Dynamically Loading Advanced User Controls in Delphi ASP.NET applications

Handling Custom User Control Events, View State, Postbacks, ...

Developing any non-trivial ASP.NET User Control involves defining properties, events, binding methods ...

Even the simplest Calculator User Control takes some time to be developed :(

Fist, let's add the code to fill the operationsList list with the possible operations.

Binding to a List of Enum Values

type TOperation = (Add, Subtract, Multiply, Divide) ;

procedure TCalculator.BindOperationList;
var
   opArray : System.&Array;
begin
   opArray := Enum.GetValues(typeof(TOperation)) ;

   operationsList.DataSource := opArray;
   operationsList.DataBind;

   //no need for items to go into the view state as we are binding the list on each postback
   operationsList.EnableViewState := false;
end;

procedure TCalculator.calcButton_Click(sender: System.Object; e: System.EventArgs) ;
begin
   //should add error handling / type checking here
   case Operation of
     Calculator.Add : resultLabel.Text := Convert.ToString(System.Int32.Parse(textBox1.Text) + System.Int32.Parse(textBox2.Text)) ;
     Calculator.Subtract: ... ;
     Calculator.Multiply: ... ;
     Calculator.Divide: ... ;
   end;

function TCalculator.GetOperation: TOperation;
var
   sOp : string;
begin
   sOp := operationsList.SelectedValue;
   //cast it to TOperation
   result := TOperation(Enum.Parse(typeof(TOperation),sOp)) ;
end;

Adding a User Control to a Page

When to "LoadControl"?

If you decide to dymaically add a User Control in the Page's Load event handler, the User Control might not work correctly. The reason to use the Init event is related to understanding ViewState and Postback Processing in ASP.NET applications

For example: the BackColor property of the Calculator User Control wraps the Panel's BackColor proerty. Since this property for the Panel component is stored in the ViewState, if you set the BackColor property after the LoadViewState, that is in the Page's Load event, the previous BackColor value will be lost.

Here's a procedure you need to call in the Page's Init event handler to succesfully load the Calculator user control.

procedure TUserControlTest.AddCalculator;
var
   calc : TCalculator;
begin
   calc := TCalculator(Page.LoadControl('~/Controls/Calculator.ascx')) ;

   Panel1.Controls.Add(calc) ;

   calc.BackColor := Color.Green;
   calc.Operation := TOperation.Add;
   calc.ID := 'calc';

   //add 2 event handlers
   Include(calc.CalculationComplete, HandleCalcComplete2) ;
   Include(calc.CalculationComplete, HandleCalcComplete) ;
end;

The code above also assigns two event handling procedures for the CalculationComplete event of the TCalculator class. Since .Net events are multicast, the Include and Exclude standard procedures are used to include and exclude event handlers at runtime.

Here's the implemntation of one of the event handling procedures, chaning the BackColor property depending on the selected operation:

procedure TUserControlTest.HandleCalcComplete2(Sender: TObject) ;
var
   calc : TCalculator;
begin
   calc := Panel1.FindControl('calc') as TCalculator;

   case calc.Operation of
     Calculator.Add: calc.BackColor := Color.Yellow ;
     Calculator.Subtract: calc.BackColor := Color.Navy;
     Calculator.Multiply: calc.BackColor := Color.Aqua;
     Calculator.Divide: calc.BackColor := Color.Red;
   end;
end;

Make sure you download the source code accompanying this chapter. Try placing a call to AddCalculator procedure in the Page Load event handler .. add some properties of your own ...

To the next chapter...

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