Understanding the TimerOnElapsed Function in Blazor WebAssembly: Why Your Function Gets Called

Discover why the `addText` function increments a variable in Blazor WebAssembly, despite not being explicitly called, and learn how rendering and state changes work in this powerful framework.
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This video is based on the question https://stackoverflow.com/q/69906182/ asked by the user 'E.D.' ( https://stackoverflow.com/u/7327933/ ) and on the answer https://stackoverflow.com/a/69906398/ provided by the user 'Brian Parker' ( https://stackoverflow.com/u/1492496/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Unraveling the Mystery of TimerOnElapsed in Blazor WebAssembly

If you've been experimenting with Blazor WebAssembly and specifically working with timers, you may have encountered a confusing situation. You might wonder why a function (like addText) that isn’t explicitly called still seems to increment a variable and update your UI. Let’s dive into the inner workings of Blazor WebAssembly to demystify this behavior.

The Setup: What’s Happening?

In your code, you have defined a timer that is set to trigger every second. The purpose of this timer is to update a variable (sometext) that you wish to display. However, you’re perplexed because the addText function is not called directly within your timer function, but the displayed text still increments every second!

A Closer Look at the Blazor WebAssembly Code

Here’s a simplified overview of the relevant code structure:

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

Understanding StateHasChanged

What It Does

When StateHasChanged() is called, it instructs Blazor to re-render the component, which means all the code you define within your HTML markup will be evaluated again. This includes any functions you've referenced in that markup like addText.

Function Calls at Render Time

In Blazor, anytime a component re-renders, the markup and all inline C# code is executed.

Thus, your addText function seems like it’s being called because the return value is generated during every re-render.

Why Use MarkupString?

You've attempted to change the rendering method to:

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

By doing this, you may not see the desired increment because this will not call the function addText() and, instead, directly use sometext. If sometext is simply incremented in your timer, and not displayed via addText, the correct value isn’t fetched for display.

Revising Your Code for Correct Increment

To ensure your counter increments properly, you can adjust your code in the TimerOnElapsed method:

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

In this setup:

sometext is directly incremented in the elapsed event.

Calling StateHasChanged() will trigger a re-render, allowing sometext to be shown in the UI.

Conclusion: Embrace the Rendering

Understanding how rendering works in Blazor is crucial for managing state and updating the user interface successfully. The confusion around your initial observations is common among developers diving into Blazor WebAssembly. The re-rendering nature of this framework promotes efficient updates, but it also requires continuous function evaluation during those cycles.

Take the time to experiment with StateHasChanged() and the various ways to update your UI—from direct variable manipulation to custom functions—as you get more comfortable with Blazor WebAssembly.

Now you're equipped with a clearer understanding of how function calls work during rendering in Blazor! Happy coding!

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