add svelte/motion, implement tween function

motion.mjs

@@ -0,0 +1,149 @@
+import { writable } from './store';
+import { assign } from './internal';
+import { linear } from './easing';
+
+const tasks = new Set();
+let running = false;
+
+function run_tasks() {
+	tasks.forEach(task => {
+		if (!task[0]()) {
+			tasks.delete(task);
+			task[1]();
+		}
+	});
+
+	running = tasks.size > 0;
+	if (running) requestAnimationFrame(run_tasks);
+}
+
+function add_task(fn) {
+	let task;
+
+	if (!running) {
+		running = true;
+		requestAnimationFrame(run_tasks);
+	}
+
+	return {
+		promise: new Promise(fulfil => {
+			tasks.add(task = [fn, fulfil]);
+		}),
+		abort() {
+			tasks.delete(task);
+		}
+	};
+}
+
+function is_date(obj) {
+	return Object.prototype.toString.call(obj) === '[object Date]';
+}
+
+function get_interpolator(a, b) {
+	if (a === b || a !== a) return () => a;
+
+	const type = typeof a;
+
+	if (type !== typeof b || Array.isArray(a) !== Array.isArray(b)) {
+		throw new Error('Cannot interpolate values of different type');
+	}
+
+	if (Array.isArray(a)) {
+		const arr = b.map((bi, i) => {
+			return get_interpolator(a[i], bi);
+		});
+
+		return t => arr.map(fn => fn(t));
+	}
+
+	if (type === 'object') {
+		if (!a || !b) throw new Error('Object cannot be null');
+
+		if (is_date(a) && is_date(b)) {
+			a = a.getTime();
+			b = b.getTime();
+			const delta = b - a;
+			return t => new Date(a + t * delta);
+		}
+
+		const keys = Object.keys(b);
+		const interpolators = {};
+
+		keys.forEach(key => {
+			interpolators[key] = get_interpolator(a[key], b[key]);
+		});
+
+		return t => {
+			const result = {};
+			keys.forEach(key => {
+				result[key] = interpolators[key](t);
+			});
+			return result;
+		};
+	}
+
+	if (type === 'number') {
+		const delta = b - a;
+		return t => a + t * delta;
+	}
+
+	throw new Error(`Cannot interpolate ${type} values`);
+}
+
+export function tween(value, defaults = {}) {
+	const store = writable(value);
+
+	let task;
+	let target_value = value;
+
+	function set(new_value, opts) {
+		target_value = new_value;
+
+		let previous_task = task;
+		let started = false;
+
+		let {
+			delay = 0,
+			duration = 400,
+			easing = linear,
+			interpolate = get_interpolator
+		} = assign(assign({}, defaults), opts);
+
+		const start = window.performance.now() + delay;
+		let fn;
+
+		task = add_task(() => {
+			const time = window.performance.now();
+			if (time < start) return true;
+
+			if (!started) {
+				fn = interpolate(value, new_value);
+				if (typeof duration === 'function') duration = duration(value, new_value);
+				started = true;
+			}
+
+			if (previous_task) {
+				previous_task.abort();
+				previous_task = null;
+			}
+
+			const elapsed = time - start;
+
+			if (elapsed > duration) {
+				store.set(value = new_value);
+				return false;
+			}
+
+			store.set(value = fn(easing(elapsed / duration)));
+			return true;
+		});
+
+		return task.promise;
+	}
+
+	return {
+		set,
+		update: fn => set(fn(target_value, value)),
+		subscribe: store.subscribe
+	};
+}