Chris Barr

Scrolling a overflow:auto; element on a touch screen device

Monday, May 03, 2010

In mobile Safari on the iPhone, iPod Touch, and iPad (as well as the webkit based browser on Android phones) it's not immediately obvious how to scroll a div that has overflow:auto; set on it. If this were a desktop browser you would see scrollbars and be able to manipulate those or even use your mouse wheel. No such concepts exist on a touch screen device!

To scroll the entire page you just touch it and move your finger. But when you touch the element that would normally scroll, the entire page scrolls instead. This is a little bit broken in my opinion since there's no visual indicator that you aren't seeing all the content. However, if you are on a site and you know there's a scrollable div there is a simple (but not obvious) workaround. Simple use two fingers at the same time and scroll them in the same direction.

This works OK but like I said it's not obvious, there's still no indicator that the content is scrollable, and when you use more than one finger you might accidentally trigger some other gesture like scaling the page. I recently ran into this exact issue at work and came up with a pretty solid solution in javascript. I broke this down into two simple functions, but the last one is where the magic happens.

This first function simply attempts to create a new touch event. Only touch screen browsers like mobile Safari have these events, so if it doesn't throw an error then we are using a touch screen device. Otherwise it's probably a desktop browser.

function isTouchDevice(){
	try{
		document.createEvent("TouchEvent");
		return true;
	}catch(e){
		return false;
	}
}

Next, this function calls the isTouchDevice() function, and if it succeeds we attach some special touch events to the page. First when a touch event begins (the touchstart event) we get the current scroll position and prevent the default browser behavior, which in this case would be to scroll the page. Next when you move your finger the touchmove event is called. Here we just subtract the position of your finger from the scroll position we saved earlier and again prevent the page from scrolling.

function touchScroll(id){
	if(isTouchDevice()){ //if touch events exist...
		var el=document.getElementById(id);
		var scrollStartPos=0;

		document.getElementById(id).addEventListener("touchstart", function(event) {
			scrollStartPos=this.scrollTop+event.touches[0].pageY;
			event.preventDefault();
		},false);

		document.getElementById(id).addEventListener("touchmove", function(event) {
			this.scrollTop=scrollStartPos-event.touches[0].pageY;
			event.preventDefault();
		},false);
	}
}

So that's it, just include these functions on your page and just call it by passing in the ID of the element you want to scroll. Like so: touchScroll("MyElement");. You can see a working demo here: http://chris-barr.com/files/touchScroll.htm. I feel like this is a better way of doing things because it's more intuitive since you're just using one finger, and it's potentially more obvious. Even if you don't immediately know there's hidden content, you might accidentally touch this while scrolling the page and realize there's more to see in this div.