WebF Async Rendering

Note: WebF development is nearly identical to web development - you use the same tools (Vite, npm, Vitest), same frameworks (React, Vue, Svelte), and same deployment services (Vercel, Netlify). This skill covers one of the 3 key differences: WebF's async rendering model. The other two differences are API compatibility and routing.

This is the #1 most important concept to understand when moving from browser development to WebF.

The Fundamental Difference

In Browsers (Synchronous Layout)

When you modify the DOM, the browser immediately performs layout calculations:

// Browser behavior
const div = document.createElement('div');
document.body.appendChild(div);
console.log(div.getBoundingClientRect()); // ✅ Returns real dimensions

Layout happens synchronously - you get dimensions right away, but this can cause performance issues (layout thrashing).

In WebF (Asynchronous Layout)

When you modify the DOM, WebF batches the changes and processes them in the next rendering frame:

// WebF behavior
const div = document.createElement('div');
document.body.appendChild(div);
console.log(div.getBoundingClientRect()); // ❌ Returns zeros! Not laid out yet.

Layout happens asynchronously - elements exist in the DOM tree but haven't been measured/positioned yet.

Why Async Rendering?

Performance: WebF's async rendering is 20x cheaper than browser synchronous layout!

• DOM updates are batched together
• Multiple changes processed in one optimized pass
• Eliminates layout thrashing
• No need for DocumentFragment optimizations

Trade-off: You must explicitly wait for layout to complete before measuring elements.

The Solution: onscreen/offscreen Events

WebF provides two non-standard events to handle the async lifecycle:

Event	When It Fires	Purpose
onscreen	Element has been laid out and rendered	Safe to measure dimensions, get computed styles
offscreen	Element removed from render tree	Cleanup and resource management

Think of these like IntersectionObserver but for layout lifecycle, not viewport visibility.

How to Measure Elements Correctly

❌ WRONG: Measuring Immediately

// DON'T DO THIS - Will return 0 or incorrect values
const div = document.createElement('div');
div.textContent = 'Hello WebF';
document.body.appendChild(div);

const rect = div.getBoundingClientRect();  // ❌ Returns zeros!
console.log(rect.width);  // 0
console.log(rect.height); // 0

✅ CORRECT: Wait for onscreen Event

// DO THIS - Wait for layout to complete
const div = document.createElement('div');
div.textContent = 'Hello WebF';

div.addEventListener('onscreen', () => {
  // Element is now laid out - safe to measure!
  const rect = div.getBoundingClientRect();  // ✅ Real dimensions
  console.log(`Width: ${rect.width}, Height: ${rect.height}`);
});

document.body.appendChild(div);

React: useFlutterAttached Hook

For React developers, WebF provides a convenient hook:

❌ WRONG: Using useEffect

import { useEffect, useRef } from 'react';

function MyComponent() {
  const ref = useRef(null);

  useEffect(() => {
    // ❌ Element not laid out yet!
    const rect = ref.current.getBoundingClientRect();
    console.log(rect); // Will be zeros
  }, []);

  return <div ref={ref}>Content</div>;
}

✅ CORRECT: Using useFlutterAttached

import { useFlutterAttached } from '@openwebf/react-core-ui';

function MyComponent() {
  const ref = useFlutterAttached(
    () => {
      // ✅ onAttached callback - element is laid out!
      const rect = ref.current.getBoundingClientRect();
      console.log(`Width: ${rect.width}, Height: ${rect.height}`);
    },
    () => {
      // onDetached callback (optional)
      console.log('Component removed from render tree');
    }
  );

  return <div ref={ref}>Content</div>;
}

Layout-Dependent APIs

Only call these inside onscreen callback or useFlutterAttached:

• element.getBoundingClientRect()
• window.getComputedStyle(element)
• element.offsetWidth / element.offsetHeight
• element.clientWidth / element.clientHeight
• element.scrollWidth / element.scrollHeight
• element.offsetTop / element.offsetLeft
• Any logic that depends on element position or size

Common Scenarios

Scenario 1: Measuring After Style Changes

const div = document.getElementById('myDiv');

// ❌ WRONG
div.style.width = '500px';
const rect = div.getBoundingClientRect(); // Old dimensions!

// ✅ CORRECT
div.style.width = '500px';
div.addEventListener('onscreen', () => {
  const rect = div.getBoundingClientRect(); // New dimensions!
}, { once: true }); // Use 'once' to remove listener after first call

Scenario 2: Positioning Tooltips/Popovers

function showTooltip(targetElement) {
  const tooltip = document.createElement('div');
  tooltip.className = 'tooltip';
  tooltip.textContent = 'Tooltip text';

  tooltip.addEventListener('onscreen', () => {
    // Now we can safely position the tooltip
    const targetRect = targetElement.getBoundingClientRect();
    const tooltipRect = tooltip.getBoundingClientRect();

    tooltip.style.left = `${targetRect.left}px`;
    tooltip.style.top = `${targetRect.bottom + 5}px`;
  }, { once: true });

  document.body.appendChild(tooltip);
}

Scenario 3: React Component with Measurement

import { useFlutterAttached } from '@openwebf/react-core-ui';
import { useState } from 'react';

function MeasuredBox() {
  const [dimensions, setDimensions] = useState({ width: 0, height: 0 });

  const ref = useFlutterAttached(() => {
    const rect = ref.current.getBoundingClientRect();
    setDimensions({
      width: rect.width,
      height: rect.height
    });
  });

  return (
    <div ref={ref} style={{ padding: '20px', border: '1px solid' }}>
      <p>This box is {dimensions.width}px wide</p>
      <p>and {dimensions.height}px tall</p>
    </div>
  );
}

Performance Benefits

WebF's async rendering provides significant advantages:

1. Batched Updates: Multiple DOM changes processed together
2. No Layout Thrashing: Eliminates read-write-read-write patterns
3. Optimized Rendering: Single pass through the render tree
4. No DocumentFragment Needed: Batching is automatic

Compare to browsers where you'd need to carefully batch operations:

// Browser optimization (not needed in WebF!)
const fragment = document.createDocumentFragment();
for (let i = 0; i < 100; i++) {
  const div = document.createElement('div');
  fragment.appendChild(div);
}
document.body.appendChild(fragment); // Single layout

In WebF, just append directly - it's automatically optimized!

Common Mistakes

Mistake 1: Forgetting to Wait

// ❌ WRONG
const div = document.createElement('div');
document.body.appendChild(div);
initializeWidget(div); // Assumes div is laid out - will fail!

// ✅ CORRECT
const div = document.createElement('div');
div.addEventListener('onscreen', () => {
  initializeWidget(div); // Now it's safe!
}, { once: true });
document.body.appendChild(div);

Mistake 2: Not Cleaning Up Listeners

// ❌ WRONG - Memory leak
element.addEventListener('onscreen', handleLayout);
// Listener never removed!

// ✅ CORRECT
element.addEventListener('onscreen', handleLayout, { once: true });
// OR
element.addEventListener('onscreen', handleLayout);
// Later...
element.removeEventListener('onscreen', handleLayout);

Mistake 3: Using IntersectionObserver for Layout

// ❌ WRONG - IntersectionObserver is for viewport visibility, not layout
const observer = new IntersectionObserver((entries) => {
  // This fires based on viewport, not layout completion!
});

// ✅ CORRECT - Use onscreen for layout lifecycle
element.addEventListener('onscreen', () => {
  // Element is laid out
});

Debugging Tips

If you're getting zero or incorrect dimensions:

1. Check if you're waiting for onscreen: Most common issue
2. Verify element is actually added to DOM: Must be in document tree
3. Confirm element has display style: display: none elements don't layout
4. Use console.log in onscreen callback: Verify callback fires

element.addEventListener('onscreen', () => {
  console.log('✅ onscreen fired');
  console.log(element.getBoundingClientRect());
}, { once: true });

Resources

• Core Concepts - Async Rendering: https://openwebf.com/en/docs/developer-guide/core-concepts#async-rendering
• Debugging & Performance: https://openwebf.com/en/docs/developer-guide/debugging-performance
• @openwebf/react-core-ui: Install with npm install @openwebf/react-core-ui

Key Takeaways

✅ DO:

• Use onscreen event or useFlutterAttached hook
• Wait for layout before measuring elements
• Use { once: true } for one-time measurements

❌ DON'T:

• Measure immediately after appendChild()
• Rely on synchronous layout like browsers
• Use IntersectionObserver for layout detection
• Forget to clean up event listeners