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Published: 08 Sep 2008
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A custom control derived from the GridView that implements filtering, custom Top pager and custom Bottom pager.
While out of the box the GridView is enough for most applications, often there is a need to customize it. One such customization requirement is for the Top Pager to be different from the Bottom Pager. GridView allows you to customize the Pager by using a Template construct, but it uses that same template for the Top Pager as well as the Bottom Pager. Another feature missing from the GridVew is filtering. Filtering allows the user to restrict the row set by specifying column values.
Figure 1: The custom control in action
In order to customize the GridView we need to first understand how it builds the control structure on the server side. On the client side the GridView is rendered as a
<table> within a
<div> element. On the server side the control tree is built within the
GridView.controls member variable as shown:
Figure 2: The control tree
So we have a
ChildTable at the root with all GridViewRows as its children. The very first row is the Pager row if it is needed. Next is the Header row. Then we have one or more DataRows. This is followed by the Footer row and then the Pager row if these are needed.
Using the Code
This is how the code for the custom grid is rendered in an ASPX file. Notice that for the columns that need filtering, the
HeaderText attribute should have a space at the end. Here we have a space for the City and State columns.
The design is explained in two sections, namely Custom Paging and Custom Filtering.
When creating the Pager rows (both Top and Bottom), the
GridView calls a virtual method called
InitializePager. We override this method and provide two new virtual methods called
InitializeBottomPager as follows:
Refer to the commented line in the above code and note that we cannot use
GridView.TopPagerRow to find out if
IntializePager is being called for creating the Top or the Bottom pager. Reason being, whenever
GridView.CreateChildControls gets called, it will in turn call
InitializePager twice, first time for the top pager and second time for the bottom pager. Now since
CreateChildControls itself may get called twice during postback/callback - first while getting created from ViewState and next when creating from database (for a paged GridView user requested a new page for eg.) - the TopPagerRow would already exists from the previous run. So we directly look at the control tree. If it is empty then the GridView has started creating the Top pager row.
Now we code the top pager as shown below:
We let the
GridView create the Bottom pager by calling
base.InitializePager. The bottom pager is created as a Table (
PagerTable) with single Row (
TableRow) and a single Cell (
TableCell). Then we customize the bottom pager by adding another Cell (
goToCell) to the
TableRow as shown:
We also handle button clicks to go to a particular page by overriding the
OnRowCommand method as follows:
HandlePageCommand will then read the text from the text box and set the
GridView.PageIndex as shown below:
We add the filtering capability within the header row for each column. First we override the
InitializeRow method and if
Header then we call the
InitializeHeaderRow method in turn calls
AddGlyphs to add up/down arrows for sorting, and then calls
AddFilters to find if filtering is enabled for columns; and if it is, then it calls
AddFilter to add a
TextBox and a
DropDownList to that columns header cell. To enable filtering for a column include a space at the end of column header text in ASPX file or in codebehind (this is kludgy).
Notice that we are triggering the filter command when the user selects an operator from
DropDownList, this done by making the
DropDownList to postback whenever its selection changes.
Here is our handler for filter:
The important thing to note here is that we set
RequiresDataBinding to true, this statement is checked by
GridView duringPreRender stage and it rebinds the data to the grid if it is
true. This is exactly what we want: We don’t want to bind it immediately, we delay it until the very last stage. Meanwhile we also provide a virtual method called
OnFilterCommand for derived classes/Page to hook into our filtering.
Following is the
Here we call the
GetFilterCommand method, which loops through all the columns to form a where clause and returns it as
filterCommand. Note that we check for a space character at the end of
Columns[i].HeaderText to find if the user has enabled filtering for
We then use
ApplyFilterCommand to change the DataSourceView’s
SelectCommand. Note that we only support SqlDataSource. I have to look into supporting ObjectDataSource.
Things that we can improve
- Better way of specifying the columns that need filtering in ASPX file. We can derive from BoundColumn and implement it.
- Instead of DropDownList using postback, it can use callback mechanism so that the whole page doesn't repainted each time you set a column filter. For this we can implement a custom DropDownList.
- Suport for ObjectDataSource.
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