Set the version number of the MyPhotoAlbum library to 17.3.

SUPPORT THE IEDITABLEOBJECT INTERFACE IN THE PHOTOGRAPH CLASS

SUPPORT THE IEDITABLEOBJECT INTERFACE IN THE PHOTOGRAPH CLASS (continued)

The IEditableObject interface is now fully implemented. Another useful change in our library is the ability to identify if a PhotoAlbum, and not just a Photograph, has been modified. We can do this by continuing the previous steps to add a

HasEdits method in the PhotoAlbum class.

ADD A HASEDITS PROPERTY TO THE PHOTOALBUM CLASS

The MyPhotoAlbum library is ready to go. Make sure the library compiles with no errors. The next step is to make use of these changes in our MyAlbumData application. This is taken up in the next section.

17.3.3USING EDITABLE OBJECTS

Typically, you do not actually use the editable object methods directly. These are used

internally by Windows Forms as required for the task at hand. In this case, our DataGrid control automatically recognizes that our PhotoAlbum object supports this interface, and calls BeginEdit whenever a user initiates a change to a row in the grid. If a user cancels an edit by pressing the Esc key, then CancelEdit is called. When the

user finishes an edit by pressing the Enter key or selecting an alternate row, the EndEdit method is invoked. The EndEdit method makes the changes to the object permanent within the object itself.

In most applications, there is an operation or class that coordinates the in-mem- ory version of an object with the permanent version of an object. In our application, the in-memory version is our PhotoAlbum class, while the permanent version is our album file. The Save method updates the album file with the version in memory, while the Open method fills the in-memory version with the recorded version in the album file.

This is true in the System.Data namespace as well. While we have avoided discussing this namespace in any real depth, it is useful to understand how the classes in this namespace relate to our discussion. The abstract DataAdaptor class is the coordinator between the in-memory version of the database, typically a DataSet instance, and the permanent version is an external database. The DataAdaptor class provides a Fill method to populate a DataSet with the external values, and an Update method to save modifications in the DataSet into the external database.

For our purposes, we have provided the HasEdits method in our in-memory objects in order to identify whether any changes must be saved into the album file. We can do this in the SelectedIndexChanged event handler before the new album is bound to our data grid.

The following table details the steps required to save our PhotoAlbum instance into its associated album file:

Set the version number of the MyAlbumData application to 17.3.

SAVE A MODIFIED ALBUM (continued)

With these modifications, our edits are now saved. Compile and run to verify that the application works as advertised.

More .NET Additional information on the DataGrid control is available in the .NET documentation and in various sample programs freely available on the Internet. One such example is the CdrCatalog program built by Andrew Skowronski that is available at http://cdrcatalog.sourceforge.net/. This program manages offline media, such as a collection of recordable compact discs, and makes use of a number of Windows Forms classes in addition to the DataGrid control. The data source used by this application is a DataSet object loaded from a local XML file.

17.4SIMPLE DATA BINDING

Binding data to a data grid is referred to as complex data binding, since multiple values are bound to a single control. Complex data binding also refers to binding objects to a list control, such as a list box or combo box. For example, if we had actually implemented the AlbumCollection class I keep mentioning to contain an array of PhotoAlbum objects, then the line to assign the DataSource for our combo box could instead be written as:

cmbxAlbum.DataSource = myAlbumCollection;

cmbxAlbum.DataMember = "FileName";

This would display the collection of PhotoAlbum objects in the myAlbumCollection variable in the cmbxAlbum combo box, using the FileName property as the

name to display. The result would be the same as that which currently appears in our application. This type of binding to a list control can also be done with database objects, such as binding the entries in a ListBox control to the set of customer names found in one column of a database table.

Simple data binding is used for binding single property values to a specific data source. This type of binding is supported by the Control class directly, and is therefore inherited by and available in all controls in Windows Forms. The concepts and techniques for so-called simple data binding are fairly identical to those we have already discussed for the DataGrid control.

In this section we will alter our application to permit some simple data binding to a photo album. We will see how to perform simple binding; update bound controls dynamically, including the image associated with a photograph; and save changes to bound controls.

17.4.1ALTERING THE MYALBUMDATA APPLICATION

Before we get into the details of exactly how simple data binding is performed, let’s whip through some changes to our MyAlbumData application in preparation for this discussion. The change we will make is to place our existing DataGrid control within a TabPage object, and add a new tab to display the Photograph information for an album one photo at a time. Figure 17.6 shows the modified application we will build throughout this and the next few sections.

Figure 17.6 These controls on the Photo tab are bound to their corresponding values in a Photograph object.