我有Singleton类 APIHTTPClient 负责使用AFHTTPRequestOperation**进行 Restful 调用。我有一个场景，用户单击按钮查看视图并调用 APIHTTPClient 以获取新数据以在该视图中显示它，但用户也可以单击后退按钮单击另一个视图并调用 APIHTTPClient 获取新视图的新数据。
当用户点击返回调用另一个restful Request时，如何取消之前的AFHTTPRequestOperation请求。
-(void)CallRemoteService:(int) iModuleID { static NSString * const BaseURLString = @" http://www.raywenderlich.com/demos/weather_sample/ "; NSString *string = [NSString stringWithFormat:@"%@weather.php?format=xml", BaseURLString];
NSURL *url = [NSURL URLWithString:string];
NSURLRequest *request = [NSURLRequest requestWithURL:url];
AFHTTPRequestOperation *operation = [[AFHTTPRequestOperation alloc] initWithRequest:request];
// Make sure to set the responseSerializer correctly
// operation.responseSerializer = [AFXMLParserResponseSerializer 序列化器];
[operation setCompletionBlockWithSuccess:^(AFHTTPRequestOperation *operation, id responseObject) {
self.queue = [[NSOperationQueue alloc] init];