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At times, applications need to retrieve a collection of child objects directly. To do this, you need to create a root collection object. For instance, the application may have a WPF UI consisting of a ListBox control that displays a collection of Contact objects. If the root object is a collection of child Contact objects, the UI developer can simply bind the collection to the ListBox (with an appropriate data template), and the user can edit all the objects in the list. This approach means that all the child objects are handled as a single unit in terms of data access. They are loaded into the collection to start with, so the user can interact with all of them and then save them as a batch when all edits are complete. This is only subtly different from having a regular root object that has a collection of child objects. Figure 5-1 shows the regular root object approach on the left, and the collection root object approach on the right.
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CHAPTER 5 CSLA . NET OBJECT TEMPLATES
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Figure 5-1. Comparing simple root objects (left) and collection root objects (right) This approach isn t recommended when there are large numbers of potential child objects, because the retrieval process can become too slow. However, it can be useful in cases where you can specify criteria to limit the number of objects returned. To create an editable root collection object, use a template like this: [Serializable] public class EditableRootList : BusinessListBase<EditableRootList, EditableChild> { #region Authorization Rules private static void AddObjectAuthorizationRules() { // TODO: add authorization rules //AuthorizationRules.AllowGet(typeof(EditableRootList), "Role"); } #endregion #region Factory Methods public static EditableRootList NewEditableRootList() { return DataPortal.Create<EditableRootList>(); } public static EditableRootList GetEditableRootList(int id) { return DataPortal.Fetch<EditableRootList>( new SingleCriteria<EditableRootList, int>(id)); }
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CH A PT ER 5 C SLA .NET O BJECT T EMPLAT ES
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private EditableRootList() { /* Require use of factory methods */ } #endregion #region Data Access private void DataPortal_Fetch( SingleCriteria<EditableRootList, int> criteria) { RaiseListChangedEvents = false; // TODO: load values into memory object childData = null; foreach (var item in (List<object>)childData) this.Add(EditableChild.GetEditableChild(childData)); RaiseListChangedEvents = true; } #endregion } The Authorization Rules region implements the AddObjectAuthorizationRules() method to define the roles that can interact with the object. This is the same as with an editable root object, and the UI developer can use this information to enable and disable UI elements accordingly. Also, the data portal uses these rules to ensure only authorized users are able to create, get, edit, or delete the collection object. The one difference is that this AddObjectAuthorizationRules() method only needs to define the roles allowed to get the object. The Factory Methods region implements factory methods to create, retrieve, and (optionally) delete the collection. The methods rely on the data portal to do much of the work, ultimately delegating the call to the appropriate DataPortal_XYZ method. In the Data Access region, the DataPortal_Fetch() method is responsible for getting the data from the database using whatever data access technology you choose. This is often LINQ to SQL, the ADO.NET Entity Framework, or raw ADO.NET. In any case, the Data Access layer must return the child data as a result so it can use it to load each individual child object with data. You load each child object by calling the child object s factory method, passing the object s data as a parameter. The resulting child object is added to the collection. The DataPortal_Fetch() method sets the RaiseListChanged Events property to false before changing the collection, and then restores it to true once the operation is complete. Setting this property to false tells the base BindingList<T> class to stop raising the ListChanged event. When doing batches of updates or changes to a collection, this can increase performance. The BusinessListBase class includes a default implementation of the DataPortal_Update() method, which loops through all child objects (and deleted child objects) to ensure the appropriate Child_XYZ method is called on each when the collection is being saved. Normally this is the desired behavior, but you can override DataPortal_Update() if you need to take control of the update process for some unusual scenario. As with the editable root stereotype, you can use the ObjectFactory attribute to have the data portal invoke an object factory rather than invoke the DataPortal_XYZ methods directly in the business class. Collection objects that inherit from BusinessListBase automatically support data binding in WPF, Windows Forms, and Web Forms. However, if you bind a collection to a Windows Forms DataGrid control, you may be surprised to find that the user can t just add new items by going to the bottom of the grid. You need to write a bit of extra code in your collection to enable this behavior.
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