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C HAPTE R 10 DA TA BINDIN G
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The INotifyPropertyChanging Interface
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The INotifyPropertyChanging interface is almost identical to INotifyPropertyChanged. The only real difference is the timing of when the PropertyChanging event is raised as opposed to the PropertyChanged event. When a property value is about to be changed, the PropertyChanging event is raised first. Then the value is actually changed, and then the PropertyChanged event is raised. The methods in BusinessBase, such as the LoadPropertyValue() method, handle this. That method contains this code: OnPropertyChanging(propertyInfo.Name) FieldManager.SetFieldData(Of P)( _ propertyInfo, newValue) PropertyHasChanged(propertyInfo.Name) You can see the steps here clearly, including how both events are raised I won t walk through the code for INotifyPropertyChanging here, because it is so similar to INotifyPropertyChanged. You can look at the code in BindableBase on your own.
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The IEditableObject Interface
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The IEditableObject interface allows data binding to tell your object to undo recent changes or accept recent changes to the object s properties. It is used in many scenarios by Windows Forms data binding, including when an object is bound to a form using a BindingSource control and when a collection of objects is bound to any grid control that supports in-place editing of the data. The IEditableObject interface appears simple on the surface, but it s actually the most complex interface I ll discuss in this chapter. This is because there are a number of subtle idiosyncrasies in how this interface is called by data binding, and any object implementing the interface must deal with all the resulting edge cases. The interface defines the three methods listed in Table 10-2.
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Table 10-2. Methods Defined by IEditableObject
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BeginEdit CancelEdit EndEdit
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Called by data binding to tell the object to take a snapshot of its property values Called by data binding to tell the object to undo any changes, restoring the property values to the snapshot taken when BeginEdit() was called Called by data binding to tell the object to accept any changes, effectively discarding the snapshot taken when BeginEdit() was called
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This doesn t sound that hard, until you read the fine print in the documentation and start using the interface and then discover the even finer print that isn t actually in the documentation at all.
Note Many of the lessons I ve learned and the issues I ve solved in the data binding implementation in CSLA .NET are due to a lot of hard work and research by the CSLA .NET community at http://forums.lhotka.net. Without the strong support and involvement by numerous people, many of the hard challenges would likely still be unsolved.
CHAPTER 10 DATA B INDING
For example, it turns out that BeginEdit() may be called any number of times by data binding, but your implementation should honor only the first call. That first call to BeginEdit() must be balanced out by a subsequent call to either CancelEdit() or EndEdit(), at which point the next BeginEdit() call should be honored. It is also the case that EndEdit() can be called multiple times, though only the first call should be honored. Worse yet, there are scenarios where neither CancelEdit() nor EndEdit() are called at all, leaving the object under the impression that it is still in the process of being edited. This interface is implemented in BusinessBase. The implementation of this interface is related to the implementation of n-level undo, which I ll discuss in 13. The n-level undo feature already has the capability of taking a snapshot of the object s property values (actually its field values) and restoring them later if an undo is requested. The only difference is that IEditableObject is a single level of undo, while the n-level undo feature obviously supports multiple levels of undo.
The DisableIEditableObject Property
Some people have found the IEditableObject interface to be extremely challenging, especially if they are trying to create a UI that doesn t want to use all the data binding features. Windows Forms data binding uses this interface in numerous scenarios, and it can sometimes make life harder rather than easier. To minimize this pain, it is possible to disable the interface by setting a DisableIEditableObject property on a specific business object.
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