Vielzeug

Responsive Viewport Tracking ​

Problem ​

A page needs one current breakpoint value that updates when viewport dimensions change. createViewport() supplies the measurements and owns the browser listeners.

Solution ​

Render the current breakpoint into an output element and clean up the subscription and Sentinel on page exit.

ts
import { createViewport } from '@vielzeug/sentinel';

const output = document.createElement('output');
document.body.append(output);

function observeBreakpoint(): () => void {
  const viewport = createViewport();
  const render = () => {
    const width = viewport.getSnapshot().width;
    output.value = width < 640 ? 'mobile' : width < 1024 ? 'tablet' : 'desktop';
  };

  render();
  const unsubscribe = viewport.subscribe(render);

  return () => {
    unsubscribe();
    viewport.dispose();
  };
}

const stopObserving = observeBreakpoint();
// Call stopObserving() when this output is removed.

Pitfalls ​

  • Use CSS media queries when only presentation changes.
  • Read the new snapshot from viewport.getSnapshot() inside the listener.
  • Dispose the Sentinel and unsubscribe the listener when their owner ends.