Create onUserInputChange.md

snippets/onUserInputChange.md

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+### onUserInputChange
+
+Will run the callback whenever the user changes their input type (either `mouse` or `touch`). This is useful
+if you want to disable certain code depending on if the user is using touch as input or a mouse (including trackpads).
+
+Use two event listeners. Assume `mouse` input initially and bind a `touchstart` event listener to the document. 
+On `touchstart`, the callback is run and supplied with the current input type as an argument. 
+Then, add a `mousemove` event listener to listen for two consecutive `mousemove` events firing within 20ms 
+using `performance.now()` (browsers recently fire them every animation frame). Run the callback and supply the new type
+`mouse` as the argument. This process needs to happen dynamically because of hybrid devices (such as a touchscreen laptop), 
+where the user can switch between either input type at will.
+
+```js
+const onUserInputChange = callback => {
+  let type = 'mouse';
+
+  const mousemoveHandler = (() => {
+    let lastTime = 0;
+    return () => {
+      const now = performance.now();
+      if (now - lastTime < 20) {
+        type = 'mouse';
+        callback(type);
+        document.removeEventListener('mousemove', mousemoveHandler);
+      }
+      lastTime = now;
+    }
+  })();
+
+  document.addEventListener('touchstart', () => {
+    if (type === 'touch') return;
+    type = 'touch';
+    callback(type);
+    document.addEventListener('mousemove', mousemoveHandler)
+  });
+};
+```
+
+```js
+onUserInputChange(type => {
+  console.log('The user is now using', type, 'as an input method.');
+});
+```