Supporting Object Edit Notifications with Property Change Events in .NET

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Supporting Object Edit Notifications with Property Change Events
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Another problem people frequently encounter when first trying to use collections of objects as their data source is detecting changes in the bound data objects if those changes aren't made directly through the bound control getting or setting properties on the object When you change an object property by editing it through a bound control, the control will change the values through the property descriptors for the object Containers for those objects, such as the binding source, can hook up a callback to be notified when the property values are changed through the property descriptor (as discussed in 7 and demonstrated later in this chapter) When this happens, other controls that are bound directly or indirectly to the same source can be updated by the container raising ListChanged events on the IBindingList interface implementation However, if an object property's value changes behind the scenes due to other means, the values presented in a bound control won't automatically be updated For example, you could have a background thread that is continuously updating the objects based on some activity that it is monitoring, or you could have another control in the form, such as a button, invoke some code that modifies an object in memory directly For example, if you add a button to your form in the CustomBusinessObjectsClient example, and call the following code from that button's Click event handler, the first object in the customers list collection would have its CustomerName property changed to IDesign However, the customers grid would not immediately update with the changed value You would have to perform some interaction with the grid or form to make it refresh its data binding to see the changed customer value private void button1_Click(object sender, EventArgs e) { List<Customer> custs = m_CustomersBindingSourceDataSource as List<Customer>; custs[0]CustomerName = "IDesign"; }
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One way to address this problem is to implement property change events on the Customer object (or any object that you plan to support data binding against) Property change events are events that you define on an object that notify anyone who is interested any time a particular property on the object changes For the data-binding mechanisms in Windows Forms, the expected naming convention for these events is to publish a <propertyname>Changed event for each property, where <propertyname> is replaced with the name of the property in question For your Customer object and your limited support for changes against theCustomerName and CustomerId properties, this means defining two events on the class: CustomerNameChanged and CustomerIdChanged These events need to be of type EventHandler and should be fired whenever the corresponding properties change This can easily be done from the set blocks of the properties themselves, as shown in the following modified code for the Customer class: public class Customer : IEditableObject { // other fields public event EventHandler CustomerIdChanged; public event EventHandler CustomerNameChanged; public int CustomerId { get { return m_CustomerId; } set { m_CustomerId = value; if (CustomerIdChanged != null) CustomerIdChanged(this, EventArgsEmpty);
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This document was created by an unregistered ChmMagic, please go to http://wwwbisentercom to register it Thanks } } public string CustomerName { get { return m_Name; } set { m_Name = value; if (CustomerNameChanged != null) CustomerNameChanged(this, EventArgsEmpty); } } // other methods and properties }
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By making this simple change to the class, any bound controls will be automatically notified whenever an object's properties change, whether it is due to editing those objects in a data-bound control or due to some other code execution that sets the properties in memory Basically, the data-binding mechanisms detect the presence of these methods and set up subscriptions to them so that they can refresh their values when the data values change Note that if you write your own data-bound controls from scratch as discussed in the last chapter, you will have to take responsibility for performing these subscriptions yourself So with this code in place, if you press the button added earlier that directly modified properties on objects in the collection of customers, the grid would immediately reflect the change You will also see that if you edit a cell in the customers grid, then press Esc to abort the change, the Customer field in the orders grid will also update based on that rollback, because the control will call CancelEdit If you hadn't done the rollback by setting the properties in the implementation of CancelEdit as mentioned earlier, then the property change events wouldn't fire, and any other controls bound to the same objects wouldn't be immediately updated You could alternatively have fired the appropriate property change events from CancelEdit instead, but that would result in more code duplication than just setting the properties in CancelEdit, which call whatever related behavior is needed through the property set block
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This document was created by an unregistered ChmMagic, please go to http://wwwbisentercom to register it Thanks
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