What does this block syntax do?

Question

I have this code which is part of a (pretty long) method signature:

[...]requestCreator:(NSURLSessionDataTask *(^)(void (^)(NSURLSessionDataTask *, NSError *)))creator {
//
};

I don't quite get the fact that no variable names are there in parameters and what appears to be a type cast before.

Can someone break down this syntax and explain what's the behavior here?

Answer 1

Take the original method parameter declaration:

(NSURLSessionDataTask *(^)(void (^)(NSURLSessionDataTask *, NSError *)))creator

The form of such a declaration is (type)identifier . So, the identifier is creator and the type is:

NSURLSessionDataTask *(^)(void (^)(NSURLSessionDataTask *, NSError *))

In a local variable declaration, that would look like this possibly more-familiar format, instead:

NSURLSessionDataTask *(^creator)(void (^)(NSURLSessionDataTask *, NSError *))

Let's rework that with some typedefs:

typedef void (^TaskErrorHandlerBlock)(NSURLSessionDataTask *, NSError *);
typedef NSURLSessionDataTask* (^TaskCreatorBlock)(TaskErrorHandlerBlock);
TaskCreatorBlock creator;

I've made up the names TaskErrorHandlerBlock and TaskCreatorBlock from my guess as to what they do. A TaskCreatorBlock is a block which returns an NSURLSessionDataTask* – it creates such a task object. It takes as input a TaskErrorHandlerBlock , which is presumably a block which is called if there's an error. It takes as parameters the task which encountered the error and the error itself.

Answer 2

它看起来像一个需要另一个获得url会话和错误的块的块。