s COMPLETING THE FRAMEWORK in Visual Basic .NET

Make PDF-417 2d barcode in Visual Basic .NET s COMPLETING THE FRAMEWORK

CHAPTER 5 s COMPLETING THE FRAMEWORK
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ApplyReadRules Method
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Finally, we get to the heart of the matter: altering the target control. If the user is not allowed to read the property value, the target control must not display the value. To prevent display of the value, two things are done to the target control: it is disabled, and any values coming from the data source to the control are intercepted and replaced with an empty value. Disabling the control is easily accomplished by setting its Enabled property to False. All controls have an Enabled property, so this is not an issue. Intercepting all values from the data source before they reach the control is more complex. Fortunately, data binding offers a solution through the Format event. All Binding objects have both Format and Parse events, which can be used to alter data as it moves from the data source to the control and then back to the data source. The Format event is raised after the data value has been read from the data source, but before the value is provided to the control. The idea is that a UI developer can handle this event and use it to format the value for display. In this case, however, the value will simply be replaced with a default empty value instead, thus ensuring that the control never gets the real value that the user isn t authorized to see. To handle the Format event, a method is required: Private Sub ReturnEmpty( _ ByVal sender As Object, ByVal e As ConvertEventArgs) e.Value = GetEmptyValue(e.DesiredType) End Sub Private Function GetEmptyValue(ByVal desiredType As Type) As Object Dim result As Object = Nothing If desiredType.IsValueType Then result = Activator.CreateInstance(desiredType) End If Return result End Function The ReturnEmpty() method handles the Format event. It then calls GetEmptyValue() to get an empty value appropriate for the data type of the value read from the data source. That empty value is returned through e.Value. The result is that data binding puts this empty value rather than the original value from the data source into the control. Within the ApplyReadRules() method, if the user is not authorized to read the property, the control is disabled and the event handler is set up: ctl.Enabled = False AddHandler binding.Format, AddressOf ReturnEmpty ' clear the value displayed by the control Dim propertyInfo As PropertyInfo = _ ctl.GetType.GetProperty(binding.PropertyName, _ BindingFlags.FlattenHierarchy Or _ BindingFlags.Instance Or _ BindingFlags.Public) If propertyInfo IsNot Nothing Then propertyInfo.SetValue(ctl, _ GetEmptyValue( _ Utilities.GetPropertyType(propertyInfo.PropertyType)), _ New Object() {}) End If Of course, the control might have already contained a value, and if so, that value must be removed. To do this, the type of the property value is retrieved using reflection, and the
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CHAPTER 5 s COMPLETING THE FRAMEWORK
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GetEmptyValue() method is called to get an appropriate empty value. This value is then placed into the control, overwriting any previous value the control may have had. The reverse of the process occurs if the user is allowed to read the property. In that case, the control is enabled and the Format event handler is removed: Dim couldRead As Boolean = ctl.Enabled ctl.Enabled = True RemoveHandler binding.Format, AddressOf ReturnEmpty If Not couldRead Then binding.ReadValue() Additionally, if the control was disabled before this code was run, it is assumed that the control doesn t contain a valid value. The ReadValue() method on the Binding object is called to force data binding to reload the control with the value from the data source.
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ApplyWriteRules Method
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The ApplyWriteRules() method is very similar to ApplyReadRules(), but takes a slightly different approach. In this case, users may be able to view the data, but they certainly can t be allowed to edit the data. If the control implements a ReadOnly property, then it can be set to False; otherwise, the control must be entirely disabled through the use its Enabled property. As an optimization, if the control is a Label, the method immediately exits. Because Label controls are so common, and they are read-only by definition, it is worth this special check. The preference is to use the control s ReadOnly property if it is implemented by the control. Reflection is used to get a PropertyInfo object corresponding to the control s ReadOnly property: ' enable/disable writing of the value Dim propertyInfo As PropertyInfo = _ ctl.GetType.GetProperty("ReadOnly", _ BindingFlags.FlattenHierarchy Or _ BindingFlags.Instance Or _ BindingFlags.Public) If propertyInfo IsNot Nothing Then Dim couldWrite As Boolean = _ Not CBool(propertyInfo.GetValue(ctl, New Object() {})) propertyInfo.SetValue(ctl, Not canWrite, New Object() {}) If Not couldWrite AndAlso canWrite Then binding.ReadValue() If a ReadOnly property is found, then it is set to True or False depending on whether the user is allowed or denied write access to the business object property: propertyInfo.SetValue(ctl, Not canWrite, New Object() {}) First, though, the value of the control s ReadOnly property is retrieved. If it is False, that means that the user was already able to edit the control the user could write, so couldWrite is True. This is important, because if the user was unable to edit the control, and now is able to edit the control, data binding needs to be told to reload the data from the data source into the control: If Not couldWrite AndAlso canWrite Then binding.ReadValue() Otherwise, it is possible for the user to be placed into an empty control even though there really is a value in the business object s property. If the control doesn t have a ReadOnly property, then the Enabled property is used as a fallback. The same procedure is used, just with the Enabled property instead: Dim couldWrite As Boolean = ctl.Enabled ctl.Enabled = canWrite If Not couldWrite AndAlso canWrite Then binding.ReadValue()
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