Introduction
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
Background
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.
Design Details
The design is explained in two sections, namely Custom Paging and Custom Filtering.
Custom Paging
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 InitializeTopPager and 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:
The HandlePageCommand will then read the text from the text box and set the GridView.PageIndex as shown below:
Custom Filtering
We add the filtering capability within the header row for each column. First we override the InitializeRow method and if RowType is Header then we call the InitializeHeaderRow method.
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 OnPreRender function:
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 Columns[i].
We then use ApplyFilterCommand to change the DataSourceView’s SelectCommand. Note that we only support SqlDataSource. I have to look into supporting ObjectDataSource.
References
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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