isElementVisible method
检查元素是否可见
Check if an element is visible.
若集合为 null 或空,则所有元素默认可见。
若集合包含 key,则该元素被隐藏。
If the set is null or empty, all elements are visible by default.
If the set contains key, that element is hidden.
Implementation
bool isElementVisible(String key) {
final set = this;
if (set == null || set.isEmpty) return true;
return !set.contains(key);
}