Is it possible to limit Renderings so they can only be placed once on a page?

Question

I'm using Sitecore 8.1 rev. 151207 - And I have that rendering which only should be placed once on a page.

Is it possible to limit that in terms of letting the user not add another one of the same rendering again. The only way I can think of would lead to possible wasted Renderings and items which are put on the page but are not shown and I think thats not best practice.

I already searched for solutions but nothing showed up so far

Answer 1

Yes you can. It will take a little code and a pipeline change, but you can find the code for mvc here and webforms here.

What you want to do it count the number of times a rendering has been put on the page and stop it if it passes your max number.

public class PerformRendering : Sitecore.Mvc.Pipelines.Response.RenderPlaceholder.PerformRendering
{
    protected override IEnumerable<Rendering> GetRenderings(string placeholderName, RenderPlaceholderArgs args)
    {
        // get all renderings
        var renderings = base.GetRenderings(placeholderName, args);

        // return all renderings in Experience Editor
        if (Sitecore.Context.PageMode.IsExperienceEditor) return renderings;

        // get the maximum number of components
        var maxComponents = this.GetMaxComponents(args);

        // return only first n components
        return maxComponents > 0 ? renderings.Take(maxComponents) : renderings;
    }

    private int GetMaxComponents(RenderPlaceholderArgs args)
    {
        var maxComponents = 0;
        if (args.CustomData.ContainsKey("maxComponents"))
        {
            int.TryParse(args.CustomData["maxComponents"].ToString(), out maxComponents);
        }

        return maxComponents;
    }
}

The config change

<configuration xmlns:patch="http://www.sitecore.net/xmlconfig/">
  <sitecore>
    <pipelines>
      <mvc.renderPlaceholder>
        <processor patch:instead="*[@type='Sitecore.Mvc.Pipelines.Response.RenderPlaceholder.PerformRendering, Sitecore.Mvc']"
                   type="Website.Pipelines.RenderPlaceholder.PerformRendering, Website" />
      </mvc.renderPlaceholder>
    </pipelines>
  <sitecore>
</configuration>

Answer 2

There's nothing built in, and there's two ways you could go about it.

You could hook into the item:saving event and check the renderings there. If there are duplicates of known "single-use" renderings, you throw an error which will prevent the save. This could also be done via a custom validator.

Alternatively, you could allow the save but then only render the first instance. You should just be able to use dnstommy's answer as a base and filter out any renderings that have already executed this request, using a Set<ID> stored in HttpContext.Current.Items to keep track of them. However, you might run into problems with this approach if you have caching rules on some higher level composite renderings.