passing data from Sitecore MVC view rendering to layout rendering

I tried to use ViewBag in a Sitecore MVC view rendering to pass some data to its relevant layout rendering but it didn't work. In standard ASP.NET MVC you can do that, but is it possible to do the same in Sitecore also? If not, then how do you pass data?

Edit

to clarify my question, here is what I was trying to do: I have a layout rendering like this:

@using Sitecore.Mvc
@using Sitecore.Mvc.Analytics.Extensions
@using Sitecore.Mvc.Presentation
@model RenderingModel
@{
Layout = null;
}
<!doctype html>
<html>
<head>....</head>
<body>
@if (ViewBag.ShowNewsletterPopup == "display")
{
    <!-- render proper html markup to show newsletter popup -->
} 
else {
  <!-- render regular html markup --> 
}
<!-- some html markup here -->
@Html.Sitecore().Placeholder("widget")
<!-- some more html markup -->

And then, inside content placeholder goes a view rendering which has a code like this:

@using Sitecore.Mvc
@using Sitecore.Mvc.Presentation
@using Sitecore.Data.Items

@model RenderingModel
@{
   ViewBag.ShowNewsletterPopup = Model.Rendering.Parameters["ShowNewsletterPopup"];

So, what I am trying to achieve is switching between two blocks of html markups depending on the settings of a view rendering which sits outside those blocks. Perhaps there is a better way to do this, but I am not sure. What I see is that the view rendering code gets compiled after layout rendering, so the code really never gets executed.

Solution

I can confirm that the current behaviour of Sitecore MVC would not work. In the following scenario MVC Layout with:

@using Sitecore.Mvc
@{
    Layout = null;
    ViewBag.Message = "ViewBagData";
}
<html>
<body>
Message: @Html.Partial("View Rendering") <br>
Message: @Html.Sitecore().ViewRendering("View Rendering")
</body>
</html>

And the View Rendering:

@ViewBag.Mything

You would see the following output:

Message: ViewBagData
Message:

The underlying issue is that the ViewRendering type spins up an new HtmlHelper. Althought the ViewData is passed into this, the actual write to the dictionary backing of the ViewData is deferred (this is an MVC thing).

I would think that the desired behaviour would be for ViewBag data to behave the same as it does for @Html.Partial().

You could use something like (though it feels like a smell):

Sitecore.Context.Items["Message"]

Perhaps have a look at the this video, it has a concept around rendering interdependencies.