Two-finger swipe now switches windows (#3901)

The Alt+Up and Alt+Down keybindings on Desktop did not have an
equivalent for Mobile users. Now a two-finger swipe left on a
touchscreen is equivalent to Alt+Up (similarly swipe right is
equivalent to Alt+Down).

client/js/helpers/listenForTwoFingerSwipes.js

@@ -0,0 +1,108 @@
+"use strict";
+
+// onTwoFingerSwipe will be called with a cardinal direction ("n", "e", "s" or
+// "w") as its only argument.
+function listenForTwoFingerSwipes(onTwoFingerSwipe) {
+	let history = [];
+
+	document.body.addEventListener(
+		"touchmove",
+		function (event) {
+			if (event.touches.length !== 2) {
+				return;
+			}
+
+			const a = event.touches.item(0);
+			const b = event.touches.item(1);
+
+			const timestamp = window.performance.now();
+			const center = [(a.screenX + b.screenX) / 2, (a.screenY + b.screenY) / 2];
+
+			if (history.length > 0) {
+				const last = history[history.length - 1];
+				const centersAreEqual =
+					last.center[0] === center[0] && last.center[1] === center[1];
+
+				if (last.timestamp === timestamp || centersAreEqual) {
+					// Touches with the same timestamps or center don't help us
+					// see the speed of movement. Ignore them.
+					return;
+				}
+			}
+
+			history.push({timestamp, center});
+		},
+		{passive: true}
+	);
+
+	document.body.addEventListener(
+		"touchend",
+		function () {
+			if (event.touches.length >= 2) {
+				return;
+			}
+
+			try {
+				const direction = getSwipe(history);
+
+				if (direction) {
+					onTwoFingerSwipe(direction);
+				}
+			} finally {
+				history = [];
+			}
+		},
+		{passive: true}
+	);
+
+	document.body.addEventListener(
+		"touchcancel",
+		function () {
+			history = [];
+		},
+		{passive: true}
+	);
+}
+
+// Returns the cardinal direction of the swipe or null if there is no swipe.
+function getSwipe(hist) {
+	// Speed is in pixels/millisecond. Must be maintained throughout swipe.
+	const MIN_SWIPE_SPEED = 0.2;
+
+	if (hist.length < 2) {
+		return null;
+	}
+
+	for (let i = 1; i < hist.length; ++i) {
+		const previous = hist[i - 1];
+		const current = hist[i];
+
+		const speed =
+			distance(previous.center, current.center) /
+			Math.abs(previous.timestamp - current.timestamp);
+
+		if (speed < MIN_SWIPE_SPEED) {
+			return null;
+		}
+	}
+
+	return getCardinalDirection(hist[0].center, hist[hist.length - 1].center);
+}
+
+function distance([x1, y1], [x2, y2]) {
+	return Math.hypot(x1 - x2, y1 - y2);
+}
+
+function getCardinalDirection([x1, y1], [x2, y2]) {
+	// If θ is the angle of the vector then this is tan(θ)
+	const tangent = (y2 - y1) / (x2 - x1);
+
+	// All values of |tan(-45° to 45°)| are less than 1, same for 145° to 225°
+	if (Math.abs(tangent) < 1) {
+		return x1 < x2 ? "e" : "w";
+	}
+
+	return y1 < y2 ? "s" : "n";
+}
+
+export default listenForTwoFingerSwipes;

client/js/keybinds.js

@@ -6,6 +6,7 @@ import store from "./store";
 import {switchToChannel} from "./router";
 import isChannelCollapsed from "./helpers/isChannelCollapsed";
 import isIgnoredKeybind from "./helpers/isIgnoredKeybind";
+import listenForTwoFingerSwipes from "./helpers/listenForTwoFingerSwipes";
 
 // Switch to the next/previous window in the channel list.
 Mousetrap.bind(["alt+up", "alt+down"], function (e, keys) {
@@ -13,11 +14,22 @@ Mousetrap.bind(["alt+up", "alt+down"], function (e, keys) {
 		return true;
 	}
 
+	navigateWindow(keys.split("+").pop() === "up" ? -1 : 1);
+
+	return false;
+});
+
+listenForTwoFingerSwipes(function (cardinalDirection) {
+	if (cardinalDirection === "e" || cardinalDirection === "w") {
+		navigateWindow(cardinalDirection === "e" ? -1 : 1);
+	}
+});
+
+function navigateWindow(direction) {
 	if (store.state.networks.length === 0) {
-		return false;
+		return;
 	}
 
-	const direction = keys.split("+").pop() === "up" ? -1 : 1;
 	const flatChannels = [];
 	let index = -1;
 
@@ -44,9 +56,7 @@ Mousetrap.bind(["alt+up", "alt+down"], function (e, keys) {
 	index = (((index + direction) % length) + length) % length;
 
 	jumpToChannel(flatChannels[index]);
-
-	return false;
-});
+}
 
 // Switch to the next/previous lobby in the channel list
 Mousetrap.bind(["alt+shift+up", "alt+shift+down"], function (e, keys) {