.NET Table 17.8 CurrencyManager class

The CurrencyManager class represents a binding manager that is associated with a data source supporting the IList interface. This class is part of the System.Windows.Forms namespace, and inherits from the BindingManagerBase class. See .NET Table 17.7 for the members inherited from the base class.

From the table, it appears that the Refresh method is the solution we need. This method updates the bound controls with the underlying data. For classes that support the IBindingList interface, most notably the database-related objects, this method is not generally needed since the ItemChanged event will occur whenever the database object itself is modified. Do not confuse the ItemChanged event with the PositionChanged event, which occurs when a new item, or row, in the associated list is selected; or with the CurrentChanged event, which occurs when the control’s bound property is altered.

For our purposes, the default behavior that occurs when the Position or Current properties change will suffice. Since we do not support the IBindingList interface, we need to call the Refresh method directly when our PhotoAlbum is altered internally. In our current interface, this occurs each time a new album file is selected in the Album combo box control.

The following table continues our prior steps to alter our Changed event handler to invoke the Refresh method.

UPDATE THE SELECTEDINDEXCHANGED EVENT HANDLER

UPDATE THE SELECTEDINDEXCHANGED EVENT HANDLER (continued)

The controls in the Photo tab now update properly in all cases. You can compile and run this to experience the magic. Note how the index into the album, based on the Position property, is preserved when you change albums. If the second item in an album is shown and a new album is selected, the second item in the new album is selected. What happens when the number of photos in the current album is more than the number in a newly selected album?

TRY IT! We have not discussed the data binding for list controls very much here. As a way to see this in action, replace the txtPhotographer control with a combo box called cmbxPhotographer. Use data binding to automatically fill this list with the Photographer entries from the current album. This should be done in the OnLoad method and should look like the following:

cmbxPhotographer.DataSource = _album;

cmbxPhotographer.DataMember = "Photographer";

As you will see if you compile this change, all photographers assigned to the album are listed, even if they occur multiple times. Even so, this is a good example of how to populate a list quickly with values from a data source.

To provide a more robust implementation, implement a GetPhotographers method in the PhotoAlbum class that returns an array of unique photographer strings in the album. This array can then be set as the DataSource for the combo box. Be careful here, as you will need to update this setting in the SaveChanges method to accommodate any changes made to the list.

As already mentioned, when binding to a database, the IBindingList interface will ensure that the bound properties and the database object stay in sync. When either object is modified, the other is automatically updated.

One issue that remains in our current application is the display of our image in the PictureBox control. We will address this topic next.

17.4.4DISPLAYING THE IMAGE

While our application is working quite well, there is the small matter of our PictureBox control. As a simple solution, we could modify the SizeMode property of our PictureBox control to use the StretchImage value. This would stretch our image to fit the window, but would not preserve the aspect ratio of our images.

A better solution, as we know very well, is to scale the image to preserve the aspect ratio within the window. We can do this if we paint the image ourselves rather than provide a value for the Image property. The Paint event for the PictureBox control can be used to perform this painting, although we still need to know which image to paint. For this, we will use the Tag property to keep track of the current image.

Of course, as we mentioned earlier in the book, it would be nice to build a “PhotoBox” control that did this automatically. Such a control would extend the Windows Forms PictureBox control to scale an image as we do in the subsequent table. Coding this by hand is not much extra work, so we did this explicitly here. A short discussion on how to build such a “PhotoBox” control is given at the end of section 18.2.

The following steps are required to scale the image within the pboxPhoto control.

MODIFY THE PICTUREBOX CONTROL TO DISPLAY A SCALED IMAGE

MODIFY THE PICTUREBOX CONTROL TO DISPLAY A SCALED IMAGE (continued)

The photograph is now scaled and displayed within our PictureBox control. Note how we used the entire client rectangle rather than the rectangle provided in the Graphics object to ensure that the entire control area is redrawn.

It is worth noting here that binding to the Tag property is not strictly required. Since we access the current Photograph object here, we could simply load the associated image correctly. There may be a slight performance advantage here in having the image already available in the Tag property, so we selected the approach shown in the table.