跨執行緒存取UIKit會crash的解法

//把你要執行的程式包在 dispatch_async( dispatch_get_main_queue(), ^{......});裡面， 以主執行緒去執行。
dispatch_async( dispatch_get_main_queue(), ^{
                                                           [[NSNotificationCenter defaultCenter] postNotificationName:@"presentOfficialWebview" object:nil];        
                                                                });