'Cancel' for PromiseKit --

* Eliminate duplicate code in the cancellable extensions
* When using the 'cancellable' function with the extensions, ensure that the underlying task (if any) is always cancelled
* When using the 'cancellable' function, call 'reject' on the underlying promise (wherever possible) if 'cancel' is invoked
* Create cancellable wrappers for the extensions only for the cases where there is an underlying task that supports cancellation

Author: dougzilla32

Sources/UIViewPropertyAnimator+Promise.swift

@@ -4,31 +4,28 @@ import PromiseKit
 import UIKit
 
 @available(iOS 10, tvOS 10, *)
-public extension UIViewPropertyAnimator {
+public extension UIViewPropertyAnimator: CancellableTask {
     func startAnimation(_: PMKNamespacer) -> Guarantee<UIViewAnimatingPosition> {
-        return Guarantee {
+        return Guarantee(cancellableTask: self) {
             addCompletion($0)
             startAnimation()
         }
     }
-}
-
-//////////////////////////////////////////////////////////// Cancellation
-
-@available(iOS 10, tvOS 10, *)
-extension UIViewPropertyAnimator: CancellableTask {
+    
     public func cancel() {
         stopAnimation(true)
     }
 
     public var isCancelled: Bool {
         return (state == .inactive) && (fractionComplete < 1.0)
     }
-    
+}
+
+//////////////////////////////////////////////////////////// Cancellable wrapper
+
+@available(iOS 10, tvOS 10, *)
+extension UIViewPropertyAnimator {
     public func cancellableStartAnimation(_: PMKNamespacer) -> CancellablePromise<UIViewAnimatingPosition> {
-        return CancellablePromise(task: self) {
-            addCompletion($0.fulfill)
-            startAnimation()
-        }
+        return cancellable(startAnimation(.promise))
     }
 }