Pattern for implementing a custom HtmlCacheClearer

InSitecore/December 12, 2019/4 Min read

While there are many blog posts detailing how to implement a custom HtmlCacheClearer, some of these blog posts omit the fact that the out of the box HtmlCacheClearer can be used with multiple type of EventArgs. The reason for multiple types of EventArgs is due to the separation of how the cache is being cleared through the event manager, either from a local event or from a remote event. Note that generally local events are executed on the machine that initiated the action, whereas, a remote event will be executed on the machine that is listening to these events - rule of thumb, your CM server listens to local events and your CD server will listen to the remote events. You typically only care about accessing the EventArgs when you need to know more about what parameters were used when clearing the cache, such as the Target database, the language, the root item, etc.

With this post, I want to provide a base implementation of the HtmlCacheClearer that provides a clean separation for processing local and remote events.

public abstract class BaseHtmlCacheClearer<T1, T2> 
    where T1: SitecoreEventArgs
    where T2: PublishEndRemoteEventArgs
{
    public virtual void ClearCache(object sender, EventArgs args)
    {
        if (args == null)
        {
            return;
        }

        switch (args)
        {
            case T1 t1Args:
            {
                this.ClearCache(sender, t1Args);
                return;
            }
            case T2 t2Args:
            {
                this.ClearCache(sender, t2Args);
                return;
            }
            default:
            {
                throw new NotImplementedException($"Unsupported type found, {args.GetType().FullName}");
            }
        }
    }

    protected abstract void ClearCache(object sender, T1 args);

    protected abstract void ClearCache(object sender, T2 args);
}

The base implementation exposes separate methods for clearing local and remote events. To implement a custom HtmlCacheClearer we simply need to create a derived type of BaseHtmlCacheClearer and set the generic types we are targeting. For instance, we could do the following:

public class HtmlCacheClearer : BaseHtmlCacheClearer<SitecoreEventArgs, PublishEndRemoteEventArgs>
{
    protected override void ClearCache(object sender, SitecoreEventArgs args)
    {
        // Write your code here for the local event
    }

    protected override void ClearCache(object sender, PublishEndRemoteEventArgs args)
    {
        // Write your code here for the remote event
    }
}

As a bonus, here is another example of an implementation using the BaseHtmlCacheClearer that pulls parameters from both EventArgs and centralizes the logic for executing your custom code:

public class HtmlCacheClearer : BaseHtmlCacheClearer<SitecoreEventArgs, PublishEndRemoteEventArgs>
{
    protected override void ClearCache(object sender, SitecoreEventArgs args)
    {
        var publisher = Event.ExtractParameter(args, 0) as Publisher;

        if (publisher == null)
        {
            // Log a useful message
            return;
        }

        var database = publisher.Options.TargetDatabase;
        var rootItem = publisher.Options.RootItem;
        var language = publisher.Options.Language;

        this.ClearCache(database, rootItem, language);
    }

    protected override void ClearCache(object sender, PublishEndRemoteEventArgs args)
    {
        var database = Database.GetDatabase(args.TargetDatabaseName);
        var language = Language.Parse(args.LanguageName);
        var rootItemId = ID.Parse(args.RootItemId);
        var rootItem = database.GetItem(rootItemId, language);

        this.ClearCache(database, rootItem, language);
    }

    protected virtual void ClearCache(Database database, Item rootItem, Language language)
    {
        Assert.IsNotNull(database, "database != null");
        Assert.IsNotNull(rootItem, "rootItem != null");
        Assert.IsNotNull(language, "language != null");

        // Write your custom code
    }
}

Hope you enjoyed this post!

patterns publishing

Kyle KingsburyA Sitecore web development blog

Pattern for implementing a custom HtmlCacheClearer