Blazor RenderFragment – How To Use Plugins To Generate HTML

ASP.NET Core Blazor is a revolutionary framework from Microsoft that allows developers to build interactive web applications using C# instead of JavaScript. This shift has enabled .NET developers to use their existing skills to create rich, client-side web UI, eliminating the need to juggle between different languages. With the power of .NET on both the server and client side, Blazor has quickly gained traction, offering a seamless experience for building web applications. In this article, we’ll focus on the Blazor RenderFragment and its use cases.

At the heart of Blazor’s component architecture is the RenderFragment. It’s a delegate that represents a segment of UI content, essentially a piece of Razor markup. This Razor markup gets transformed into a structure that Blazor can use to update the DOM. In simpler terms, RenderFragment is a tool that allows developers to define and manipulate HTML content directly within their C# code.

Understanding Razor Components

Before we jump into what a Blazor RenderFragment is in more detail, we need should understand the basics of razor components. Razor components are a fundamental aspect of Blazor, encapsulating the HTML markup and the corresponding logic into a reusable piece. When developing with Blazor, you’ll find that razor components significantly streamline the process, promoting a clean separation of concerns between the UI and the backend logic.

Razor components are self-contained, making it a breeze to manage and evolve your application’s functionality incrementally. They are the building blocks that foster a modular design, which is invaluable in constructing intuitive and maintainable Blazor applications. The simplicity yet effectiveness of razor components make them a quintessential feature of the Blazor framework, allowing developers to craft interactive web UIs effortlessly.

What is a Blazor RenderFragment?

A Blazor RenderFragment is a built-in delegate type in Blazor, representing a block of Razor content. It’s a crucial part of the Blazor component model, allowing developers to encapsulate and render UI content dynamically. When a component renders, it produces a sequence of RenderFragment, which Blazor then uses to update the DOM.

Unlike traditional rendering mechanisms in ASP.NET Core, where views are separate from the logic, RenderFragment allows for a more integrated approach. This integration is what makes Blazor’s component-based architecture so powerful and flexible.

The Mechanics of Using Blazor RenderFragment

To use RenderFragment in a Blazor application, you typically define it within a component. Here’s a basic example:

@code {
    private RenderFragment DynamicContent = builder =>
    {
        builder.OpenElement(0, "h1");
        builder.AddContent(1, "Hello from RenderFragment!");
        builder.CloseElement();
    };
}

In the component’s Razor view, you can then use the defined RenderFragment:

@DynamicContent

When this component is rendered, it will produce an <h1> element with the text “Hello from RenderFragment!”.

Plugins and Dynamic HTML Generation

Blazor’s flexibility doesn’t stop at built-in components. With the power of a Blazor RenderFragment, developers can also integrate plugins that generate dynamic HTML content. For instance, a markdown plugin could be used to convert markdown content into HTML directly within a Blazor component.

The beauty of RenderFragment is that it can accept and render this dynamic content seamlessly. Whether it’s content from first-party libraries, third-party libraries, plugins, or other external sources, a Blazor RenderFragment acts as a bridge, facilitating the integration of dynamic HTML generation into Blazor applications.

We can use dependency injection frameworks like Autofac to create plugins that perform some functionality to output HTML. From there, our core application can be responsible for wrapping the HTML in a RenderFragment. You could consider instances where you want to offer user input that gets transformed into HTML, and you have a plugin for each type of input. For example, converting Markdown or raw HTML user input and visualizing right in the application. You may just want to have plugins that can populate parts of the UI with the total flexibility of having raw HTML get dropped in too!

Have a look at this video series walking through plugins in Blazor:

Benefits of Using RenderFragment in Blazor

Blazor, a standout feature of ASP.NET Core, has revolutionized the way developers approach web application development. One of its most powerful tools is the RenderFragment. Let’s delve into its benefits:

• Enhanced Flexibility in Component Rendering: With RenderFragment, developers can dynamically construct and render UI segments, allowing for a more adaptable component design.
• Streamlined Integration with Plugins and Third-party Libraries: RenderFragment acts as a bridge, making it easier to incorporate dynamic content from plugins or external sources directly into Blazor components.
• Potential for Creating More Interactive and Dynamic User Interfaces: By leveraging RenderFragment, developers can craft more responsive and interactive UIs, enhancing the user experience.

Plugin & Blazor RenderFragment Considerations

While RenderFragment offers numerous advantages, it’s not without its challenges. The same thing can be said about leveraging a plugin architecture. Here are some common pitfalls and how to navigate them:

• Overcomplication: It’s easy to get carried away with dynamic rendering, leading to unnecessarily complex components. It’s crucial to strike a balance and use RenderFragment judiciously. If you have no bounds on how many plugins you have, consider what scenario you may find yourself in.
• Performance Concerns: Dynamic content generation can sometimes lead to performance bottlenecks. Developers should be mindful of the impact on rendering times and overall application responsiveness. Similarly to the previous point, what happens when you have more and more plugins? How can you be sure the plugins are “well behaved” and give you adequate HTML to render?

You should certainly consider testing the performance impact when introducing dynamic content. Consider scenarios with more plugins being added and how performance will take a hit. Maybe you want to use BenchmarkDotNet to set up some baselines!

Additionally, consider how you are using plugins in your application. Does every spot actually need to be dynamically populated with raw HTML directly from a plugin, or is this pattern maybe being taken too far? Check out this video for an example of navigating Blazor plugin complexity:

Practical Example: Using RenderFragment with Plugins

Imagine you’re working on a Blazor project where you want to allow users to input text in different formats, and have HTML rendered for it. We could have a markdown plugin that can convert to HTML and then we could see the rendered HTML in real-time. Using RenderFragment can be a great solution for this, and we can use plugins to extend it to other formats as well.

Let’s consider some example code that can resolve plugins, pass it user input, and then render the resulting HTML:

@code {
    private string userInput = "## Hello, Blazor!";
    private RenderFragment dynamicContent => builder =>
    {
        var converter = ResolveConverterPlugin();
        var htmlContent = converter .ConvertToHtml(userInput);
        builder.AddMarkupContent(0, htmlContent);
    };
}

<div>
    <textarea @bind="userInput"></textarea>
    <div>@dynamicContent</div>
</div>

In this example, the RenderFragment named dynamicContent is used to convert the user input content from the userInput variable into HTML. It does this by resolving a plugin by calling a hypothetical method named ResolveConverterPlugin. I’m leaving out some of the implementation details here because you can look into things like loading plugins with Autofac separately. This HTML is then rendered dynamically in the Blazor component.

Keep in mind that the logic to resolve plugins may look very different from application to application. For example, by the time we’re executing this code in a particular app, we might already have the plugin resolved for us. In other situations, maybe we have something like a dropdown UI control that can change the conversion type from user input to HTML. For that, we would need to look up the appropriate plugin dynamically based on that selection. These implementation details are for you to decide!

Best Practices for Using RenderFragment in Blazor

Plugins aside, here are some best practices to consider when using the ASP.NET Core Blazor RenderFragment:

• Keep It Simple: While RenderFragment offers dynamic rendering, it’s essential to avoid overcomplicating the component logic.
• Performance Testing: Always test the impact of dynamic content on the application’s performance. This ensures that the user experience remains smooth.
• Modular Design: Break down components into smaller, reusable segments. This not only makes the code more maintainable but also optimizes the rendering process.
• Thorough Testing: Given the dynamic nature of the content, rigorous testing is crucial to catch and rectify any rendering anomalies.

Wrapping Up the Blazor RenderFragment

The RenderFragment in ASP.NET Core Blazor offers developers flexibility in crafting dynamic and interactive web applications. While it comes with its set of challenges, the benefits far outweigh the drawbacks. Plugins are no different in that they can be powerful but can create a lot of complexity if overused. These two things together can work very well OR if misused can create a pile of headaches!

For those diving into Blazor, embracing the Blazor RenderFragment can open doors to innovative and responsive web application designs. And if you’re new to playing with plugin architectures, there’s a whole lot to learn! Dive in, experiment, and harness the power of these two tools in your next Blazor project!

If you want to keep up to date with more posts and videos that include topics like Blazor and plugins, subscribe to my weekly newsletter! Each weekend I’ll send you helpful software engineering content to help you level up.

Nick Cosentino Principal Software Engineering Manager

Principal Software Engineering Manager at Microsoft. Views are my own.

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