How to Handle PreRendering in Blazor to Avoid Duplicate Initialization Code

Discover how to manage the two-pass rendering in Blazor applications effectively, ensuring your initialization code only runs once during pre-rendering.
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Tackling PreRendering Challenges in Blazor

Blazor Server applications offer dynamic and interactive web experiences, but they come with their own set of challenges, particularly when it comes to rendering. One common issue developers face is retaining variable values between the two rendering passes, especially during pre-rendering. In this guide, we will explore the problem of variable persistence in Blazor's rendering process and provide actionable solutions to help you run initialization code just once.

Understanding the Problem

When developing Blazor applications, you might notice that they are rendered in two passes:

First Pass (PreRendering): In this phase, your components are rendered on the server, and the initial HTML is sent to the client.

Second Pass (Client-Side): After the client receives the initial HTML, the Blazor framework takes over, establishing a real-time connection with the server, which is when the full interactivity is enabled.

Many developers encounter issues during this process:

Variables set during the first pass do not persist to the second pass.

Boolean flags intended to control initialization do not retain their values between the two passes.

This can lead to scenarios where initialization code runs multiple times or errors arise from untracked state, ultimately causing development errors.

The Solution: Leveraging IHttpContextAccessor

To resolve the issue of variable persistence between rendering passes, we can utilize the IHttpContextAccessor service, which allows us to detect the rendering context. By properly configuring this service, we can efficiently manage our code's initialization phase.

Step-by-Step Implementation

Add the IHttpContextAccessor Service

[[See Video to Reveal this Text or Code Snippet]]

Inject the Service in Your Component

Decide where you want to implement this check. Here’s an example of how to use it in the Index component:

[[See Video to Reveal this Text or Code Snippet]]

Understanding the Code

Check Rendering Context: The line _isServerRender = !(HttpContextAccessor.HttpContext is not null && HttpContextAccessor.HttpContext.Response.HasStarted); determines whether the current context is server-side rendering.

Conditional Execution: Based on the value of _isServerRender, you can decide whether to execute the full initialization code or just the minimal requirements to display the page.

Conclusion

By implementing the solution detailed above, you can effectively manage your initialization logic in Blazor applications, avoiding the pitfalls of duplicate execution during the two-pass rendering process. Utilizing the IHttpContextAccessor, you can create a more efficient rendering flow that works harmoniously with Blazor's architecture.

Whether you place the conditional logic in specific components like Index or globally in App, this approach will ensure you maintain control over your application's state, leading to a seamless user experience.

Take control of your Blazor Server application’s rendering logic and avoid those pesky duplicate initialization issues!