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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() End If 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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The ApplyWriteRules Method
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The ApplyWriteRules() method is 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 of its Enabled property. As an optimization, if the control is a Label, the method will exit immediately. 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() End If End If 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() End If 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.
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CHAPTER 10 DATA B INDING
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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() End If The end result is that when the user is denied write access to a business object s property, controls bound to that property are either set to ReadOnly or are disabled. And if the user is denied read access to a business object s property, controls bound to that property are disabled and empty values are placed in the control rather than any real values from the business object.
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The BindingSourceRefresh Control
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The BindingSourceRefresh control is also an extender control, but its purpose is quite different from the ReadWriteAuthorization control. It turns out that there s a quirk in the way Windows Forms data binding works. The BindingSourceRefresh control helps work around this quirk. The quirk is that when data is changed in a business object, data binding doesn t always display the changes in the controls on the form. This occurs in the following sequence of events: 1. The user edits a value in a bound control. 2. Data binding puts the user s new value into the business object. 3. The business object alters the value in the property Set block. 4. The business object raises its PropertyChanged event. You would expect that data binding would handle the PropertyChanged event, realize that the property s data has changed, and then update the control with the new value. And that does happen for all controls except the current control. In other words, the PropertyChanged event causes data binding to refresh all other controls on the form except the control that initiated the change in the first place. Obviously, this can be problematic. Consider a TextBox control that is bound to a business object property that uses a SmartDate. I ll discuss SmartDate in 16, but one of its features is to accept the + character as input and to replace it with tomorrow s date. Because of this data binding quirk, when the user enters a + character, that value is put into the business object, which translates it to tomorrow s date but that new value is not displayed to the user. The user continues to see the + character. What s even more confusing for users is that if they edit a different control, then the date text box control will be updated with tomorrow s date. Remember that data binding updates everything except the current control when it gets a PropertyChanged event. This is the problem BindingSourceRefresh is intended to solve. It does so by interacting with the BindingSource control that manages the data binding for a given business object. While ReadWriteAuthorization extends controls like TextBox and Label, BindingSourceRefresh extends BindingSource controls. The plumbing code in this control is virtually identical to ReadWriteAuthorization, so I won t walk through all the details of the control. This control extends only BindingSource controls, and if it is enabled, it hooks the BindingComplete event raised by the BindingSource control. This event is raised by a BindingSource control after all controls have had their values updated through data binding well, all controls except the current one, of course. The Control_BindingComplete() method takes the extra step of forcing the BindingSource control to refresh the value for the current binding as well.
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