Using the QCheckBoxColumn
In this example we will take our Paginated QDataGrid, and add a column which has a
"Select" checkbox. Sound familiar? Yep, the last example achieved
the same goal by creating checkbox controls on the fly. Well, this time, we're going to do it
using a simpler method. This time around we're using the QCheckBoxColumn, and letting
it do the heavy lifting for us. :)
So instead of having to create the checkboxes ourselves, and loop through child controls,
we just create the QCheckBoxColumn, and we can use the column's GetSelectedItems() function
to get the list of selected people to fill the label with. Note that the column itself doesn't know
what object type the items in it are, so you need to pass the class name to the
GetSelectedItems function.
In the previous example, we triggered a server hit everytime a checkbox was clicked. Our first
datagrid replicates this by specifying a callback for when the checkbox is rendered
and then adding an action to each checkbox, but usually you will want to wait until a seperate save
button to be hit in order to reduce server activity. It is also a good idea to avoid click acitons
on a QCheckBoxColumn because it is impossible to add an action to the Select All checkbox.
The second example is a much more practical example. In this case, we're displaying a
many-to-many relationship and allowing the user to select a Project that should be associated with
the current one (in this case, ACME Website Redesign). This is exactly what the MetaControls do for
association tables.
First, we once again create the QCheckBoxColumn, and again we call
SetCheckboxCallback, but this time it's in order to allow us to set the initial state of
the checkbox based on if an association already exists. The most efficient way to get this
information is by setting a virtual attribute as part of the bind function, as shown.
Finally, we'll want to act on the changed checkboxes once the user clicks Go. To do so,
instead of calling GetSelectedItems like we did before, we want to call
GetChangedIds. The reason for this is that there may be associated items on later pages
that never had checkboxes created for them. And those items won't show up on a simple list of
selected checkboxes, but we don't want to accidently unassociate their related items. It's also
nice that the QCheckBoxColumn takes care of figuring out which items have been deselected
for us as well.
Once we have those changed item's Ids, we can do what we'd like with them. In this example
we just output them to the user via javascript alerts. When the MetaControls do this
for association tables, they actually perform the described associations.
Finaly, you should note that using a QCheckBoxColumn requires that the databound object's
primary key be a single column named Id.
