Revert "NSURLSession Reimplementation (#411)"

This reverts commit 07233534e6.

Source/GSTaskRegistry.m

@@ -0,0 +1,98 @@
+#import "GSTaskRegistry.h"
+#import "Foundation/NSDictionary.h"
+#import "Foundation/NSException.h"
+#import "Foundation/NSURLSession.h"
+
+
+@implementation GSTaskRegistry
+{
+  NSMutableDictionary  *_tasks;
+  void                 (^_tasksCompletion)(void);
+}
+
+- (instancetype) init 
+{
+  if (nil != (self = [super init])) 
+    {
+      _tasks = [[NSMutableDictionary alloc] init];
+    }
+  
+  return self;
+}
+
+- (void) dealloc
+{
+  DESTROY(_tasks);
+  [super dealloc];
+}
+
+- (NSArray*) allTasks 
+{
+  return [_tasks allValues];
+}
+
+- (BOOL) isEmpty 
+{
+  return [_tasks count] == 0;
+}
+
+- (void) notifyOnTasksCompletion: (void (^)(void))tasksCompletion 
+{
+  _tasksCompletion = tasksCompletion;
+}
+
+- (void) addTask: (NSURLSessionTask*)task
+{
+  NSString          *identifier;
+  NSUInteger        taskIdentifier;
+  NSURLSessionTask  *t;
+
+  taskIdentifier = [task taskIdentifier];
+
+  NSAssert(taskIdentifier != 0, @"Invalid task identifier");
+  
+  identifier = [NSString stringWithFormat: @"%lu", taskIdentifier];
+
+  if (nil != (t = [_tasks objectForKey: identifier])) 
+    {
+      if ([t isEqual: task]) 
+        {
+          NSAssert(NO,
+            @"Trying to re-insert a task that's already in the registry.");
+        } 
+      else 
+        {
+          NSAssert(NO,
+            @"Trying to insert a task, but a different task with the same"
+            @" identifier is already in the registry.");
+        }
+    }
+  
+  [_tasks setObject: task forKey: identifier];
+}
+
+- (void) removeTask: (NSURLSessionTask*)task 
+{
+  NSString          *identifier;
+  NSUInteger        taskIdentifier;
+
+  taskIdentifier = [task taskIdentifier];
+
+  NSAssert(taskIdentifier != 0, @"Invalid task identifier");
+  
+  identifier = [NSString stringWithFormat: @"%lu", taskIdentifier];
+
+  if (nil == [_tasks objectForKey: identifier])
+    {
+      NSAssert(NO, @"Trying to remove task, but it's not in the registry."); 
+    }
+
+  [_tasks removeObjectForKey: identifier];
+
+  if (nil != _tasksCompletion && [self isEmpty]) 
+    {
+      _tasksCompletion();
+    }
+}
+
+@end