取消选择段时出现越界错误

Question

每当用户重新点击选定的段时，我都会尝试以编程方式从 Xamarin 中取消选择 UISegmentedControl。我创建了一个继承类，它将确定是否需要这样做，但是每当我尝试将 SelectedSegment 设置为 -1 时，应用程序就会爆炸，抛出越界异常。

public class UIDeselectableSegmentedControl : UISegmentedControl
{
    private int previouslySelectedIndex = -1;

    private bool isIOS7
    {
        get
        {
            return UIDevice.CurrentDevice.CheckSystemVersion(7, 0);
        }
    }

    public UIDeselectableSegmentedControl()
    {
        this.ValueChanged += UIDeselectableSegmentedControl_ValueChanged;
    }

    void UIDeselectableSegmentedControl_ValueChanged(object sender, EventArgs e)
    {
        if (previouslySelectedIndex == this.SelectedSegment)
        {
            this.SelectedSegment = -1;
        }
        previouslySelectedIndex = this.SelectedSegment;
    }

    public override void TouchesBegan(NSSet touches, UIEvent evt)
    {
        int initialIndex = this.SelectedSegment;

        base.TouchesBegan(touches, evt);

        //Pre-iOS7 Segment Changes are present in Touches Began
        if (!isIOS7) CheckSelectedSegment(initialIndex, this.SelectedSegment);

    }

    public override void TouchesEnded(NSSet touches, UIEvent evt)
    {
        int initialIndex = this.SelectedSegment;

        base.TouchesEnded(touches, evt);

        //Post-iOS7 Segment Changes are present in Touches Ended
        if (isIOS7) CheckSelectedSegment(initialIndex, this.SelectedSegment);

    }

    /// <summary>
    /// Compare the SelectedSegment with the Current Index value. If it is the same, raise the Value Changed event.
    /// </summary>
    /// <param name="index">Currently Selected Segment</param>
    private void CheckSelectedSegment(int oldIndex, int newIndex)
    {
        if (oldIndex == newIndex)
        {
            this.SendActionForControlEvents(UIControlEvent.ValueChanged);
        }
    }

}

如果我将其更改为this.SelectedSegment = 0，那么每当您重新点击另一个选定项目时，该控件将成功地将 SegementedControl 强制设置为第一个项目，但如果我将其设置为 -1，则会引发以下内容例外：

* 由于未捕获的异常 'NSRangeException' 导致应用程序终止，原因：'* -[__NSArrayM objectAtIndex:]：索引 4294967295 超出范围 [0 .. 1]'

我应该以其他方式取消选择该细分吗？根据this的回答，这是Objective-C中的正确方法，SelectedSegment上的元数据评论表明这是正确的：

//
// Summary:
//     The index of the selected segment.
//
// Remarks:
//     Set to -1 to turn off the currently selected segment. If MonoTouch.UIKit.UISegmentedControl.Momentary
//     == true the SelectedSegment property is ignored.

Answer 1

以典型的形式，我想我已经解决了我自己的问题。我不确定为什么（也许有人可以澄清这一点），但这似乎与尝试在ValueChanged 事件中更改SelectedSegment 有关。本质上可能是某种突变异常。

我重新编写了我的代码，因此在 TouchesEnded/Began 中调用的方法只是直接进行更改，而不是强制调用 ValueChanged 并自行更改。在某种程度上，这更有意义，因为 ValueChanged 正在对更改做出反应 - 它可能不应该对进行更改负责：

public class UIDeselectableSegmentedControl : UISegmentedControl
{
    private bool isIOS7
    {
        get
        {
            return UIDevice.CurrentDevice.CheckSystemVersion(7, 0);
        }
    }

    public override void TouchesBegan(NSSet touches, UIEvent evt)
    {
        int initialIndex = this.SelectedSegment;

        base.TouchesBegan(touches, evt);

        //Pre-iOS7 Segment Changes are present in Touches Began
        if (!isIOS7) DeselectSegmentIfApplicable(initialIndex, this.SelectedSegment);

    }

    public override void TouchesEnded(NSSet touches, UIEvent evt)
    {
        int initialIndex = this.SelectedSegment;

        base.TouchesEnded(touches, evt);

        //Post-iOS7 Segment Changes are present in Touches Ended
        if (isIOS7) DeselectSegmentIfApplicable(initialIndex, this.SelectedSegment);

    }

    /// <summary>
    /// Compare the SelectedSegment with the Current Index value. If it is the same, raise the Deselect the Segment.
    /// </summary>
    /// <param name="oldIndex">the previously selected segment</param>
    /// <param name="newIndex">The newly selected segment</param>
    private void DeselectSegmentIfApplicable(int oldIndex, int newIndex)
    {
        if (oldIndex == newIndex)
        {
            this.SelectedSegment = -1;
        }
    }
}