'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/SKProductsRequest+Promise.swift

@@ -20,7 +20,7 @@ extension SKProductsRequest {
      - Returns: A promise that fulfills if the request succeeds.
     */
     public func start(_: PMKNamespacer) -> Promise<SKProductsResponse> {
-        let proxy = SKDelegate()
+        let proxy = SKDelegate(request: self)
         delegate = proxy
         proxy.retainCycle = proxy
         start()
@@ -29,10 +29,17 @@ extension SKProductsRequest {
 }
 
 
-fileprivate class SKDelegate: NSObject, SKProductsRequestDelegate {
+fileprivate class SKDelegate: NSObject, SKProductsRequestDelegate, CancellableTask {
     let (promise, seal) = Promise<SKProductsResponse>.pending()
+    let request: SKRequest
     var retainCycle: SKDelegate?
 
+    init(request: SKRequest) {
+        self.request = request
+        super.init()
+        promise.setCancellableTask(self, reject: seal.reject)
+    }
+
     @objc fileprivate func request(_ request: SKRequest, didFailWithError error: Error) {
         seal.reject(error)
         retainCycle = nil
@@ -42,6 +49,14 @@ fileprivate class SKDelegate: NSObject, SKProductsRequestDelegate {
         seal.fulfill(response)
         retainCycle = nil
     }
+
+    var isCancelled = false
+    
+    func cancel() {
+        request.cancel()
+        retainCycle = nil
+        isCancelled = true
+    }
 }
 
 // perhaps one day Apple will actually make their errors into Errors…
@@ -51,7 +66,7 @@ fileprivate class SKDelegate: NSObject, SKProductsRequestDelegate {
 //    }
 //}
 
-//////////////////////////////////////////////////////////// Cancellation
+//////////////////////////////////////////////////////////// Cancellable wrapper
 
 extension SKProductsRequest {
     /**
@@ -60,30 +75,6 @@ extension SKProductsRequest {
      - Returns: A cancellable promise that fulfills if the request succeeds.
      */
     public func cancellableStart(_: PMKNamespacer) -> CancellablePromise<SKProductsResponse> {
-        let proxy = SKDelegate()
-        delegate = proxy
-        proxy.retainCycle = proxy
-        let cp = CancellablePromise(task: SKRequestTask(self, proxy), promise: proxy.promise, resolver: proxy.seal)
-
-        start()
-        return cp
-    }
-}
-
-fileprivate class SKRequestTask: CancellableTask {
-    let request: SKRequest
-    let proxy: SKDelegate
-
-    init(_ request: SKRequest, _ proxy: SKDelegate) {
-        self.request = request
-        self.proxy = proxy
-    }
-    
-    var isCancelled = false
-    
-    func cancel() {
-        request.cancel()
-        proxy.retainCycle = nil
-        isCancelled = true
+        return cancellable(start(.promise))
     }
 }