从子控件检测ViewDidUnload

Question

我试图弄清楚如何在控件的superview上连接到ViewDidUnload。为了说明我为什么需要这样做，请考虑我的ControlFactory类，它生成UIButtons (以及其他控件)：

internal static class ControlFactory
{
        private static readonly IObservable<Unit> sharedDynamicTypeChanged = TinyIoCContainer.Current
            .Resolve<ISystemNotificationsService>()
            .DynamicTypeChanged
            .Publish()
            .RefCount();

        public static UIButton CreateButton()
        {
            return new DynamicTypeAwareButton(sharedDynamicTypeChanged, UIFont.PreferredHeadline);
        }
}

这里的想法是，工厂生产的每个UIButton都将根据用户的动态类型设置自动缩放其字体。我的DynamicTypeAwareButton是一个内部类，如下所示：

private sealed class DynamicTypeAwareButton : UIButton
{
    private readonly IObservable<Unit> dynamicTypeChanged;
    private readonly UIFont font;
    private IDisposable subscription;

    public DynamicTypeAwareButton(IObservable<Unit> dynamicTypeChanged, UIFont font)
    {
        this.dynamicTypeChanged = dynamicTypeChanged;
        this.font = font;
    }

    public override void MovedToSuperview()
    {
        base.MovedToSuperview();

        // TODO: figure out when to subscribe/unsubscribe
        this.subscription = this.dynamicTypeChanged
            .StartWith(Unit.Default)
            .Subscribe(_ => this.UpdateFont());
    }

    private void UpdateFont()
    {
        this.Font = this.font;
    }
}

正如注释中所指出的，问题在于，我需要知道按钮的superview何时被卸载，这样我就可以释放订阅。我可以很容易地访问superview，但是在卸载superview时，我找不到需要通知的钩子。

有人知道有什么办法可以做到这一点吗？

Answer 1

而不是MovedToSuperView使用WillMoveToSuperView -它将被调用两次，一次是当Button被添加到视图中时(即=>订阅)，另一次是当Button即将被销毁时(其中newSuperview将为null)。

此外，您还可以使用SerialDisposable编写更优雅的代码。

private sealed class DynamicTypeAwareButton : UIButton
{
    private readonly IObservable<Unit> dynamicTypeChanged;
    private readonly UIFont font;
    private SerialDisposable subscription = new SerialDisposable();

    public override void WillMoveToSuperView(UIView newView)
    {
        base.WillMoveToSuperView();

        // Whenever SerialDisposable.Disposable is assigned, it throws
        // away the previous one. That means, even if the Button gets
        // moved to a new non-null View, we're still not leaking a 
        // subscription
        if (newView != null) 
        {
            this.subscription.Disposable = this.dynamicTypeChanged
                .StartWith(Unit.Default)
                .Subscribe(_ => this.UpdateFont());
        } 
        else 
        {
            this.subscription.Disposable = Disposable.Empty;
        }
    }

    public void UpdateFont()
    {
        /* ... */
    }
}