Sitecore MVC: pass parameters to placeholders - model-view-controller

Question

As we are switching to sitecore from a java platform, I have some questions regarding parameters. This is easily done in jsp but I can't find a solution for sitecore. (the implementation is done by external partners)
In my cshtml, I include other elements via the placeholder-function: #Html.Sitecore().Placeholder("Placholdername")
The elements included as a placeholder also can include other elements as placeholders.
So the question is: can I pass some parameters along with those placeholders?
Like my parent element has a certain variable set, for example "i = 5", and I want to pass this variable to the elements included as placeholders and also pass it to the elements included as placeholders in the placeholders?
Something like:
A includes B as a placeholder and passes "i=5" and B includes C as a placeholder and passes "i=5" so in C the value of "i" is "5" because "i" was set to "5" in A.
On out current coremedia platform I can simply use something like:
<cm:include self="${self}" view="asdf">
<cm:param name="i" value="5"/>
</cm:include>
Edit:
What I want to achieve is the following: For example I have the following structure: the page frame cshtml with a variable i=5, which then includes a grid as a placeholder and passes the variable to the grid. The grid then does some math like i=i+5 (which should equal 10) and then includes a teaser as a placeholder and passes the new i=10 to the teaser and so on..

Answer 1

You should set the value of i in the model. Then all the different views or partial views should inherit the same model.

Answer 2

You can assign parameters to renderings, but not to placeholders. Placeholders should be seen as a hole. You can dynamically put stuff in it but you can't assign parameters to it. There's a discussion of that here: https://sitecore.stackexchange.com/questions/2098/add-rendering-parameters-to-placeholder/2101
I can think of at least two approaches to solve your problem:
Although your question is a bit lacking in detail, it kind of looks like you don't necessarily need placeholders because you already know what you're going to render inside those spaces. If that's the case, then you can statically bind your MVC views instead of using placeholders. This is not a common practice, but it is mentioned in Sitecore's latest training material and elsewhere as a way to optimize when you don't need the flexibility of placeholders. This is normally done using the #Html.Sitecore.Rendering helper.
You could use a global variable of sorts by leveraging the good old HttpContext.Items collection.

Answer 3

You can add parameters to the ViewData dictionary, in a controller action method:
public ActionResult MyPage()
{
ContextService.Get().GetCurrent().ViewData.Add("MyKey", "MyValue");
return View();
}
Then, any View Renderings can access the parameter from ViewData:
#{
var value = ViewData["MyKey"].Value;
}
Or if you're using Controller Renderings, add some code to fetch the ViewData from parent pages, and add it to the current ViewData instance:
public ActionResult ChildRendering()
{
// Get any ViewData previously added to this ViewContext
var contextViewData = ContextService.Get().GetCurrent().ViewData;
contextViewData.ToList().ForEach(x => ViewData.Add(x.Key, x.Value));
return View();
}
Your ViewData contents will now be available in view files.
This is discussed in a little more detail here: https://chrisperks.co/2017/03/06/a-workaround-for-missing-viewdata-in-sitecore-mvc/