委托，UpdatePanels和ascx控件

Question

I have built an ascx control that is part of many different components of my application. There is a Previous and Next button on this control, which should be signaled to the parent aspx page. This is done by having the parent page add some Delegates for postbacks, such as OnPreviousClicked, OnNextClicked etc.

Everything in this app is 'ajaxified' with an updatepanel. Now I notice that my app breaks if I don't set the delegates on every single Page_Load call in the parent. In other words, if I don't ALWAYS set the delegates in the Page_Load of the parent aspx, then the ascx ends up with null delegates and an exception. Am I coding stuff correctly?

Answer 1

If all you're doing is signaling the clicking of a button, I would have your ASCX control raise a simple event instead. That way each page can listen for the event if they need to and the controls can function regardless if anybody is listening.

First declare the events in your ASCX codebehind:

public event System.EventHandler NextSelected;

Then you create your button click events that raise the event.

protected void btnNextSelected_Click(object sender, EventArgs e)
{
   if (EmployeeSelected != null) 
   {
      NextSelected(this, new EventArgs());
   }
}

Then in your parent ASP.Net pages you can add your ASCX control (we'll call it NavControl) and create methods that listen for these events.

NavControl.NextSelected += new EventHandler(NextPageRedirect);

protected void NextPageRedirect(object sender, EventArgs e)
{
    Response.Redirect("~/ViewEmployee.aspx", false);

}

Note that with this you don't have to create the event handler or method and you can still use the nav control on your page. It should also eliminate the issues you are having with the delegates.

Answer 2

// inside the control

public event EventHandler OnPreviousClicked;

private void PreviousButton_OnClick(object sender, EventArgs e)
{
  if(OnPreviousClicked != null) {
    OnPreviousClicked(this, e); // or whatever args you want
  }
}

// and inside the Page code-behind

private void Page_Load(...)
{
   MyUserControl.OnPreviousClicked += new EventHandler(myHandler);
}

// OR inside the Page aspx, you also could just set the OnPreviousClicked property.

<xx:MyUserControl ID="MyUserControl1" runat="server" OnPreviousClicked="myHandler" />

See http://asp.net-tutorials.com/user-controls/events/ .

Answer 3

remember each postback has to restart your code. The event handlers don't get serialized to viewstate so you have to set them up again.

Answer 4

That is the expected behavior in asp.net.

The entire page life cycle occurs even when it is a partial postback (update panel). So in order for the events to fire, you will have to wire them up programmatically during page_load or declaratively in your markup (if those delegates you mentioned are events).