add crossfade and flip functions (sveltejs#2241)

Author: Rich-Harris

.gitignore

@@ -10,6 +10,7 @@ node_modules
 /easing.js
 /motion.*
 /transition.js
+/animate.js
 /scratch/
 /coverage/
 /coverage.lcov/

animate.mjs

@@ -0,0 +1,25 @@
+import { cubicOut } from './easing';
+import { is_function } from './internal';
+
+export function flip(node, animation, params) {
+	const style = getComputedStyle(node);
+	const transform = style.transform === 'none' ? '' : style.transform;
+
+	const dx = animation.from.left - animation.to.left;
+	const dy = animation.from.top - animation.to.top;
+
+	const d = Math.sqrt(dx * dx + dy * dy);
+
+	const {
+		delay = 0,
+		duration = d => Math.sqrt(d) * 120,
+		easing = cubicOut
+	} = params;
+
+	return {
+		delay,
+		duration: is_function(duration) ? duration(d) : duration,
+		easing,
+		css: (t, u) => `transform: ${transform} translate(${u * dx}px, ${u * dy}px);`
+	};
+}

rollup.config.js

@@ -88,7 +88,7 @@ export default [
 	},
 
 	// everything else
-	...['index', 'store', 'easing', 'transition'].map(name => ({
+	...['index', 'store', 'easing', 'transition', 'animate'].map(name => ({
 		input: `${name}.mjs`,
 		output: {
 			file: `${name}.js`,

transition.mjs

@@ -1,4 +1,5 @@
 import { cubicOut, cubicInOut } from './easing';
+import { assign, is_function } from './internal';
 
 export function fade(node, {
 	delay = 0,
@@ -79,4 +80,62 @@ export function draw(node, {
 		easing,
 		css: (t, u) => `stroke-dasharray: ${t * len} ${u * len}`
 	};
+}
+
+export function crossfade({ fallback, ...defaults }) {
+	let to_receive = new Map();
+	let to_send = new Map();
+
+	function crossfade(from, node, params) {
+		const {
+			delay = 0,
+			duration = d => Math.sqrt(d) * 30,
+			easing = cubicOut
+		} = assign(assign({}, defaults), params);
+
+		const to = node.getBoundingClientRect();
+		const dx = from.left - to.left;
+		const dy = from.top - to.top;
+
+		const style = getComputedStyle(node);
+		const transform = style.transform === 'none' ? '' : style.transform;
+
+		return {
+			delay,
+			duration: is_function(duration) ? duration(d) : duration,
+			easing,
+			css: (t, u) => `
+				opacity: ${t};
+				transform: ${transform} translate(${u * dx}px,${u * dy}px);
+			`
+		};
+	}
+
+	function transition(items, counterparts, intro) {
+		return (node, params) => {
+			items.set(params.key, {
+				rect: node.getBoundingClientRect()
+			});
+
+			return () => {
+				if (counterparts.has(params.key)) {
+					const { rect } = counterparts.get(params.key);
+					counterparts.delete(params.key);
+
+					return crossfade(rect, node, params);
+				}
+
+				// if the node is disappearing altogether
+				// (i.e. wasn't claimed by the other list)
+				// then we need to supply an outro
+				items.delete(params.key);
+				return fallback(node, params, intro);
+			};
+		};
+	}
+
+	return [
+		transition(to_send, to_receive, false),
+		transition(to_receive, to_send, true)
+	];
 }