Loading Boot Resources

Controlling Headers

Use middleware to control the headers collection:

app.Use(async (context, next) =>
    context.Response.Headers.Add("Content-Security-Policy", "{POLICY STRING}");
    await next();

In hosted Blazor WASM apps, pass StaticFileOptions to MapFallbackToFile that specifies response headers:
Server project’s Program.cs:

var staticFileOptions = new StaticFileOptions
    OnPrepareResponse = context =>
            "{POLICY STRING}");


app.MapFallbackToFile("index.html", staticFileOptions);

Loading Progress Indicators

The Blazor WASM project template contains SVG and text indicators to show loading progress. These are implemented with HTML and CSS using two CSS custom properties provided by Blazor WASM:

  • --blazor-load-percentage
  • --blazor-load-percentage-text

To create custom indicators:

// resourcesLoaded is an instantaneous count of resources loaded during startup
// totalResources is the number of resources to load
const percentage = resourcesLoaded / totalResources * 100;
document.documentElement.style.setProperty('--blazor-load-percentage', `${percentage}%`);
document.documentElement.style.setProperty('--blazor-load-percentage-text', `"${Math.floor(percentage)}%"`);

The default round progress indicator is in wwwroot/index.html. To implement a linear progress indicator:


.linear-progress {
    background: silver;
    width: 50vw;
    margin: 20% auto;
    height: 1rem;
    border-radius: 10rem;
    overflow: hidden;
    position: relative;

.linear-progress:after {
    content: '';
    position: absolute;
    inset: 0;
    background: blue;
    scale: var(--blazor-load-percentage, 0%) 100%; <!-- blazor-load-percentage is updated automatically -->
    transform-origin: left top;
    transition: scale ease-out 0.5s;