The Web Control framework handles events in two ways:

• Buttons
  Buttons are special objects and they automatically post back to the server. If no event target is found Web Connection runs through all buttons and checks to see if the button was clicked.

• __doPostBack() Client Script
  Anything but stock buttons that posts back to the server uses some special client script to post events from the client to the server by writing out event information into a few hidden variables and submitting the form. You should always use this.Page.GetPostbackEventReference to insert this script.

Firing a Button from the Client

Button clicks is completely automatic and built into both HTML and Web Connection natively. There's nothing special about triggering clicks - they just automatically fire events on the server. If you want to trigger a button to post back to the server the easiest is simply to call the specific button's 'click' method in JavaScript code:

 type="text/javascript">
function ScriptCode()
{
    document.getElementById('btnSubmit').click();
}

This fires the click of the button and simply submits it. Easy.

Firing a custom Event from the client

But buttons are usually not the target of explicitly triggered events from the client. A more common scenario is that you click an item in a datagrid and want to have it fire an event on the server that doesn't navigate to a new page and instead handles the operation on the current page.

For example, you have a grid display and have a Delete button on the grid. When you click Delete you want to post back to the current form delete the selected record and then redisplay the grid without the deleted record.

You can fire ANY server side control event from the client with this sort of script:

__doPostBack('CtlUniqueId','Event','Parameter');

So if you wanted to fire a list box OnSelectedIndexChanged event:

__doPostBack('lstCustomers','SelectedIndexChanged',this.value);

This will trigger the SelectedIndexChanged event on the lstCustomers control on the server and pass whatever this.value is on the client and pass it to the server as a string value.

If you use code like this from the client make sure that you inlude the following somewhere in your server code explicitly to ensure that the post back script code always gets embedded:

FUNCTION OnLoad
THIS.RegisterPostbackScriptCode()
...
ENDFUNC

This function ensures that the __doPostBack() client script function as well as the event variables are available on the client.

Let's look at the above example I mentioned of a delete button in a DataGrid:

 ID="gdCustomers" runat="server">

     runat="server" ID="Company" Expression="Company" >
    
     runat="server" ID="WwWebDataGridColumn1" 
                            Expression="this.Page.DeleteExpression()" 
                            HeaderText="Action" >
    

The second column is a Delete link. I'm using an Expression that points back to the page to create the output for this column which is going to be a hyperlink that calls the javascript:__doPostBack() function. I'm using an expression because there will be embedded expressions in the string and it's easier to write in VFP than in the VS.NET editor.

FUNCTION DeleteExpression()
RETURN [Delete]
ENDFUNC

FUNCTION DeleteCompany
lcResult = Request.Form("__EventParameter")

*** Delete Code would go here
this.Errordisplay.ShowMessage("you've selected " + lcResult ) 
ENDFUNC

I created two methods in the Page class. The first generates the Expression to display and as you can see it creates the __doPostBack() function call for each link in the grid. It says:

• Call the Page Class
• Fire the DeleteCompany method
• Pass the PK as a value to the method

The actual HTML generated inside of the grid column looks like this:

 href="javascript:__doPostBack('Page','DeleteCompany','32');" >Delete

Note the special name 'Page' in this example. You can use Page or This to specify that you want to fire an event on the Page object. For any other control use its UniqueID value to identify the control (ie. this.txtButton.UniqueID).

When clicked this code now goes out and fires the Page.DeleteCompany method in the page. The mthod then can use Request.Form("__EventParameter") to retrieve the parameter that the client sent. So at this point you'd be ready to delete the record with a PK value of 32. The code above merely echos back the value passed.

Note parameters are always passed and retrieved as strings.

Note that if you generate the script reference on the server (ie. you're creating a control or you're using one of the Page.RegisterClientScriptxxx functions you should NEVER directly reference '__doPostback(...). Instead use this.Page.GetPostbackEventReference() which generates this code for you and does so with an easier interface to boot. The reason for this is that there may be future changes in the way events are called and this standardizes the way the events are generated in client code.

You can also apply this in client code with something like this:

function FireServerEvent()

{  

    <%= this.Page.GetPostbackEventReference(this.txtName.UniqueId,"change") %>;   

    // generates: __doPostback("Ctl0_txtName","change");

}