feat: let interpolated values be static

In some cases, an interpolated variable may be either a static or dynamic value, so supporting both kinds makes things easier.

packages/core/src/Interpolation.ts

@@ -1,9 +1,4 @@
-import {
-  Arrify,
-  InterpolatorArgs,
-  InterpolatorFn,
-  OneOrMore,
-} from '@react-spring/types'
+import { Arrify, InterpolatorArgs, InterpolatorFn } from '@react-spring/types'
 import {
   is,
   raf,
@@ -18,6 +13,7 @@ import {
   callFluidObservers,
   addFluidObserver,
   removeFluidObserver,
+  hasFluidValue,
 } from '@react-spring/shared'
 
 import { FrameValue, isFrameValue } from './FrameValue'
@@ -51,7 +47,7 @@ export class Interpolation<In = any, Out = any> extends FrameValue<Out> {
 
   constructor(
     /** The source of input values */
-    readonly source: OneOrMore<FluidValue>,
+    readonly source: unknown,
     args: InterpolatorArgs<In, Out>
   ) {
     super()
@@ -106,7 +102,9 @@ export class Interpolation<In = any, Out = any> extends FrameValue<Out> {
   protected _attach() {
     let priority = 1
     each(toArray(this.source), source => {
-      addFluidObserver(source, this)
+      if (hasFluidValue(source)) {
+        addFluidObserver(source, this)
+      }
       if (isFrameValue(source)) {
         if (!source.idle) {
           this._active.add(source)
@@ -121,7 +119,9 @@ export class Interpolation<In = any, Out = any> extends FrameValue<Out> {
   // Stop observing our sources once we have no observers.
   protected _detach() {
     each(toArray(this.source), source => {
-      removeFluidObserver(source, this)
+      if (hasFluidValue(source)) {
+        removeFluidObserver(source, this)
+      }
     })
     this._active.clear()
     becomeIdle(this)
@@ -148,7 +148,8 @@ export class Interpolation<In = any, Out = any> extends FrameValue<Out> {
     // our value won't be updated until our parents have updated.
     else if (event.type == 'priority') {
       this.priority = toArray(this.source).reduce(
-        (max, source: any) => Math.max(max, (source.priority || 0) + 1),
+        (highest: number, parent) =>
+          Math.max(highest, (isFrameValue(parent) ? parent.priority : 0) + 1),
         0
       )
     }

packages/core/src/interpolate.ts

@@ -20,26 +20,30 @@ export const interpolate: Interpolator = (source: any, ...args: [any]) => (
 
 /** Extract the raw value types that are being interpolated */
 export type Interpolated<T extends ReadonlyArray<any>> = {
-  [P in keyof T]: T[P] extends { get(): infer U } ? U : never
+  [P in keyof T]: T[P] extends infer Element
+    ? Element extends FluidValue<infer U>
+      ? U
+      : Element
+    : never
 }
 
 /**
  * This interpolates one or more `FluidValue` objects.
  * The exported `interpolate` function uses this type.
  */
 export interface Interpolator {
-  // Single parent
-  <In, Out>(
-    parent: FluidValue<In>,
-    interpolator: InterpolatorFn<In, Out>
-  ): Interpolation<Out>
-
-  // Tuple of parents
-  <In extends ReadonlyArray<FluidValue>, Out>(
+  // Tuple of parent values
+  <In extends ReadonlyArray<any>, Out>(
     parents: In,
     interpolator: (...args: Interpolated<In>) => Out
   ): Interpolation<Out>
 
+  // Single parent value
+  <In, Out>(
+    parent: FluidValue<In> | In,
+    interpolator: InterpolatorFn<In, Out>
+  ): Interpolation<Out>
+
   // Interpolation config
   <Out>(
     parents: OneOrMore<FluidValue>,