Saturday, March 17, 2007

Implementation of paging and sorting for the GridView control that works with an array of objects


Fairly often, such a data presentation model is used: an array of objects is bound to a GridView control with predefined columns. This model was popularized by the DotNetNuke web framework and has the advantage of working with the business objects instead of nameless rows of a data table. A classic example:
<asp:GridView id="gv" runat="server" AutoGenerateColumns="False" ...
<Columns>
<asp:BoundField DataField="Name" HeaderText="Name" SortExpression="Name" />
...
</Columns>
</asp:GridView>

and elsewhere in Page_Load:
if (!IsPostBack)
{
gv.DataSource = someArray;
gv.DataBind();
}

A pretty good model, in my opinion. But, the GridView control does not have built-in ability for sorting and paging if it is bound to an array of objects. Let's implement it.

The code overview

I inherit my control from the GridView control and override the OnInit method in order to add two event handlers.
this.PageIndexChanging += new GridViewPageEventHandler(BaseGridView_PageIndexChanging);
this.Sorting += new GridViewSortEventHandler(BaseGridView_Sorting);

The paging implementation is trivial.
void BaseGridView_PageIndexChanging(object sender, GridViewPageEventArgs e)
{
this.PageIndex = e.NewPageIndex;
BindDataSource();
}

The sorting one is rather complicated. The GridView control has a sorting event handler, but it does not save information about the previous sorting state. So, I added a few variables to save the data: PreviousSortExpression, CurrentSortExpression, and CurrentSortDirection.
void BaseGridView_Sorting(object sender, GridViewSortEventArgs e)
{
//for unknown reason e.SortExpression always is Ascending
if (PreviousSortExpression == e.SortExpression)
{
e.SortDirection = SortDirection.Descending;
PreviousSortExpression = null;
}
else
PreviousSortExpression = e.SortExpression;

CurrentSortExpression = e.SortExpression;
CurrentSortDirection = e.SortDirection;

ChangeHeaders(this.HeaderRow);

BindDataSource();
}

A little something is lacking. There is no visual presentation whether the grid is sorted and how it is sorted. So, I change the sorted column header to show this information. I do it by means of two Unicode symbols: the up arrow (\u25bc) and the down arrow (\u25b2).
private void ChangeHeaders(GridViewRow headerRow)
{
for (int i = 0; i < headerRow.Cells.Count; i++)
{
if (headerRow.Cells[i] is DataControlFieldCell)
{
DataControlField field = ((DataControlFieldCell)headerRow.Cells[i]).ContainingField;
//remove all previous sorting marks if they exist
Regex r = new Regex(@"\s(\u25bc|\u25b2)");
field.HeaderText = r.Replace(field.HeaderText, "");
if (field.SortExpression != null && field.SortExpression == CurrentSortExpression)
{
//add current sorting state mark
if (CurrentSortDirection == SortDirection.Ascending)
field.HeaderText += " \u25b2";
else
field.HeaderText += " \u25bc";
}
}
}
}

Now, about filling the grid with data. I add the event that occurs when the data source is requested and the method that will handle this event.
public delegate void DataSourceRequestedEventHandler(out Array dataSource);
public event DataSourceRequestedEventHandler DataSourceRequested;

Refilling of the grid occurs when a user sorts the grid or changes the current page. For initial filling, the BindDataSource method is used.
public void BindDataSource()
{
Array dataSource = null;

//request for the data source
if (DataSourceRequested != null)
DataSourceRequested(out dataSource);

if (dataSource == null)
throw new Exception("Failed to get data source.");

//sort the data in case of need
if (CurrentSortExpression != null)
{
ArrayList ar = new ArrayList(dataSource);
ar.Sort(new ArrayComparer(CurrentSortExpression, CurrentSortDirection));
dataSource = ar.ToArray();
}

base.DataSource = dataSource;
base.DataBind();
}

ArrayComparer is the implementation of the IComparer interface to compare two objects by a specified property in a specified order (ascending or descending).

How to use it

First of all, because I hide all the paging and sorting functionality inside the new control, I have to reject the old data binding method explicitly. So, I add an event handler for all requests for the data source and the method to call the data binding. And also, it is needed to allow paging and sorting in the GridView :)
<cc:GridViewEx ID="gv" runat=server AllowPaging=True AllowSorting=True 
OnDataSourceRequested="gv_DataSourceRequested" AutoGenerateColumns="False">
<Columns>

<asp:BoundField DataField="Name" HeaderText="Name" SortExpression="Name" />
...
</Columns>>
</cc:GridViewEx>

protected void Page_Load(object sender, EventArgs e)
{
gv.DataSourceRequest+=new DataSourceRequestEventHandler(gv_DataSourceRequested);
if (!IsPostBack)
gv.BindDataSource();
}
protected void gv_DataSourceRequested(out Array dataSource)
{
dataSource = someArray;
}

That is all you need to use this control, no additional work for paging or sorting is required. The full code can be found in the above demo.

Download source/demo project - 10.5 Kb