- (BOOL)canPerformAction:(SEL)ac

重写此方法，来控制 UIMenuItem 的显示和隐藏：

- (BOOL)canPerformAction:(SEL)action withSender:(id)sender
{
    if (action == @selector(copy:))
    {
        return YES;
    }
    else if (action == @selector(select:))
    {
        return YES;
    }
    else if (action == @selector(selectAll:))
    {
        return YES;
    }
    
    return [super canPerformAction:action withSender:sender];
}

以下是系统默认的 UIMenuItem 所对应的 action：

• cut: // 剪切
• copy: // 拷贝
• select: // 选择
• selectAll: // 全选
• paste: // 粘贴
• delete: // 删除
• _promptForReplace: // Replace...
• _transliterateChinese: // 简<=>繁
• _showTextStyleOptions: // B/<u>U</u>
• _define: // Define
• _addShortcut: // Learn...
• _accessibilitySpeak: // Speak
• _accessibilitySpeakLanguageSelection: // Speak...
• _accessibilityPauseSpeaking: // Pause
• _share: // 共享...
• makeTextWritingDirectionRightToLeft: // 往右缩进
• makeTextWritingDirectionLeftToRight: // 往左缩进