Core Data

1、UIManagedDocument

- (void)saveToURL:(NSURL *)url forSaveOperation:(UIDocumentSaveOperation)saveOperation completionHandler:(void (^)(BOOL success))completionHandler

- (void)openWithCompletionHandler:(void (^)(BOOL success))completionHandler

open和save方法都是异步的asynchronous，所以其需要一个completionBlock来执行open或save后的事情