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Use passive event listeners for touchstart/touchmove #4440

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merged 3 commits into from
Jun 28, 2017
Merged

Use passive event listeners for touchstart/touchmove #4440

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7720b58
Use passive event listenes for touchstart/touchmove
mister-ben Jun 22, 2017
d67dfc4
Define passive events
mister-ben Jun 26, 2017
643d308
Wrap try-catch in IIFE
mister-ben Jun 26, 2017
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25 changes: 24 additions & 1 deletion src/js/utils/events.js
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Original file line number Diff line number Diff line change
Expand Up @@ -202,6 +202,23 @@ export function fixEvent(event) {
return event;
}

/**
* Whether passive event listeners are supported
*/
let _supportsPassive = false;

try {
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we should wrap this try/catch in an iife.

const opts = Object.defineProperty({}, 'passive', {
get() {
_supportsPassive = true;
}
});

window.addEventListener('test', null, opts);
} catch (e) {
// disregard
}

/**
* Add an event listener to element
* It stores the handler function in a separate cache object
Expand Down Expand Up @@ -273,7 +290,13 @@ export function on(elem, type, fn) {

if (data.handlers[type].length === 1) {
if (elem.addEventListener) {
elem.addEventListener(type, data.dispatcher, false);
let options = false;

if (_supportsPassive &&
['touchstart', 'touchmove'].indexOf(type) > -1) {
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I wonder if this array ought to be defined once in the parent scope rather than inline. Something like:

const passiveEvents = [
  'touchstart',
  'touchmove'
];

options = {passive: true};
}
elem.addEventListener(type, data.dispatcher, options);
} else if (elem.attachEvent) {
elem.attachEvent('on' + type, data.dispatcher);
}
Expand Down