Support for linear() easing function

packages/framer-motion/package.json

@@ -98,23 +98,23 @@
     "bundlesize": [
         {
             "path": "./dist/size-rollup-motion.js",
-            "maxSize": "33.85 kB"
+            "maxSize": "34.02 kB"
         },
         {
             "path": "./dist/size-rollup-m.js",
             "maxSize": "5.9 kB"
         },
         {
             "path": "./dist/size-rollup-dom-animation.js",
-            "maxSize": "16.9 kB"
+            "maxSize": "17 kB"
         },
         {
             "path": "./dist/size-rollup-dom-max.js",
-            "maxSize": "29 kB"
+            "maxSize": "29.1 kB"
         },
         {
             "path": "./dist/size-rollup-animate.js",
-            "maxSize": "17.7 kB"
+            "maxSize": "17.9 kB"
         },
         {
             "path": "./dist/size-rollup-scroll.js",

packages/framer-motion/src/animation/animators/AcceleratedAnimation.ts

@@ -1,3 +1,6 @@
+import { anticipate } from "../../easing/anticipate"
+import { backInOut } from "../../easing/back"
+import { circInOut } from "../../easing/circ"
 import { EasingDefinition } from "../../easing/types"
 import { DOMKeyframesResolver } from "../../render/dom/DOMKeyframesResolver"
 import { ResolvedKeyframes } from "../../render/utils/KeyframesResolver"
@@ -20,7 +23,7 @@ import {
 import { MainThreadAnimation } from "./MainThreadAnimation"
 import { acceleratedValues } from "./utils/accelerated-values"
 import { animateStyle } from "./waapi"
-import { isWaapiSupportedEasing } from "./waapi/easing"
+import { isWaapiSupportedEasing, supportsLinearEasing } from "./waapi/easing"
 import { getFinalKeyframe } from "./waapi/utils/get-final-keyframe"
 
 const supportsWaapi = /*@__PURE__*/ memo(() =>
@@ -110,6 +113,12 @@ interface ResolvedAcceleratedAnimation {
     keyframes: string[] | number[]
 }
 
+const unsupportedEasingFunctions = {
+    anticipate,
+    backInOut,
+    circInOut,
+}
+
 export class AcceleratedAnimation<
     T extends string | number
 > extends BaseAnimation<T, ResolvedAcceleratedAnimation> {
@@ -159,6 +168,22 @@ export class AcceleratedAnimation<
             return false
         }
 
+        /**
+         * If the user has provided an easing function name that isn't supported
+         * by WAAPI (like "anticipate"), we need to provide the corressponding
+         * function. This will later get converted to a linear() easing function.
+         */
+        if (
+            typeof ease === "string" &&
+            supportsLinearEasing() &&
+            ease in unsupportedEasingFunctions
+        ) {
+            ease =
+                unsupportedEasingFunctions[
+                    ease as keyof typeof unsupportedEasingFunctions
+                ]
+        }
+
         /**
          * If this animation needs pre-generated keyframes then generate.
          */

packages/framer-motion/src/animation/animators/waapi/__tests__/easing.test.ts

@@ -1,4 +1,5 @@
 import { isWaapiSupportedEasing } from "../easing"
+import { supportsFlags } from "../utils/supports-flags"
 
 test("isWaapiSupportedEasing", () => {
     expect(isWaapiSupportedEasing()).toEqual(true)
@@ -7,6 +8,9 @@ test("isWaapiSupportedEasing", () => {
     expect(isWaapiSupportedEasing("anticipate")).toEqual(false)
     expect(isWaapiSupportedEasing("backInOut")).toEqual(false)
     expect(isWaapiSupportedEasing([0, 1, 2, 3])).toEqual(true)
+    supportsFlags.linearEasing = true
+    expect(isWaapiSupportedEasing((v) => v)).toEqual(true)
+    supportsFlags.linearEasing = false
     expect(isWaapiSupportedEasing((v) => v)).toEqual(false)
     expect(isWaapiSupportedEasing(["linear", "easeIn"])).toEqual(true)
     expect(isWaapiSupportedEasing(["linear", "easeIn", [0, 1, 2, 3]])).toEqual(
@@ -15,6 +19,9 @@ test("isWaapiSupportedEasing", () => {
     expect(isWaapiSupportedEasing(["linear", "easeIn", "anticipate"])).toEqual(
         false
     )
+    supportsFlags.linearEasing = true
+    expect(isWaapiSupportedEasing(["linear", "easeIn", (v) => v])).toEqual(true)
+    supportsFlags.linearEasing = false
     expect(isWaapiSupportedEasing(["linear", "easeIn", (v) => v])).toEqual(
         false
     )

packages/framer-motion/src/animation/animators/waapi/easing.ts

@@ -1,10 +1,25 @@
 import { BezierDefinition, Easing } from "../../../easing/types"
 import { isBezierDefinition } from "../../../easing/utils/is-bezier-definition"
+import { generateLinearEasing } from "./utils/linear"
+import { memoSupports } from "./utils/memo-supports"
+
+export const supportsLinearEasing = /*@__PURE__*/ memoSupports(() => {
+    try {
+        document
+            .createElement("div")
+            .animate({ opacity: 0 }, { easing: "linear(0, 1)" })
+    } catch (e) {
+        return false
+    }
+    return true
+}, "linearEasing")
 
 export function isWaapiSupportedEasing(easing?: Easing | Easing[]): boolean {
     return Boolean(
-        !easing ||
-            (typeof easing === "string" && easing in supportedWaapiEasing) ||
+        (typeof easing === "function" && supportsLinearEasing()) ||
+            !easing ||
+            (typeof easing === "string" &&
+                (easing in supportedWaapiEasing || supportsLinearEasing())) ||
             isBezierDefinition(easing) ||
             (Array.isArray(easing) && easing.every(isWaapiSupportedEasing))
     )
@@ -25,22 +40,22 @@ export const supportedWaapiEasing = {
     backOut: /*@__PURE__*/ cubicBezierAsString([0.33, 1.53, 0.69, 0.99]),
 }
 
-function mapEasingToNativeEasingWithDefault(easing: Easing): string {
-    return (
-        (mapEasingToNativeEasing(easing) as string) ||
-        supportedWaapiEasing.easeOut
-    )
-}
-
 export function mapEasingToNativeEasing(
-    easing?: Easing | Easing[]
+    easing: Easing | Easing[] | undefined,
+    duration: number
 ): undefined | string | string[] {
     if (!easing) {
         return undefined
+    } else if (typeof easing === "function" && supportsLinearEasing()) {
+        return generateLinearEasing(easing, duration)
     } else if (isBezierDefinition(easing)) {
         return cubicBezierAsString(easing)
     } else if (Array.isArray(easing)) {
-        return easing.map(mapEasingToNativeEasingWithDefault)
+        return easing.map(
+            (segmentEasing) =>
+                (mapEasingToNativeEasing(segmentEasing, duration) as string) ||
+                supportedWaapiEasing.easeOut
+        )
     } else {
         return supportedWaapiEasing[easing as keyof typeof supportedWaapiEasing]
     }

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

@@ -17,7 +17,7 @@ export function animateStyle(
     const keyframeOptions: PropertyIndexedKeyframes = { [valueName]: keyframes }
     if (times) keyframeOptions.offset = times
 
-    const easing = mapEasingToNativeEasing(ease)
+    const easing = mapEasingToNativeEasing(ease, duration)
 
     /**
      * If this is an easing array, apply to keyframes, not animation as a whole

packages/framer-motion/src/animation/animators/waapi/utils/__tests__/linear.test.ts

@@ -0,0 +1,14 @@
+import { noop } from "../../../../../utils/noop"
+import { generateLinearEasing } from "../linear"
+
+describe("generateLinearEasing", () => {
+    test("Converts easing function into string of points", () => {
+        expect(generateLinearEasing(noop, 110)).toEqual(
+            "linear(0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1)"
+        )
+        expect(generateLinearEasing(() => 0.5, 200)).toEqual(
+            "linear(0.5, 0.5, 0.5, 0.5, 0.5, 0.5, 0.5, 0.5, 0.5, 0.5, 0.5, 0.5, 0.5, 0.5, 0.5, 0.5, 0.5, 0.5, 0.5, 0.5)"
+        )
+        expect(generateLinearEasing(() => 0.5, 0)).toEqual("linear(0.5, 0.5)")
+    })
+})

packages/framer-motion/src/animation/animators/waapi/utils/linear.ts

@@ -0,0 +1,19 @@
+import { EasingFunction } from "../../../../easing/types"
+import { progress } from "../../../../utils/progress"
+
+// Create a linear easing point for every 10 ms
+const resolution = 10
+
+export const generateLinearEasing = (
+    easing: EasingFunction,
+    duration: number // as milliseconds
+): string => {
+    let points = ""
+    const numPoints = Math.max(Math.round(duration / resolution), 2)
+
+    for (let i = 0; i < numPoints; i++) {
+        points += easing(progress(0, numPoints - 1, i)) + ", "
+    }
+
+    return `linear(${points.substring(0, points.length - 2)})`
+}

packages/framer-motion/src/animation/animators/waapi/utils/memo-supports.ts

@@ -0,0 +1,10 @@
+import { memo } from "../../../../utils/memo"
+import { supportsFlags } from "./supports-flags"
+
+export function memoSupports<T extends any>(
+    callback: () => T,
+    supportsFlag: keyof typeof supportsFlags
+) {
+    const memoized = memo(callback)
+    return () => supportsFlags[supportsFlag] ?? memoized()
+}

packages/framer-motion/src/animation/animators/waapi/utils/supports-flags.ts

@@ -0,0 +1,7 @@
+/**
+ * Add the ability for test suites to manually set support flags
+ * to better test more environments.
+ */
+export const supportsFlags: Record<string, boolean | undefined> = {
+    linearEasing: undefined,
+}