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