Using the DataGridCheckboxColumn
In this example we will take our Paginated DataGrid, and add a column which has a
"Select" checkbox. Checkbox columns that are part of a paginated datagrid can be tricky to manage.
Remember that a paginated control only shows a portion of the overall data. Since the data could be
10,0000 records or more, we want to avoid having to query the entire database every time a checkbox changes. However,
we might have to do that if the user checks the "Check All" box.
Further complicating the situation is that more than one person may be changing the data at the same time.
In our examples here, we address two situations:
- The checkboxes represent a selection of rows which you would like to act upon later. In this situation, the
state of the selection is unique to each user, and a private copy of the selection is maintained in the _SESSION
- The checkboxes represent a boolean value that is in the database itself. Changing the checkbox immediately changes the
corresponding value in the database.
A situation we don't address here is if the checkboxes represent boolean values that are part of the data, but changing
a checkbox will not change the corresponding data in the database immediately, but only after a Save button is pressed.
This is certainly doable, but the
problem with this scenario is that in a multi-user environment, the last person who presses the save button might
unknowingly replace values recently changed by another user. Its better to have real-time awareness of the values when two
people are editing the same data.
The DataGridCheckboxColumn is designed to help manage a column of checkboxes.
By default, it acts as a selection list, and maintains its own record of what is checked,
managing the display of checked items
as the user pages through the data. To get the current list of what is checked, call getCheckedItemIds().
The DataGridCheckboxColumn can display a checkbox in the header that will check all, or check none,
by setting the ShowCheckAll property to true.
In order to implement this, DataGridCheckboxColumn must be able to get an id
for each possible row shown, so that when the user pages through data, they can see what is checked. In order to do this,
you must subclass DataGridCheckboxColumn and implement the getAllIds() method.
In the first example, we are using the default functionality of the DataGridCheckboxColumn class
to create a selection list that will be acted on later. The DataGridCheckboxColumn keeps a record
of what is checked in the _SESSION variable. It uses a subclass of DataGridCheckboxColumn to get
all the ids if the user clicks the Check All button.
The second example is a simulation of changing the data in real-time. 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). Checking a box will immediately associate the selected project
with the ACME project. To support this type of interaction, we subclass DataGridCheckboxColumn and
implement the GetItemCheckedState and SetItemCheckedState methods.