WinUI 3自定义控件事件处置：如何正确取消事件订阅？

问题根源 #

WinUI 3的Unloaded事件并非控件销毁的标志——当控件被临时移出视觉树（比如布局调整、父控件切换可视状态）时就会触发，所以在这个事件里取消订阅会导致控件后续正常使用时事件失效。

正确解决方案 #

1. 每次模板应用时先取消旧订阅，再订阅新事件

OnApplyTemplate可能会被多次调用（比如模板刷新、控件样式变更），所以每次获取新的Popup实例前，必须先取消旧实例的事件订阅，避免重复订阅和内存泄漏：

private Popup? _popup;
private Button? toggleButton;
private ListView? listView;
private Grid? popupGrid;
private TextBlock? textBlock;

protected override void OnApplyTemplate()
{
    base.OnApplyTemplate();
    
    // 先取消旧Popup的事件订阅
    if (_popup != null)
    {
        _popup.Closed -= Popup_Closed;
        _popup.Opened -= Popup_Opened;
    }

    // 获取模板中的控件实例
    toggleButton = GetTemplateChild("PART_ToggleButton") as Button;
    _popup = GetTemplateChild("PART_Popup") as Popup;
    listView = GetTemplateChild("PART_ListView") as ListView;
    popupGrid = GetTemplateChild("PART_PopupGrid") as Grid;
    textBlock = GetTemplateChild("PART_ContentTextBlock") as TextBlock;

    // 订阅新Popup的事件
    if (_popup != null)
    {
        _popup.Closed += Popup_Closed;
        _popup.Opened += Popup_Opened;
    }
}

2. 在控件销毁时取消订阅

通过实现IDisposable接口，在控件真正被销毁时清理事件订阅，确保资源释放：

public sealed class YourCustomControl : Control, IDisposable
{
    // ... 上述成员变量和OnApplyTemplate代码 ...

    public void Dispose()
    {
        Dispose(true);
        GC.SuppressFinalize(this);
    }

    ~YourCustomControl()
    {
        Dispose(false);
    }

    private void Dispose(bool disposing)
    {
        if (disposing)
        {
            // 清理托管资源：取消Popup事件订阅
            if (_popup != null)
            {
                _popup.Closed -= Popup_Closed;
                _popup.Opened -= Popup_Opened;
                _popup = null;
            }
            // 可在此处清理其他托管资源
        }
        // 若有非托管资源，在此处清理
    }
}

备选方案：使用WeakEventManager避免内存泄漏

如果担心忘记手动取消订阅，可使用WeakEventManager来订阅事件，它不会因未取消订阅导致内存泄漏：

// 订阅事件
if (_popup != null)
{
    WeakEventManager<Popup, object>.AddHandler(_popup, nameof(Popup.Opened), Popup_Opened);
    WeakEventManager<Popup, object>.AddHandler(_popup, nameof(Popup.Closed), Popup_Closed);
}

// 取消订阅
if (_popup != null)
{
    WeakEventManager<Popup, object>.RemoveHandler(_popup, nameof(Popup.Opened), Popup_Opened);
    WeakEventManager<Popup, object>.RemoveHandler(_popup, nameof(Popup.Closed), Popup_Closed);
}

内容的提问来源于stack exchange，提问作者Bevara Satyasairam