Spring animations now take velocity into account when determining dur…

…ation. (#56)

* Spring animations now take velocity into account when determining duration.

Prior to this change, spring animations were calculating their settling time prior to having set their initial velocity.

After this change, spring animations will calculate their settling time after having set the initial velocity.

* Add tests.

src/private/CABasicAnimation+MotionAnimator.m

@@ -51,13 +51,8 @@
       spring.mass = timing.curve.data[MDMSpringMotionCurveDataIndexMass];
       spring.stiffness = timing.curve.data[MDMSpringMotionCurveDataIndexTension];
       spring.damping = timing.curve.data[MDMSpringMotionCurveDataIndexFriction];
+      spring.duration = timing.duration;
 
-      // This API is only available on iOS 9+
-      if ([spring respondsToSelector:@selector(settlingDuration)]) {
-        spring.duration = spring.settlingDuration;
-      } else {
-        spring.duration = timing.duration;
-      }
       animation = spring;
       break;
     }
@@ -229,4 +224,17 @@ void MDMConfigureAnimation(CABasicAnimation *animation,
       springAnimation.initialVelocity = absoluteInitialVelocity / displacement;
     }
   }
+
+  // Update the animation's duration to match the proposed settling duration.
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wpartial-availability"
+  if ([animation isKindOfClass:[CASpringAnimation class]]) {
+    CASpringAnimation *springAnimation = (CASpringAnimation *)animation;
+#pragma clang diagnostic pop
+
+    // This API is only available on iOS 9+
+    if ([springAnimation respondsToSelector:@selector(settlingDuration)]) {
+      animation.duration = springAnimation.settlingDuration;
+    }
+  }
 }

tests/unit/InitialVelocityTests.swift

@@ -121,6 +121,19 @@ class InitialVelocityTests: XCTestCase {
     }
   }
 
+  func testVelocityInfluencesDuration() {
+    let velocity: CGFloat = 50
+    animate(from: 0, to: 100, withVelocity: velocity)
+
+    XCTAssertEqual(addedAnimations.count, 3)
+    addedAnimations.flatMap { $0 as? CASpringAnimation }.forEach { animation in
+      XCTAssertEqual(animation.duration, animation.settlingDuration,
+                     "from: \(animation.fromValue!), "
+                      + "to: \(animation.toValue!), "
+                      + "withVelocity: \(velocity)")
+    }
+  }
+
   private func animate(from: CGFloat, to: CGFloat, withVelocity velocity: CGFloat) {
     let timing = MotionTiming(delay: 0,
                               duration: 0.7,