Fix for reverse animation with negative speed

dev/examples/Animation-reverse.tsx

@@ -0,0 +1,29 @@
+import { useAnimate } from "framer-motion"
+import * as React from "react"
+
+export const App = () => {
+    const [scope, animate] = useAnimate()
+
+    return (
+        <div className="App" ref={scope}>
+            <div
+                className="four"
+                style={{ width: 50, height: 50, backgroundColor: "blue" }}
+            ></div>
+            <p>reverse</p>
+            <button
+                onClick={() => {
+                    const animation = animate(
+                        ".four",
+                        { x: 90 },
+                        { duration: 2 }
+                    )
+                    animation.time = animation.duration
+                    animation.speed = -1
+                }}
+            >
+                play
+            </button>
+        </div>
+    )
+}

dev/tests/animate-reverse.tsx

@@ -0,0 +1,53 @@
+import { motion, animate } from "framer-motion"
+import * as React from "react"
+import { useEffect, useState } from "react"
+import styled from "styled-components"
+
+const Container = styled.section`
+    position: relative;
+    display: flex;
+    flex-direction: column;
+    padding: 100px;
+
+    div {
+        width: 100px;
+        height: 100px;
+        background-color: red;
+    }
+`
+
+export const App = () => {
+    const [count, setCount] = useState(0)
+    const [result, setResult] = useState("")
+
+    useEffect(() => {
+        if (count % 2 === 0) return
+
+        const output: number[] = []
+        const controls = animate(0, 100, {
+            duration: 0.5,
+            ease: "linear",
+            onUpdate: (v: number) => output.push(v),
+            onComplete: () =>
+                setResult(
+                    output[1] === 100 && output.length !== 2
+                        ? "Success"
+                        : "Fail"
+                ),
+        })
+        controls.time = controls.duration
+        controls.speed = -1
+
+        return controls.stop
+    }, [count])
+
+    return (
+        <Container>
+            <button id="action" onClick={() => setCount((c) => c + 1)}>
+                Animate
+            </button>
+            <input id="result" readOnly value={result} />
+            <motion.div className="box" layout />
+        </Container>
+    )
+}

packages/framer-motion/cypress/integration/animate-reverse.ts

@@ -0,0 +1,13 @@
+describe("animate() x layout prop in reverse speed", () => {
+    it("animate() plays as expected when layout prop is present", () => {
+        cy.visit("?test=animate-reverse")
+            .wait(1000)
+            .get("#action")
+            .trigger("click", 1, 1, { force: true })
+            .wait(600)
+            .get("#result")
+            .should(([$element]: any) => {
+                expect($element.value).to.equal("Success")
+            })
+    })
+})

packages/framer-motion/src/animation/animators/js/__tests__/animate.test.ts

@@ -1232,6 +1232,45 @@ describe("animate", () => {
         expect(output).toEqual([0, 20, 40, 20, 0])
     })
 
+    test("Reverse animation from the end", async () => {
+        const output: number[] = []
+
+        const animation = animateValue({
+            keyframes: [0, 100],
+            driver: syncDriver(20),
+            duration: 100,
+            ease: linear,
+            onUpdate: (v) => {
+                output.push(v)
+            },
+        })
+        animation.time = 100
+        animation.speed = -1
+
+        await animation
+
+        expect(output).toEqual([100, 80, 60, 40, 20, 0])
+    })
+
+    test("Reverse animation from the end with half speed", async () => {
+        const output: number[] = []
+
+        const animation = animateValue({
+            keyframes: [0, 100],
+            driver: syncDriver(20),
+            duration: 100,
+            ease: linear,
+            onUpdate: (v) => {
+                output.push(v)
+            },
+        })
+        animation.time = 100
+        animation.speed = -0.5
+
+        await animation
+        expect(output).toEqual([100, 90, 80, 70, 60, 50, 40, 30, 20, 10, 0])
+    })
+
     test("Correctly ends animations with duration: 0", async () => {
         const animation = animateValue({
             keyframes: [0, 100],

packages/framer-motion/src/animation/animators/js/index.ts

@@ -143,16 +143,22 @@ export function animateValue<V = number>({
          * a pending operation that gets resolved here.
          */
         if (speed > 0) startTime = Math.min(startTime, timestamp)
+        if (speed < 0)
+            startTime = Math.min(timestamp - totalDuration / speed, startTime)
 
         if (holdTime !== null) {
             currentTime = holdTime
         } else {
-            currentTime = (timestamp - startTime) * speed
+            // Rounding the time because floating point arithmetic is not always accurate, e.g. 3000.367 - 1000.367 =
+            // 2000.0000000000002. This is a problem when we are comparing the currentTime with the duration, for
+            // example.
+            currentTime = Math.round(timestamp - startTime) * speed
         }
 
         // Rebase on delay
-        const timeWithoutDelay = currentTime - delay
-        const isInDelayPhase = timeWithoutDelay < 0
+        const timeWithoutDelay = currentTime - delay * (speed >= 0 ? 1 : -1)
+        const isInDelayPhase =
+            speed >= 0 ? timeWithoutDelay < 0 : timeWithoutDelay > totalDuration
         currentTime = Math.max(timeWithoutDelay, 0)
 
         /**
@@ -240,14 +246,12 @@ export function animateValue<V = number>({
         let { done } = state
 
         if (!isInDelayPhase && calculatedDuration !== null) {
-            done = currentTime >= totalDuration
+            done = speed >= 0 ? currentTime >= totalDuration : currentTime <= 0
         }
 
         const isAnimationFinished =
             holdTime === null &&
-            (playState === "finished" ||
-                (playState === "running" && done) ||
-                (speed < 0 && currentTime <= 0))
+            (playState === "finished" || (playState === "running" && done))
 
         if (onUpdate) {
             onUpdate(state.value)