fix: allow the "immediate" prop to stop an animation at its current value

Closes pmndrs#884

Author: aleclarson

packages/core/src/SpringValue.test.ts

@@ -116,6 +116,22 @@ function describeImmediateProp() {
     it.todo('still resolves the "start" promise')
     it.todo('never calls the "onStart" prop')
     it.todo('never calls the "onRest" prop')
+
+    it('stops animating', async () => {
+      const spring = new SpringValue(0)
+      spring.start(2)
+      await advanceUntilValue(spring, 1)
+
+      // Use "immediate" to emulate the "stop" method. (see #884)
+      const value = spring.get()
+      spring.start(value, { immediate: true })
+
+      // The "immediate" prop waits until the next frame before going idle.
+      mockRaf.step()
+
+      expect(spring.idle).toBeTruthy()
+      expect(spring.get()).toBe(value)
+    })
   })
 }
 

packages/core/src/SpringValue.ts

@@ -640,13 +640,21 @@ export class SpringValue<T = any> extends FrameValue<T> {
       node.setValue((value = from as T))
     }
 
+    const immediate =
+      // Sometimes the value is not animatable.
+      !(toConfig || is.num(goal) || is.arr(goal)) ||
+      !!matchProp(get('immediate'), key)
+
+    if (immediate !== anim.immediate) {
+      anim.immediate = immediate
+      if (!is.und(to)) {
+        started = true
+      }
+    }
+
     if (started) {
       anim.values = node.getPayload()
       anim.toValues = toConfig ? null : toArray(goal)
-      anim.immediate =
-        // Sometimes the value is not animatable.
-        !(toConfig || is.num(goal) || is.arr(goal)) ||
-        !!matchProp(get('immediate'), key)
     }
 
     // Event props are replaced on every update.