UpdatePanel with TextBox.OnTextChanged Async Event not firing - asp.net

Question

I have a textbox for inputting a desired username. When the user types in their name and the input loses focus, I want to send an async postback to check whether their username is already taken. This works fine in the cases that no error is thrown, but I want to handle errors, as well.
Per MSDN, I've added an AsyncPostBackError handler to the ScriptManager. I've tried this both on every page load and only on non-postback page loads.
I have my TextBox in an UpdatePanel like so:
<asp:UpdatePanel ID="upLogin" runat="server" ChildrenAsTriggers="true" UpdateMode="Conditional">
<ContentTemplate>
<asp:TextBox ID="tbLogin" runat="server" placeholder="username"
OnTextChanged="tbLogin_TextChanged" required />
</ContentTemplate>
<Triggers>
<asp:AsyncPostBackTrigger ControlID="tbLogin" EventName="TextChanged" />
</Triggers>
</asp:UpdatePanel>
In the code-behind, I have:
protected void Page_Load(object sender, EventArgs e)
{
ScriptManager.GetCurrent(Page).AsyncPostBackError +=
BootstrapLogin_AsyncPostBackError;
}
void BootstrapLogin_AsyncPostBackError(object sender, AsyncPostBackErrorEventArgs e)
{
tbLogin.Attributes.Remove("data-success");
tbLogin.Attributes["data-error-message"] = myErrorMessage;
}
protected void tbLogin_TextChanged(object sender, EventArgs e)
{
throw new Exception("what?");
}
I also added Async="true" to my Default.aspx Page markup.
When I have AutoPostBack="true" on my TextBox, the error is thrown and the AsyncPostBackError handler is never called. For some reason, during the postback, Page.IsAsync is false. I suspect this is the reason (or indication of why) it's not being handled.
When I set AutoPostBack="false", no postback is fired at all, despite the specified trigger.
Ideas?

Answer 1

It looks like following code doesn't work as intended
ScriptManager.GetCurrent(Page).AsyncPostBackError +=
BootstrapLogin_AsyncPostBackError;
Instead of that I just set the OnAsyncPostBackError property of the ScriptManager in markup and it worked with AutoPostback = "True" in your TextBox.
<asp:ScriptManager ID="ScriptManager1" runat="server" OnAsyncPostBackError="BootstrapLogin_AsyncPostBackError"></asp:ScriptManager>
Not sure why your code didn't work. It picks up the correct ScriptManager.
UPDATE
Solved it!
I should refresh my knowledge on Page lifecycle :(
Actually, we should add your code into Page_Init rather than in Page_Load. Following works perfectly.
protected override void OnInit(EventArgs e)
{
base.OnInit(e);
ScriptManager.GetCurrent(Page).AsyncPostBackError +=
BootstrapLogin_AsyncPostBackError;
}
UPDATE 2
See this screen capture
Hope this helped!