1.4.2.4. SAVING（Core Data 应用程序实践指南）

- (void)saveContext {
    if (debug == 1) {
        NSLog(@"Running %@ ‘%@‘",self.class, NSStringFromSelector(_cmd));
    }
    
    if ([_context hasChanges]) {
        NSError *error;
        
        if ([_context save:&error]) {
            NSLog(@"_context save changes to persistent store.");
        }else{
            NSLog(@"Faild to save changes: %@.",error);
        }
    }else{
        NSLog(@"SKIPPED _context save ,there are no changes!");
    }
    
}