Experience Sitecore !

One day I was working on a page that had way too many Content Editor section opened, plus those plenty coming out from Standard fields also added frustration. I thought it would be great to have Collapse All button implemented, that closes all the sections in order to help navigation. 

I went checking the way Content Editor works this out and later wrote a JavaScript snippet, that implements and wires desired functionality. I also added Expand All button bringing the reverse behavior. Here's the code:

scContentEditor.prototype.onDomReady = function (evt) {
    this.addCollapser(window.jQuery || window.$sc);
};
scContentEditor.prototype.addCollapser = function ($) {
    $ = $ || window.jQuery || window.$sc;
    if (!$) { return; }

    $('#EditorTabs').append("<style>.toggler { border: 1px solid #bdbdbd; box-shadow: 0 1px #ffffff inset; cursor: pointer; height: 35px; margin: 16px 1px 0; }</style>");
    $('#EditorTabs').append("<button id='expander' class='toggler'>Expand all</button><button id='collapser' class='toggler'>Collapse all</button>");
    $('#EditorTabs').on("click", "#collapser", function () {

        $('.scEditorSectionCaptionExpanded').each(function () {
            var script = $(this).attr("onclick");
            eval(script);
        });
        return false;
    });
    $('#EditorTabs').on("click", "#expander", function () {

        $('.scEditorSectionCaptionCollapsed').each(function () {
            var script = $(this).attr("onclick");
            eval(script);
        });
        return false;
    });
};

All you need to do is to append to the bottom of <your_web_root>\sitecore\shell\Applications\Content Manager\Content Editor.js and that's it!

For those like me who like automation, I am attaching this JavaScript file, right click this link and save it, and then use PowerShell:

$file2 = Get-Content "ExpanderCollapser.js"
Add-Content "C:\inetpub\wwwroot\<YOUR_WEB_ROOT>\sitecore\shell\Applications\Content Manager\Content Editor.js" $file2

Once done, you'll see the result in Content:

There is also package available for download (compatible with Sitecore 9.0.* - 9.2)

I was working on implementing a subscription model system, where authenticated users visit website with a specific role coming from Identity Server (or, unauthenticated - anonymous, of course), so that I can apply personalization of content, as we normally do.

The difference was, however, that subscription level were logical units, more complicated and not matching IDS roles. They also should be adjustable from Sitecore by business users. That made using personalization by these users type quite complicated, due to complex rules creation, especially those with inverted logic except when. But even with that in mind, I could not simply use personalization for preventing unauthorised users (for example, those registered and logged, but still having insufficient permissions) from accessing specific types of content. The business requirement demands all the pages to be accessible by anyone, but when users don't have required access level - most of content apart from few teasing paragraphs in the beginning, needs to be greyed out by a components encouraging them to increase their subscription level in order to get full access.

So, in order to address these requirements, I decided to implemented a simple role-mapping Subscription Model, something as could be described by this template:

But wait! There is no possibility to use Sitecore security roles in an item!

So I decided to implement the one. After quick googling I came across Mike Reynold's experiments with fields and templates and went similar way on implementing Role Multilist Selector field. 

Once done, core database needs to get a new field type registered - Roles, which is implemented in a way of traditional multi-select field:

So now, I can use it as an ordinary Sitecore item field. Please note, that Source column at first screenshot above contains Domain=ids - that is a set of parameters passed in a format of URL string (UrlString is .NET class that accepts these parameters in the code). I've implemented that as a Sitecore domain filtering parameter, where ids is the domain name.

Now we can select roles - they will be stored in pipe-separated format in given field:

Finally, after implementing a logical layer of Subscription Model, I also had to create custom rules conditions to apply personalization operating these logical subscriptions, but that made business users' life way easier.

Hope this helps!

Image Link rendering variant field

This is quite powerful and at the same time very simple rendering variant field - it nicely renders <img> tag surrounded with <a> anchor tag without any of other unwanted wrappings normally coming when nesting components in SXA, as below:

<a href="http://link.to/internal-or-external-item" class="individual-class-for-anchor">
    <img src="/-/Media-item-from-sitecore" class="individual-class-for-image"/>
</a>

When starting working with SXA I faced some lack of good guidance and walkthrough (with an exception of excellent series by Adam Najmanowicz). Today I am going to eliminate this gap by adding my own walkthrough of implementing a footer, what could be simpler, I thought. After reading some official tutorials I expected it to be an exercise of dropping structure components (ie. splitters with rows and columns) and assigning link lists into them so that it could be later styled by front-end team. Wrong! Things appeared to be not as easy.

To start with, my strict front-end team came to me with quite a precise requirement for a footer, and below is they demand from me.

Requirements

They send me an image with all the blocks assigned:

It was accompanied by the HTML output expected for me to achieve:

Minimal and effective, nice job, front-enders! Now the ball is on my side in order to get all that implemented.

Implementation

1. As per Sitecore SXA recommendations, I create Footer partial design and open it in Experience Editor for editing. Footer component will be created for that partial design, which itself will be used to construct a resulting page.

2. Not to mess with any OOB components, I create Footer rendering by cloning one of the existing renderings with datasource, so that I ensured a copy of datasource template and folder created. Also make sure this rendering stays outside of Experience Accelerator folder, but on feature layer where serialization enabled.

3. Assign new rendering to Available Renderings (/sitecore/content/Tenant/Site/Presentation/Available Renderings/Module) so that it appears in my custom components sectionand also in Toolbox

4. Now it is a good time to adjust a Template for Footer,it was created automatically by cloning, but this is how I defined it:

What is important to explain here - Elements field is pointing to Footer Elements Folder. At a first glance, that folder contains just a collection of link lists. But that's not right! There are at least 3 types of footer blocks: first four blocks are indeed Link List blocks,however,followed by Rich Text blocks, and finally, there is Social Presence block and all three can be added intoFooter Elements Folder. These three types are defined along with Footer template, you may see on the screenshots below.

5. Elements templates. This is how I defined them, Link ist footer element:

Rich text footer element:

Social presence footer element:

6. Insert Options to be configured for Footer folder to accept both Footer items and Footer Elements Folder. Configure Footer Elements Folder to accept these 3 types. Now one can insert the data. Once done, that will be how the data folder looks like:

First 4 items (About Us, Hot topics, Other sites, Help and Support) reference corresponding Link Lists as defined as below:

Next two items point to reusable Rich Text items under /Data folder. And the last one is a reference to a Social Presence I have implemented previously.

After items of all three types are created, we can assign them into footer datasource item itself:

7. Rendering variants. In the previous steps, we have defined rendering, templates, folders and actual data. Now it is time to make it all together work to produce the output by creating rendering variants. This is the most tricky part of the current walkthrough

Default is the main and only rendering variant to be called or footer. Other three variants are "service" variants and designed to be called from Defaultinternally in a loop, being assigned to a component rendering field with personalisation applied (see image above).

Here's how these three other variants look like:

Link list footer element switches to a referenced Link List item and uses Query variant field to iterate its children.

Rich Text footer element simply references to a Rich Text (reusable) item under /Data folder in the same manner.

As for Social presence footer element rendering variant, it defines a component that references Social buttons rendering with Social presence rendering variant, that I have described in one of my previous posts.

Lastly, footer-info__copyright field renders copyright lines at the very bottom.

8. In order to avoid confusion for your editors, it makes sense to restrict rendering variant by selecting Allowed in templates field leaving only Default variant since other three variants are internally called from Default using Component variant field. 

9. Apply new rendering to a Partial Design it in Experience Editor. Do not forget to configure footer placeholder to accept only Footer component by creating a placeholder setting,

Result

Save the page and enjoy the result:

No single line of back-end code!

When dealing with a rendering variant field, it is not a big deal to set few data-attributes to it - those inputs are located at the very bottom of Variant Details section. You can do it like that:

But what if you need data attributes to the top level of component, which it Rendering Variant item itself? There isn't such an option!

Requirements are

1. An id attribute (ie. section-1, section-2, ... section-N)
2. One or many data-attributes (ie. " User-friendly title", "Another user-friendly title", etc.)
3. CSS class section-with-anchor on those instances, which have both previous requirements implemented

Solution

public static MvcHtmlString RenderAllDataAttributes(this HtmlHelper helper)
{
    var rendering = Sitecore.Mvc.Presentation.RenderingContext.Current.Rendering;

    string additionalAttributes = String.Empty;
    if (rendering?.Parameters != null)
    { 
        foreach (KeyValuePair<string, string> parameter in rendering.Parameters)
        {
            if (parameter.Key.ToLower().StartsWith("data-"))
            {
                additionalAttributes += $"{parameter.Key}=\'{parameter.Value}\' ";
            }
        }
    }

    return new MvcHtmlString(additionalAttributes.Trim());
}

<div @Html.RenderAllDataAttributes() @Html.Sxa().Component(Model.Rendering.RenderingCssClass ?? "default-class", Model.Attributes)>
    <div class="component-content">
        ...
    </div>
</div>

Result

Finally, I got it all rendered as expected:

Yet another rendering variant field came to my to-do list for implementation - Script rendering variant field. Why would I need one at all? 

I came across two uses cases where implementing this field type made my job done and that's just recently. Some developers have (reasonable) biases against having JavaScript code inline instead of referencing JS-file at the bottom of a page, and I do understand their point - Google may penalise your site for overusing inline JS. So use it responsibly, don't overuse. Keeping in mind that given field comes up as a part of a component dynamically added to a page - there isn't a big choice on how to extend running website with some additional client-side functionality. I am presenting both cases below, let's take a look at them. Would you know the better way of achieving these goals - please let me know via Slack or Twitter. Also, the code of Script Variant Field is at the very bottom of the page.

Use case 1: implementing in-page navigation panel

As usual, I got a precise requirement from my strict front-end team to implement such a piece of code as a component. It has a UL-tag that will keep a link list to other components of this same page (prefixed with #) created client-side dynamically, and a script that does the actual job. When rendering the in-page navigation component by the backend, we're now aware of other components and their attributes, so the walk-around was handling page loaded event and identifying all the components that have IDs set and class section-with-anchor.

<div class="component content col-12 content-section">
    <div class="component-content">
        <div class="anchor-panel">
            <ul class="anchor-panel__list"></ul>
            <script defer>
                document.addEventListener('DOMContentLoaded', function(){
                    if (!$('.section-with-anchor').length || !$('.section-with-anchor').length) return;
                    $('.section-with-anchor').each(function(index, el) {
                        var anchor = '#' + $(el).attr('id');
                        var text = $(el).attr('data-text');
                        var $anchorList = $('.anchor-panel__list');
                        var $anchorItem = $('<li class="anchor-panel__item"></li>');
                        var $anchorLink = $('<a href=""></a>')
                        $anchorItem.append($anchorLink.text(text).attr('href', anchor));
                        $anchorList.append($anchorItem);
                    });
                });
            </script>
        </div>
    </div>
</div>

Quite obviously, my rendering variant will contain 2 fields: a UL-tag section field for in-page navigation that takes the links dynamically and script variant field, containing the client-side logic. You can test this script in action by this link. That is how it looks in Content Editor:

Since I have 5 other sections qualifying script requirements - they all have been identified by the script and added to in-page navigation panel. Here's how the result looks like on a styled page for me:

Once again, I decided to implement a new script variant reference field because the component is subject to some seldom minor JavaScript changes, but should be configurable. Also, at given use case it can be dropped only once into a page, so there's no problem with multiple instances of the same script for me, but using it you might need to check if that applies for your scenarios. But even with that in mind, I'd probably not bothered creating yet another new rendering variant field, if not few more potential usages I have in backlog.

Use case 2: accessing client side URL hash-key parameter and presenting it on a page

Recently I implemented a URL query string parameter variant field, where one can set any parameter and the field present its URL-decoded value on a page in any given tag and style. A colleague of mine who develops search results page with SXA asked if that's doable to extract a hash-key URL parameter and show it on a page along with other content, however that a fully client-side parameter that never got posted to the server.

I decided to give a try and managed to confirm with a small proof of concept. I quickly wrote this script (so please be forgiving for it being just a quick PoC and me not being the proper FED).

window.addEventListener("hashchange", function () {
    var h1 = document.getElementsByClassName("updatedHashValue");
    if (h1.length > 0) {
        h1[0].innerHTML = getHashValue("param");
    }
    function getHashValue(parameter) {
        var hashValues = window.location.hash.substr(1);
        var result = hashValues.split('&').reduce(function (result, item) {
                var parts = item.split('=');
                result[parts[0]] = parts[1];
                return result;
        }, {});
        return result[parameter];
    };
});

That's how it looks implemented as the part of rendering variant. It creates an H1 element to store the value, extracted out of hash parameters stored at URL bar and updated without any postback to the server, and of source script variant field:

Testing. After adding a component to a page, saving and selecting the above rendering variant, I got the page reloaded as anticipated with no visual changes. Then I open a console from browser dev.tools and enter:

window.location.hash = "param=Successful!"

From dev.tools that was easy to confirm that there was no postback done to the server, however, browser navigation bar predictably changed, appending new hash parameters pair: 

And guess what? H1 element immediately got that value displayed. Love that magic!

The code is very simple, similar to other variant fields I've blogged previously. Create the model and implement property for a field:

public class VariantScript : VariantField
{
    public string Script { get; set; }
}

Reference ID of that field from a template and ID of a template itself:

public static class Constants
{
    public static class RenderingVariants
    {
        public static class Templates
        {
            public static ID Script = new ID("...");
        }
        public static class Fields
        {
            public static class Script
            {
                public static ID ScriptField { get; } = new ID("...");
            }
        }
    }
}

Here's a parser

public class ParseScript : ParseField
{
    public override ID SupportedTemplateId => Constants.RenderingVariants.Templates.Script;

    public override void TranslateField(ParseVariantFieldArgs args)
    {
        ParseVariantFieldArgs variantFieldArgs = args;

        variantFieldArgs.TranslatedField = new VariantScript
        {
            Script = args.VariantItem[Constants.RenderingVariants.Fields.Script.ScriptField]
        };
    }
}

and a renderer:

public class RenderScript : RenderVariantField
{
    public override Type SupportedType => typeof(VariantScript);

    public override RendererMode RendererMode => RendererMode.Html;

    public override void RenderField(RenderVariantFieldArgs args)
    {
        var variantField = args.VariantField as VariantScript;
        if (variantField != null)
        {
            args.ResultControl = RenderScriptField(variantField, args);
            args.Result = RenderControl(args.ResultControl);
        }
    }

    protected virtual Control RenderScriptField(VariantScript variantScript, RenderVariantFieldArgs args)
    {
        if (!string.IsNullOrWhiteSpace(variantScript.Script))
        {
            var tag = new HtmlGenericControl("script") { InnerHtml = variantScript.Script };
            tag.Attributes.Add("defer", String.Empty);
            return tag;
        }

        return new LiteralControl();
    }
}

New Script Rendering Variant field, that enhances your tooling for implementing more modern-looking websites. And as usual - please use it responsibly!

public class VariantItemReference : BaseVariantField
{
    public string PassThroughItem { get; set; }

    public IEnumerable<BaseVariantField> NestedFields { get; set; }
}

public static class Constants
{
    public static class RenderingVariants
    {
        public static class Templates
        {
            public static ID ItemReference = new ID("...");
        }

        public static class Fields
        {
            public static class ItemReference
            {
                public static ID Item { get; } = new ID("...");
            }
        }
    }
}

public class ParseItemReference : ParseVariantFieldProcessor
{
    public override ID SupportedTemplateId => Constants.RenderingVariants.Templates.ItemReference;

    public override void TranslateField(ParseVariantFieldArgs args)
    {
        ParseVariantFieldArgs variantFieldArgs = args;

        var reference = new VariantItemReference();
        reference.ItemName = args.VariantItem.Name;
        reference.PassThroughItem = args.VariantItem[Constants.RenderingVariants.Fields.ItemReference.Item];

        reference.NestedFields = args.VariantItem.Children.Count > 0
            ? ((IVariantFieldParser)ServiceLocator.ServiceProvider.GetService(typeof(IVariantFieldParser))).ParseVariantFields(args.VariantItem, args.VariantRootItem, false)
            : new List<BaseVariantField>();

        variantFieldArgs.TranslatedField = reference;
    }
}

public class RenderItemReference : RenderVariantFieldProcessor
{
    public override Type SupportedType => typeof(VariantItemReference);

    public override RendererMode RendererMode => RendererMode.Html;
      
    public override void RenderField(RenderVariantFieldArgs args)
    {
        var control = new PlaceHolder();

        var variantItemReference = args.VariantField as VariantItemReference;
        if (!string.IsNullOrWhiteSpace(variantItemReference?.PassThroughItem))
        {
            var newContextItem = Sitecore.Context.Database.GetItem(new ID(variantItemReference.PassThroughItem));
            if (newContextItem != null)
            {
                foreach (BaseVariantField referencedItem in variantItemReference.NestedFields)
                {
                    RenderVariantFieldArgs variantFieldArgs = new RenderVariantFieldArgs
                    {
                        VariantField = referencedItem,
                        Item = newContextItem,
                        HtmlHelper = args.HtmlHelper,
                        IsControlEditable = args.IsControlEditable,
                        IsFromComposite = args.IsFromComposite,
                        RendererMode = args.RendererMode,
                        Model = args.Model
                    };

                    CorePipeline.Run("renderVariantField", variantFieldArgs);
                    if (variantFieldArgs.ResultControl != null)
                    {
                        control.Controls.Add(variantFieldArgs.ResultControl);
                    }
                }
            }
        }

        args.ResultControl = control;
        args.Result = RenderControl(args.ResultControl);
    }
}

Got a requirement to display a value passed with a URL parameter on a page (URL decoded of course). I want to achieve the goal as quick as possible, with the minimum steps, ideally. So, here we go..

1. Just use existing VariantField (located at /sitecore/templates/Foundation/Experience Accelerator/Rendering Variants/VariantField). When copying, ensure you keep new variant field template outside of Experience Accelerator folder (to avoid it being lost on future SXA upgrade), ideally somewhere within a folder with serialization configured.

2. Remove unwanted fields from a new template. I left only a few I felt necessary, but you may end up having even less. Another thing I've done at this step was adding a user-friendly label to the field, to avoid editors' confusion with a misleading naming:

3. Then you need to create a model. As the simplest, I inherited from VariantField

public class VariantQueryString : VariantField
{
    public VariantQueryString(Item variantItem) : base(variantItem)
    {
    }
}

With a parser, I only process those fields I will be using - what I kept from the previous step: 

public class ParseQueryStringField : ParseField
{
    public override ID SupportedTemplateId => Constants.RenderingVariants.Templates.QueryString;

    public override void TranslateField(ParseVariantFieldArgs args)
    {
        ParseVariantFieldArgs variantFieldArgs = args;

        variantFieldArgs.TranslatedField = new VariantQueryString(args.VariantItem)
        {
            // this property is reused under different purpose rather than named - it stores URL parameter name
            FieldName = args.VariantItem[Constants.RenderingVariants.Fields.QueryField.FieldName],

            Tag = args.VariantItem.Fields[Constants.RenderingVariants.Fields.QueryField.Tag].GetEnumValue(),
            CssClass = args.VariantItem[Constants.RenderingVariants.Fields.QueryField.CssClass],
            Prefix = args.VariantItem[Constants.RenderingVariants.Fields.QueryField.Prefix],
            Suffix = args.VariantItem[Constants.RenderingVariants.Fields.QueryField.Suffix],
            RenderIfEmpty = args.VariantItem[Constants.RenderingVariants.Fields.QueryField.RenderIfEmpty] == "1"
        };
    }
}

Also reference IDs of these fields within Constants class:

public static class Constants
{
    public static class RenderingVariants
    {
        public static class Fields
        {
            public static class QueryField
            {
                public static ID Tag { get; } = new ID("{F556DEDD-5D6B-4FFF-A904-E4C65AB0E698}");
                public static ID CssClass { get; } = new ID("{B3B6B300-1704-493B-999C-AD21CCE58FEF}");
                public static ID FieldName { get; } = new ID("{9D3717EF-CD09-4D98-8C4A-914299503626}");
                public static ID Prefix { get; } = new ID("{5844C4AA-5E48-4721-8E30-91646E430C83}");
                public static ID Suffix { get; } = new ID("{0EFAF48E-E0DD-4408-80D3-4C001101E834}");
                public static ID RenderIfEmpty { get; } = new ID("{E14C3930-FE6C-48AC-99D8-C6867B489066}");
            }
        }
    }
}

4. Finally, implementing RenderQueryStringField class. Nothing complex - just getting a value from the query string, wrapping it with a selected tag/styles and rendering it into the page. Also, there's a fallback scenario for Experience Editor when a given parameter is missing from URL but component needs to be visible.

public class RenderQueryStringField : RenderVariantField
{
    public override Type SupportedType => typeof(VariantQueryString);

    public override RendererMode RendererMode => RendererMode.Html;

    public override void RenderField(RenderVariantFieldArgs args)
    {
        var variantField = args.VariantField as VariantQueryString;
        if (variantField != null)
        {
            args.ResultControl = RenderQueryStringValue(variantField, args);
            args.Result = RenderControl(args.ResultControl);
        }
    }

    protected virtual Control RenderQueryStringValue(VariantField variantField, RenderVariantFieldArgs args)
    {
        var queryString = HttpContext.Current.Request.QueryString;

        if (!string.IsNullOrEmpty(variantField.FieldName) && !string.IsNullOrWhiteSpace(variantField.Tag))
        {
            if (queryString.HasKeys() && queryString[variantField.FieldName] != null)
            {
                var tag = new HtmlGenericControl(variantField.Tag);
                AddClass(tag, (variantField.CssClass + " " + GetFieldCssClass(variantField.FieldName)).Trim());
                tag.InnerText = queryString[variantField.FieldName];
                return tag;
            }

            if (args.IsControlEditable && PageMode.IsExperienceEditorEditing)
            {
                return GetVariantFieldNameLiteral(variantField.FieldName);
            }
        }

        return new LiteralControl();
    }

    protected virtual HtmlGenericControl GetVariantFieldNameLiteral(string parameterName)
    {
        var missingField = new HtmlGenericControl("span");
        missingField.Attributes.Add("class", "missing-field-hint");
        missingField.InnerText = $"[{parameterName}] URL paramenter";
        return missingField;
    }
}

5. I am adding new field variant into existing Foundation project, but if you haven't got one - you might create it. Do not forget to include config patches. something like below:

?xml version="1.0" encoding="utf-8" ?>
<configuration xmlns:patch="http://www.sitecore.net/xmlconfig/">
  <sitecore>
    <pipelines>
      <parseVariantFields>
        <processor type="Platform.Foundation.Variants.Pipelines.VariantFields.QueryString.ParseQueryStringField, Platform.Foundation.Variants" resolve="true" patch:before="processor[@type='Sitecore.XA.Foundation.RenderingVariants.Pipelines.ParseVariantFields.ParseField, Sitecore.XA.Foundation.RenderingVariants']" />
      </parseVariantFields>
      <renderVariantField>
        <processor type="Platform.Foundation.Variants.Pipelines.VariantFields.QueryString.RenderQueryStringField, Platform.Foundation.Variants" resolve="true" patch:before="processor[@type='Sitecore.XA.Foundation.RenderingVariants.Pipelines.RenderVariantField.RenderVariantField, Sitecore.XA.Foundation.RenderingVariants']"  />
      </renderVariantField>
    </pipelines>
  </sitecore>
</configuration>

6. You might consider adding a new variant field into Insert options. Once done, add it as you normally do with variant fields:

Result. That is how it works when passing a URL parameter with a value:

When a parameter is missing from URL, this is how it looks like in Experience Editor, at least making component selectable:

Hope you find this post helpful!

Got a weird exception, very similar to the one described at StackExchange (SXA Error when open page in Experience Editor). It happened at my CI server, so decided to reproduce locally, deploying on top of a clean instance of Sitecore, and got an issue reproduced.

Here's a call stack:

17264 22:03:56 ERROR Failed to render rendering
Exception: System.Web.HttpUnhandledException
Message: An unhandled exception occurred.
Source: Sitecore.Mvc
   at Sitecore.Mvc.Pipelines.MvcEvents.Exception.ShowAspNetErrorMessage.ShowErrorMessage(ExceptionContext exceptionContext, ExceptionArgs args)
   at Sitecore.Mvc.Pipelines.MvcEvents.Exception.ShowAspNetErrorMessage.Process(ExceptionArgs args)
   at (Object , Object )
   at Sitecore.Pipelines.CorePipeline.Run(PipelineArgs args)
   at Sitecore.Pipelines.DefaultCorePipelineManager.Run(String pipelineName, PipelineArgs args, String pipelineDomain, Boolean failIfNotExists)
   at Sitecore.Pipelines.DefaultCorePipelineManager.Run(String pipelineName, PipelineArgs args, String pipelineDomain)
   at Sitecore.Mvc.Pipelines.PipelineService.RunPipeline[TArgs](String pipelineName, TArgs args)
   at Sitecore.Mvc.Filters.PipelineBasedRequestFilter.OnException(ExceptionContext exceptionContext)
   at System.Web.Mvc.ControllerActionInvoker.InvokeExceptionFilters(ControllerContext controllerContext, IList`1 filters, Exception exception)
   at System.Web.Mvc.ControllerActionInvoker.InvokeAction(ControllerContext controllerContext, String actionName)
   at System.Web.Mvc.Controller.ExecuteCore()
   at System.Web.Mvc.ControllerBase.Execute(RequestContext requestContext)
   at Sitecore.Mvc.Controllers.ControllerRunner.ExecuteController(Controller controller)
   at Sitecore.Mvc.Controllers.ControllerRunner.Execute(TextWriter writer)
   at Sitecore.Mvc.Pipelines.Response.RenderRendering.ExecuteRenderer.Render(Renderer renderer, TextWriter writer, RenderRenderingArgs args)

Nested Exception

Exception: System.NullReferenceException
Message: Object reference not set to an instance of an object.
Source: Sitecore.XA.Foundation.Grid
   at Sitecore.XA.Foundation.Grid.Commands.ShowGridPropertiesDialog.QueryState(CommandContext context)
   at Sitecore.Shell.Framework.Commands.CommandManager.QueryState(Command command, CommandContext context)
   at Sitecore.Pipelines.GetChromeData.GetChromeDataProcessor.QueryButtonState(WebEditButton button, CommandContext context, String click)
   at Sitecore.Pipelines.GetChromeData.GetChromeDataProcessor.AddButtonToChromeData(WebEditButton button, GetChromeDataArgs args)
   at Sitecore.Pipelines.GetChromeData.GetChromeDataProcessor.AddButtonsToChromeData(IEnumerable`1 buttons, GetChromeDataArgs args)
   at Sitecore.Pipelines.GetChromeData.GetRenderingChromeData.Process(GetChromeDataArgs args)
   at (Object , Object )
   at Sitecore.Pipelines.CorePipeline.Run(PipelineArgs args)
   at Sitecore.Pipelines.DefaultCorePipelineManager.Run(String pipelineName, PipelineArgs args, String pipelineDomain, Boolean failIfNotExists)
   at Sitecore.Pipelines.DefaultCorePipelineManager.Run(String pipelineName, PipelineArgs args, String pipelineDomain)
   at Sitecore.Pipelines.GetChromeData.GetChromeDataPipeline.Run(GetChromeDataArgs args)
   at Sitecore.Mvc.ExperienceEditor.Presentation.RenderingMarker.GetClientData()
   at Sitecore.Mvc.ExperienceEditor.Presentation.RenderingMarker.get_ClientData()
   at Sitecore.Mvc.ExperienceEditor.Presentation.RenderingMarker.GetStart()
   at Sitecore.Mvc.ExperienceEditor.Presentation.Wrapper..ctor(TextWriter writer, IMarker marker)
   at Sitecore.Mvc.ExperienceEditor.Pipelines.Response.RenderRendering.AddWrapper.Process(RenderRenderingArgs args)
   at (Object , Object )
   at Sitecore.Pipelines.CorePipeline.Run(PipelineArgs args)
   at Sitecore.Pipelines.DefaultCorePipelineManager.Run(String pipelineName, PipelineArgs args, String pipelineDomain, Boolean failIfNotExists)
   at Sitecore.Pipelines.DefaultCorePipelineManager.Run(String pipelineName, PipelineArgs args, String pipelineDomain)
   at Sitecore.Mvc.Pipelines.PipelineService.RunPipeline[TArgs](String pipelineName, TArgs args)
   at Sitecore.Mvc.Pipelines.Response.RenderPlaceholder.PerformRendering.Render(String placeholderName, TextWriter writer, RenderPlaceholderArgs args)
   at (Object , Object )
   at Sitecore.Pipelines.CorePipeline.Run(PipelineArgs args)
   at Sitecore.Pipelines.DefaultCorePipelineManager.Run(String pipelineName, PipelineArgs args, String pipelineDomain, Boolean failIfNotExists)
   at Sitecore.Pipelines.DefaultCorePipelineManager.Run(String pipelineName, PipelineArgs args, String pipelineDomain)
   at Sitecore.Mvc.Pipelines.PipelineService.RunPipeline[TArgs](String pipelineName, TArgs args)
   at Sitecore.Mvc.Helpers.SitecoreHelper.RenderPlaceholderCore(String placeholderName, TextWriter writer)
   at Sitecore.Mvc.Helpers.SitecoreHelper.Placeholder(String placeholderName)
   at ASP._Page_Views_Shared_Partial_Design_Dynamic_Placeholder_cshtml.Execute() in c:\inetpub\wwwroot\RssbPlatform.dev.local\Views\Shared\Partial Design Dynamic Placeholder.cshtml:line 5
   at System.Web.WebPages.WebPageBase.ExecutePageHierarchy()
   at System.Web.Mvc.WebViewPage.ExecutePageHierarchy()
   at System.Web.WebPages.WebPageBase.ExecutePageHierarchy(WebPageContext pageContext, TextWriter writer, WebPageRenderingBase startPage)
   at System.Web.Mvc.ViewResultBase.ExecuteResult(ControllerContext context)
   at System.Web.Mvc.ControllerActionInvoker.InvokeActionResultFilterRecursive(IList`1 filters, Int32 filterIndex, ResultExecutingContext preContext, ControllerContext controllerContext, ActionResult actionResult)
   at System.Web.Mvc.ControllerActionInvoker.InvokeActionResultFilterRecursive(IList`1 filters, Int32 filterIndex, ResultExecutingContext preContext, ControllerContext controllerContext, ActionResult actionResult)
   at System.Web.Mvc.ControllerActionInvoker.InvokeActionResultFilterRecursive(IList`1 filters, Int32 filterIndex, ResultExecutingContext preContext, ControllerContext controllerContext, ActionResult actionResult)
   at System.Web.Mvc.ControllerActionInvoker.InvokeActionResultWithFilters(ControllerContext controllerContext, IList`1 filters, ActionResult actionResult)
   at System.Web.Mvc.ControllerActionInvoker.InvokeAction(ControllerContext controllerContext, String actionName)

No single reference to my code, or whatsoever prompting to the actual reason.

Troubleshooting this was not an easy walk out. Logs also contained nothing relevant to the issue. Since I always keep at least daily backups done with a helpful tool Sif0n, it was relatively easy to substitute a web root folder from a fully functional solution. but the problem persisted. Then it is likely to be caused by something stored in a database.

I suspected something could be wrong with my serialization, I checked the settings but those were all correct. Then I restored fully functional database along with existing instance, calling it master_OK and started comparing those items one by one. And finally, I found the difference:

Reason for error: Rendering parameters template was missing due not being serialized and synced on that new environment. How could that happen? Don't know for sure, but a very alike suspect is that when using a clone rendering SXA script I accidentally missed out secondary options, so that script cloned rendering parameters for a new rendering, but left it underneath Experience Accelerator folder. Which is kept outside of serialization (as I more or less correctly predicted at the beginning of my investigation). So rendering parameters template was referenced, but did not exist on the target system. 

After fixing it, the site worked well. I do understand that the call stack I included above and the error messages are quite generic and may be met but any other scenarios, so probably Sitecore needs to assert and explicitly log such errors. At least I shared the way of dealing with it and my exact reason with you, hope it may help someone.

Imagine a case - there is a datasource item, that has a Droplink field to pointing to another item, let's say a Rich Text with an address (as on an image below):

That means, in your rendering variant you simply use Reference Variant Field referencing an item behind Rich text droplink, and within that Rich text item you simply render a field, as normal:

Seems obvious, right? Let's increase complexity!

Scenario one

We change scenario so that Droplink would now point not to an item having the field to render, but rather to an item, containing other children you might need to display:

So instead of referencing a Rich text item, we'll reference Link list item, where you need to iterate child items (links) of a referenced Link List item, and show all these links on a page with a rendering variant:

Question: how do I iterate children of my referenced item?

Okay, that's where your new friend comes to help you. Welcome, Query Rendering Variant Field!

What it actually does is also switching a context to a single item / multi items, similarly to Reference Variant Field, but instead of taking switching context from a link-powered field, it uses Sitecore search query and switches context to its execution result. Here's how one can add Query Variant Field into a rendering variant:

So for my simple example with Link List children, I use a query to search all the children of a current item (Link List):

That does the job and withing this query item you may use fields related to items coming out from a search.

One more helpful tip: when working with references, queries or whatsoever changes the context, you would often wondering iа changing context worked well and referencing what you intend to. The trick I use is inserting a debugging template field exposing ID and/or a name of where we've switched to (or any other universal property all the items do have and that are accessible from NVelocity $item):

This takes only 5 seconds of my time to implement but immediately helps to visualise a context on output. Very helpful!

Second scenario

... where Query Field Variant helps me so much. I need to implement a Subscription Level identifier on every page, so that it shows which privilege a current user has (obviously, he/she will see an only single one of the labels show below at one time):

But there is a requirement for that labels itself to be changeable and kept somewhere under site's settings item - subject to alterations at a later stage. So I already have a separate folder with such items:

With using a Reference Variant Field one would need to have rendering having a datasource item with a field pointing to one of those subscription items, which is not an elegant scenario. And what if my rendering does not accept datasources? Or it simply breaks sense, as in a described scenario?

Since we know the IDs of these subscription items we can reference them directly and that's where Query Variant Template also helps us. Also, search query by an ID always returns not more than one item, and since we know item exists we can use it as direct linking. That's how I do that:

Works like a charm!

I have just shown only two real-life examples of using Query Variant Field, but the potential area of their usage is virtually unlimited - as much as you can query you indexes. This gives an ultimate tool for comfy switching context as much as you may need that, but I'd warn you from overusing it too much.