Fix PFPurchase block nullability annotations.

Author: nlutsenko

Parse/PFPurchase.h

@@ -17,6 +17,10 @@
 
 PF_ASSUME_NONNULL_BEGIN
 
+typedef void (^PFPurchaseProductObservationBlock)(SKPaymentTransaction *transaction);
+typedef void (^PFPurchaseBuyProductResultBlock)(NSError *PF_NULLABLE_S error);
+typedef void (^PFPurchaseDownloadAssetResultBlock)(NSString *PF_NULLABLE_S filePath, NSError *PF_NULLABLE_S error);
+
 /*!
  `PFPurchase` provides a set of APIs for working with in-app purchases.
 
@@ -35,16 +39,15 @@ PF_ASSUME_NONNULL_BEGIN
  @param productIdentifier the product identifier
  @param block The block to be run when buying a product.
  */
-+ (void)addObserverForProduct:(NSString *)productIdentifier
-                        block:(void(^)(SKPaymentTransaction *transaction))block;
++ (void)addObserverForProduct:(NSString *)productIdentifier block:(PFPurchaseProductObservationBlock)block;
 
 /*!
  @abstract *Asynchronously* initiates the purchase for the product.
 
  @param productIdentifier the product identifier
  @param block the completion block.
  */
-+ (void)buyProduct:(NSString *)productIdentifier block:(void(^)(NSError *error))block;
++ (void)buyProduct:(NSString *)productIdentifier block:(PFPurchaseBuyProductResultBlock)block;
 
 /*!
  @abstract *Asynchronously* download the purchased asset, which is stored on Parse's server.
@@ -55,7 +58,7 @@ PF_ASSUME_NONNULL_BEGIN
  @param completion the completion block.
  */
 + (void)downloadAssetForTransaction:(SKPaymentTransaction *)transaction
-                         completion:(void(^)(NSString *filePath, NSError *error))completion;
+                         completion:(PFPurchaseDownloadAssetResultBlock)completion;
 
 /*!
  @abstract *Asynchronously* download the purchased asset, which is stored on Parse's server.
@@ -67,7 +70,7 @@ PF_ASSUME_NONNULL_BEGIN
  @param progress the progress block, which is called multiple times to reveal progress of the download.
  */
 + (void)downloadAssetForTransaction:(SKPaymentTransaction *)transaction
-                         completion:(void(^)(NSString *filePath, NSError *error))completion
+                         completion:(PFPurchaseDownloadAssetResultBlock)completion
                            progress:(PF_NULLABLE PFProgressBlock)progress;
 
 /*!

Parse/PFPurchase.m

@@ -24,7 +24,7 @@ @implementation PFPurchase
 #pragma mark - Public
 ///--------------------------------------
 
-+ (void)addObserverForProduct:(NSString *)productIdentifier block:(void(^)(SKPaymentTransaction *))block {
++ (void)addObserverForProduct:(NSString *)productIdentifier block:(PFPurchaseProductObservationBlock)block {
     // We require the following method to run on the main thread because we want to add the observer
     // *after* all products handlers have been added. Developers might be calling this method multiple
     // times; and if the observer is added after the first call, the observer might not know how to
@@ -37,7 +37,7 @@ + (void)addObserverForProduct:(NSString *)productIdentifier block:(void(^)(SKPay
     [[Parse _currentManager].purchaseController.transactionObserver handle:productIdentifier block:block];
 }
 
-+ (void)buyProduct:(NSString *)productIdentifier block:(void(^)(NSError *))completion {
++ (void)buyProduct:(NSString *)productIdentifier block:(PFPurchaseBuyProductResultBlock)completion {
     [[[self _purchaseController] buyProductAsyncWithIdentifier:productIdentifier] continueWithBlock:^id(BFTask *task) {
         if (completion) {
             completion(task.error);
@@ -50,12 +50,13 @@ + (void)restore {
     [[self _purchaseController].paymentQueue restoreCompletedTransactions];
 }
 
-+ (void)downloadAssetForTransaction:(SKPaymentTransaction *)transaction completion:(PFStringResultBlock)completion {
++ (void)downloadAssetForTransaction:(SKPaymentTransaction *)transaction
+                         completion:(PFPurchaseDownloadAssetResultBlock)completion {
     [self downloadAssetForTransaction:transaction completion:completion progress:nil];
 }
 
 + (void)downloadAssetForTransaction:(SKPaymentTransaction *)transaction
-                         completion:(PFStringResultBlock)completion
+                         completion:(PFPurchaseDownloadAssetResultBlock)completion
                            progress:(PFProgressBlock)progress {
     @weakify(self);
     [[[PFUser _getCurrentUserSessionTokenAsync] continueWithBlock:^id(BFTask *task) {