touchmove event has empty event data

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07-10-2024

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The TouchMove Event's Empty Data: A Deep Dive

Problem: You're using the touchmove event in JavaScript to track finger movements on a touch screen, but the event.touches object is empty, causing your code to malfunction.

Rephrased: Imagine trying to track a moving finger on a touchscreen, but you're handed an empty map! That's what happens when the touchmove event provides empty touch data – your code can't understand where the finger is going.

The Scenario:

Let's say you have a simple HTML element where you want to move it based on finger movements:

<div id="draggable">Drag Me!</div>

You might use the following JavaScript code:

const draggable = document.getElementById("draggable");

draggable.addEventListener('touchmove', (event) => {
  event.preventDefault(); // Prevent scrolling
  const touch = event.touches[0]; // Get the first touch
  if (touch) {
    draggable.style.left = touch.clientX + 'px';
    draggable.style.top = touch.clientY + 'px';
  } else {
    console.error("No touch data found!");
  }
});

This code aims to update the draggable element's position based on the touch coordinates. However, if event.touches is empty, the touch variable will be undefined, and the code won't work as intended.

Analyzing the Issue:

The issue lies in how browsers handle touch events. The touchmove event is triggered when the finger is actively moving across the screen. However, the browser might not provide touch data for every single movement.

Here are some common reasons why event.touches could be empty:

• Event Propagation: If the touch event is being handled by a parent element and the touchmove event is being stopped (e.g., using event.stopPropagation()), the child element might receive an empty event.touches object.
• Browser Optimization: To improve performance, browsers might skip some touchmove events, especially during rapid finger movements or when multiple touch events occur in a short time.
• Touchscreen Hardware: Some touchscreens might have limitations in their touch tracking capability, leading to missing touch data.

Solutions:

1. Check for Touch Support: Before using touch events, it's essential to verify if the device supports touch input using Modernizr or similar libraries.

2. Track Movement Using Other Events: Since touchmove might be unreliable, consider using touchstart and touchend to track touch events more reliably. The touch position can be calculated based on the starting and ending points.

3. Use a Touch Library: Libraries like HammerJS can simplify touch event handling and provide robust solutions for tracking finger movements, including situations where event.touches is empty.

Additional Value:

• Touch Events Explained: Dive deeper into the world of touch events with resources like MDN's Touch Events documentation (https://developer.mozilla.org/en-US/docs/Web/API/Touch_events).
• Best Practices for Touch Events: Learn about efficient handling of touch events and explore techniques for optimizing performance, including event throttling and debouncing.

By understanding the reasons behind empty touch data in touchmove events and adopting alternative approaches, you can build robust and reliable touch-based interactions for your web applications.