Return nil CAAction when swapping implementation (#109)

When swapping the implementation of `actionForKey:`, returning `NSNull`
would result in a crash because it could not respond to other messages
being passed (like `runAction:forKey:`). Instead, returning `nil` will
safely receive additional messages. This was easily reproducible by
running the unit test suite on iOS 8.1 or 8.3 simulators.

Returning `nil` matches the documented behavior of `actionForKey:` in
the CALayer headers:

> Returns the action object associated with the event named by the
> string 'event'. The default implementation searches for an action
> object in the following places:
>
> 1. if defined, call the delegate method -actionForLayer:forKey:
> 2. look in the layer's `actions' dictionary
> 3. look in any `actions' dictionaries in the `style' hierarchy
> 4. call +defaultActionForKey: on the layer's class
>
> If any of these steps results in a non-nil action object, the
> following steps are ignored. If the final result is an instance of
> NSNull, it is converted to `nil'.
>
> - (nullable id<CAAction>)actionForKey:(NSString *)event;

Also fixed a crash with CASpringAnimation on iOS 8.x because the desired
selectors aren't available.

src/private/CABasicAnimation+MotionAnimator.m

@@ -138,7 +138,8 @@ void MDMConfigureAnimation(CABasicAnimation *animation, MDMAnimationTraits * tra
   // linking against the public API on iOS 9+.
 #pragma clang diagnostic ignored "-Wpartial-availability"
   BOOL isSpringAnimation = ([animation isKindOfClass:[CASpringAnimation class]]
-                            && [traits.timingCurve isKindOfClass:[MDMSpringTimingCurve class]]);
+                            && [traits.timingCurve isKindOfClass:[MDMSpringTimingCurve class]]
+                            && [animation respondsToSelector:@selector(setInitialVelocity:)]);
   MDMSpringTimingCurve *springTimingCurve = (MDMSpringTimingCurve *)traits.timingCurve;
   CASpringAnimation *springAnimation = (CASpringAnimation *)animation;
 #pragma clang diagnostic pop

src/private/MDMBlockAnimations.m

@@ -124,7 +124,7 @@ @interface MDMLayerDelegate: NSObject <CALayerDelegate>
   // animations, we queue up the modified actions and then add them all at the end of our
   // MDMAnimateImplicitly invocation.
   [context addActionForLayer:layer keyPath:event];
-  return [NSNull null];
+  return nil;
 }
 
 NSArray<MDMImplicitAction *> *MDMAnimateImplicitly(void (^work)(void)) {

tests/unit/InitialVelocityTests.swift

@@ -49,10 +49,12 @@ class InitialVelocityTests: XCTestCase {
 
     XCTAssertEqual(addedAnimations.count, 3)
     addedAnimations.flatMap { $0 as? CASpringAnimation }.forEach { animation in
-      XCTAssertEqual(animation.initialVelocity, 0.5,
-                     "from: \(animation.fromValue!), "
-                      + "to: \(animation.toValue!), "
-                      + "withVelocity: \(velocity)")
+      if (animation.responds(to: #selector(getter: CASpringAnimation.initialVelocity))) {
+        XCTAssertEqual(animation.initialVelocity, 0.5,
+                       "from: \(animation.fromValue!), "
+                        + "to: \(animation.toValue!), "
+                        + "withVelocity: \(velocity)")
+      }
     }
   }
 
@@ -62,10 +64,12 @@ class InitialVelocityTests: XCTestCase {
 
     XCTAssertEqual(addedAnimations.count, 3)
     addedAnimations.flatMap { $0 as? CASpringAnimation }.forEach { animation in
-      XCTAssertEqual(animation.initialVelocity, 0.5,
-                     "from: \(animation.fromValue!), "
-                      + "to: \(animation.toValue!), "
-                      + "withVelocity: \(velocity)")
+      if (animation.responds(to: #selector(getter: CASpringAnimation.initialVelocity))) {
+        XCTAssertEqual(animation.initialVelocity, 0.5,
+                       "from: \(animation.fromValue!), "
+                        + "to: \(animation.toValue!), "
+                        + "withVelocity: \(velocity)")
+      }
     }
   }
 
@@ -75,10 +79,12 @@ class InitialVelocityTests: XCTestCase {
 
     XCTAssertEqual(addedAnimations.count, 3)
     addedAnimations.flatMap { $0 as? CASpringAnimation }.forEach { animation in
-      XCTAssertGreaterThan(animation.initialVelocity, 0,
-                           "from: \(animation.fromValue!), "
-                            + "to: \(animation.toValue!), "
-                            + "withVelocity: \(velocity)")
+      if (animation.responds(to: #selector(getter: CASpringAnimation.initialVelocity))) {
+        XCTAssertGreaterThan(animation.initialVelocity, 0,
+                             "from: \(animation.fromValue!), "
+                              + "to: \(animation.toValue!), "
+                              + "withVelocity: \(velocity)")
+      }
     }
   }
 
@@ -88,10 +94,12 @@ class InitialVelocityTests: XCTestCase {
 
     XCTAssertEqual(addedAnimations.count, 3)
     addedAnimations.flatMap { $0 as? CASpringAnimation }.forEach { animation in
-      XCTAssertLessThan(animation.initialVelocity, 0,
-                        "from: \(animation.fromValue!), "
-                          + "to: \(animation.toValue!), "
-                          + "withVelocity: \(velocity)")
+      if (animation.responds(to: #selector(getter: CASpringAnimation.initialVelocity))) {
+        XCTAssertLessThan(animation.initialVelocity, 0,
+                          "from: \(animation.fromValue!), "
+                            + "to: \(animation.toValue!), "
+                            + "withVelocity: \(velocity)")
+      }
     }
   }
 
@@ -101,10 +109,12 @@ class InitialVelocityTests: XCTestCase {
 
     XCTAssertEqual(addedAnimations.count, 3)
     addedAnimations.flatMap { $0 as? CASpringAnimation }.forEach { animation in
-      XCTAssertGreaterThan(animation.initialVelocity, 0,
-                           "from: \(animation.fromValue!), "
-                            + "to: \(animation.toValue!), "
-                            + "withVelocity: \(velocity)")
+      if (animation.responds(to: #selector(getter: CASpringAnimation.initialVelocity))) {
+        XCTAssertGreaterThan(animation.initialVelocity, 0,
+                             "from: \(animation.fromValue!), "
+                              + "to: \(animation.toValue!), "
+                              + "withVelocity: \(velocity)")
+      }
     }
   }
 
@@ -114,10 +124,12 @@ class InitialVelocityTests: XCTestCase {
 
     XCTAssertEqual(addedAnimations.count, 3)
     addedAnimations.flatMap { $0 as? CASpringAnimation }.forEach { animation in
-      XCTAssertLessThan(animation.initialVelocity, 0,
-                        "from: \(animation.fromValue!), "
-                          + "to: \(animation.toValue!), "
-                          + "withVelocity: \(velocity)")
+      if (animation.responds(to: #selector(getter: CASpringAnimation.settlingDuration))) {
+        XCTAssertLessThan(animation.initialVelocity, 0,
+                          "from: \(animation.fromValue!), "
+                            + "to: \(animation.toValue!), "
+                            + "withVelocity: \(velocity)")
+      }
     }
   }
 
@@ -127,10 +139,12 @@ class InitialVelocityTests: XCTestCase {
 
     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)")
+      if (animation.responds(to: #selector(getter: CASpringAnimation.settlingDuration))) {
+        XCTAssertEqual(animation.duration, animation.settlingDuration,
+                       "from: \(animation.fromValue!), "
+                        + "to: \(animation.toValue!), "
+                        + "withVelocity: \(velocity)")
+      }
     }
   }