Appcelerator Titanium code for managing scrollviews inside of scrollviews

NestedScrollViewManager.js

function NestedScrollViewManager (parent_view, child_views)
{
	// Appcelerator Titanium code for managing scrollviews inside of scrollviews (not true Android
	// NestedScrollViews, but just a horizontal scrollview inside of a vertical scrollview).
	//
	// If you want to put a horizontal scrollview inside a vertical scrollview (like the Netflix app UI),
	// it seems to work reasonably well on iOS.  But on android, the user experience is very janky.
	// Unless the user's drag movements are nearly exactly horizontal, there will be some movement of
	// the parent scrollview, and then it becomes very difficult to scroll the child view.  Flinging is
	// almost impossible.
	//
	// With a little code to detect the horizontal movement, we can temporarily lock the parent view
	// from scrolling, making the behavior closer to that of ios.
	//
	// call it like this:
	//
	// var NestedScrollViewManager = require ('/path/to/NestedScrollViewManager');
	// var nsvm = new NestedScrollViewManager (vertical_scrollview, horizontal_scrollview);
	//
	// (assuming vertical_scrollview is the parent scrollview containing horizontal_scrollview)

	if (Ti.Platform.osname !== 'android')
	{
		return;
	}

	if (!_.isArray(child_views)) {
		child_views = [child_views];
	}

	var startx,
	starty,
	direction_detected = false;

	parent_view.addEventListener ('touchstart', function (e) {
		parent_view.scrollingEnabled = true;
		_.each(child_views, function(child_view) {
			child_view.scrollingEnabled = true;
		});
	});

	_.each(child_views, function(child_view) {
		child_view.addEventListener('touchstart', function (e) {
			startx = e.x;
			starty = e.y;
			direction_detected = false;

			parent_view.scrollingEnabled = false;
			child_view.scrollingEnabled = true;

			e.cancelBubble = true;
		});

		child_view.addEventListener('touchmove', function (e) {
			if (direction_detected)
			{
				return;
			}

			var deltax = Math.abs (e.x - startx);
			var deltay = Math.abs (e.y - starty);
			var distance = Math.sqrt (deltax * deltax + deltay * deltay);

			if (distance > 10)
			{
				if (deltax > deltay)
				{
					parent_view.scrollingEnabled = false;
					child_view.scrollingEnabled = true;
				}
				if (deltay > deltax)
				{
					parent_view.scrollingEnabled = true;
					child_view.scrollingEnabled = false;
				}
				direction_detected = true;
			}
		});
	});

}

module.exports = NestedScrollViewManager;