are no special validation requirements, or if you need to update dependent properties on the root item of the device, only write requests to child item properties will be processed.

  3. Create a WIA property context to access values that changed during WIA property validation, which is necessary to update the dependent properties for a WIA item. Use the wiasCreatePropContext and wiasGetChangedValueXxx service functions.

  4. Update dependent properties using the WIA service functions, wiasWriteMultiple or wiasWritePropXxx, which includes updating any valid values that might have changed as a result of setting a property. For example, if your WIA minidriver supports setting the WIA_IPA_DEPTH property, you must update the valid list of bit depths when the application changes the WIA_IPA_DATATYPE property.

    When the value of the WIA_IPA_DATATYPE property changes from WIA_DATA_THRESHOLD to WIA_DATA_COLOR, the related WIA_IPA_DEPTH property changes from reporting one-bit color depth, to reporting 24 bits or 48 bits.

  5. Call the wiasValidateItemProperties service function to let the WIA service validate all other property requests. This is a "catch-all" case; the WIA service has built-in property validation.