Added "cancelled" property in net task.

Send "error" when net task is cancelled when using RAC.

Author: kevin-lyn

STNetTaskQueue/STNetTask.h

@@ -18,6 +18,9 @@ FOUNDATION_EXPORT NSString *const STNetTaskUnknownError;
         [subscriber sendNext:TASK];\
         [subscriber sendCompleted]; \
     }]; \
+    [[[[RACObserve(TASK, cancelled) skip:1] ignore:@NO] deliverOnMainThread] subscribeNext:^(id x) { \
+        [subscriber sendError:nil];\
+    }]; \
     return nil; \
 }]
 #endif
@@ -47,6 +50,12 @@ FOUNDATION_EXPORT NSString *const STNetTaskUnknownError;
  */
 @property (atomic, assign) BOOL pending;
 
+/*
+ Indicates if the net task is cancelled.
+ This value would be "NO" by default after net task is created, even the net task is not added to queue.
+ */
+@property (atomic, assign) BOOL cancelled;
+
 /* Indicates if the net task is finished(no matter it's successful or failed). */
 @property (atomic, assign) BOOL finished;
 

STNetTaskQueue/STNetTaskQueue.m

@@ -73,7 +73,7 @@ - (void)performUsingBlock:(void(^)())block
 - (void)addTask:(STNetTask *)task
 {
     NSAssert(self.handler, @"STNetTaskQueueHandler is not set.");
-    NSAssert(!task.finished, @"STNetTask is finished, please recreate a net task.");
+    NSAssert(!task.finished && !task.cancelled, @"STNetTask is finished/cancelled, please recreate a net task.");
 
     task.pending = YES;
     [self performInThread:self.thred usingBlock:^{
@@ -110,6 +110,7 @@ - (void)_cancelTask:(STNetTask *)task
     task.pending = NO;
 
     [self.handler netTaskQueue:self didCancelTask:task];
+    task.cancelled = YES;
 }
 
 - (BOOL)_retryTask:(STNetTask *)task withError:(NSError *)error