禁用UITextField的copy，paste，copy的menu

只需覆盖canPerformAction:withSender方法就可以，canPerformAction:withSender属于UIResponder类的。

如下：

-(BOOL)canPerformAction:(SEL)action withSender:(id)sender {

UIMenuController *menuController = [UIMenuController sharedMenuController];

if (menuController) {

[UIMenuController sharedMenuController].menuVisible = NO;

}

return NO;

}