block注意执行时，所在的线程

block作为回调使用时，需要注意block所在的线程；

比如，使用AFNetworking接口时，有些block 不在主线程里：

/**
Creates and runs an `NSURLSessionDataTask` with a `GET` request.

@param URLString The URL string used to create the request URL.
@param parameters The parameters to be encoded according to the client request serializer.
@param downloadProgress A block object to be executed when the download progress is updated. <span style="color:#ff0000;background-color: rgb(255, 255, 102);">Note this block is called on the session queue, not the main queue.</span>
@param success A block object to be executed when the task finishes successfully. This block has no return value and takes two arguments: the data task, and the response object created by the client response serializer.
@param failure A block object to be executed when the task finishes unsuccessfully, or that finishes successfully, but encountered an error while parsing the response data. This block has no return value and takes a two arguments: the data task and the error describing the network or parsing error that occurred.

@see -dataTaskWithRequest:uploadProgress:downloadProgress:completionHandler:
*/
- (nullable NSURLSessionDataTask *)GET:(NSString *)URLString
parameters:(nullable id)parameters
progress:(nullable void (^)(NSProgress *downloadProgress))downloadProgress
success:(nullable void (^)(NSURLSessionDataTask *task, id _Nullable responseObject))success
failure:(nullable void (^)(NSURLSessionDataTask * _Nullable task, NSError *error))failure;