c# – 何时何地调用RemoveObserver

我有一个UITextView子类,我添加了一个NSNotificationCenter观察器.但是我又在哪里删除了观察者呢？

我的代码：

_textDidChangeNotification = UITextView.Notifications.ObserveTextDidChange(TextDidChange);

在Objective C中,我会在dealloc方法中执行此操作,但我不确定在C#中的相同位置

据我所知,我应该打电话给我

_textDidChangeNotification.dispose()

我曾尝试过

protected override void dispose(bool disposing)
    {
        base.dispose(disposing);
        if (disposing)
        {
            _textDidChangeNotification.dispose();
        }
    }

但它永远不会被称为.

完整的课程,按要求：

public class PlaceholderTextView : UITextView
{
    public string Placeholder 
    { 
        get { return PlaceholderLabel.Text; }
        set
        { 
            PlaceholderLabel.Text = value; 
            PlaceholderLabel.SizetoFit();
        }
    }

    protected UILabel PlaceholderLabel { get; set; }

    protected NSObject _textDidChangeNotification;

    public override string Text
    {
        get
        {
            return base.Text;
        }
        set
        {
            base.Text = value;
            AdjustPlaceholderHidden();
        }
    }

    public PlaceholderTextView() 
    {
        SetupLayout();

        _textDidChangeNotification
        = UITextView.Notifications.ObserveTextDidChange(TextDidChange);
    }

    protected override void dispose(bool disposing)
    {
        base.dispose(disposing);
        _textDidChangeNotification.dispose();
    }

    protected void SetupLayout()
    {
        PlaceholderLabel = new UILabel(new CGRect(0,9,0));
        PlaceholderLabel.TextColor = UIColor.FromWhitealpha(0.702f,1f);

        AddSubview(PlaceholderLabel);
    }

    protected void AdjustPlaceholderHidden()
    {
        if (Text.Length > 0)
        {
            PlaceholderLabel.Hidden = true;
        }
        else
        {
            PlaceholderLabel.Hidden = false;
        }
    }

    protected void TextDidChange(object sender,Foundation.NSNotificationEventArgs args)
    {
        AdjustPlaceholderHidden();
    }       
}

解决方法

我会在ViewWilldisappear中这样做：

public override void ViewWillAppear(bool animated)
    {
        base.ViewWillAppear (animated);

        SubscribeMessages ();
    }

    public override void ViewWilldisappear(bool animated)
    {
        base.ViewWilldisappear(animated);
        UnSubscribeMessages ();
    }

    public void SubscribeMessages ()
    {
        _hideObserver = NSNotificationCenter.DefaultCenter.Addobserver(UIKeyboard.WillHideNotification,OnKeyboardNotification);
        _showObserver = NSNotificationCenter.DefaultCenter.Addobserver(UIKeyboard.WillShowNotification,OnKeyboardNotification);
    }

    public void UnSubscribeMessages ()
    {
        if (_hideObserver != null) NSNotificationCenter.DefaultCenter.RemoveObserver (_hideObserver);
        if (_showObserver != null) NSNotificationCenter.DefaultCenter.RemoveObserver(_showObserver);
    }

或者像Xamarin示例代码here中的ViewDiddis似

更新
我明白你的意思了,我怀疑某些事情阻止了自定义视图被垃圾收集.你有没有看过这个blog post可能会有所帮助.

同样从这个sample code看起来你正在调用dispose但是它们在ViewDidUnload here上清空了自定义视图：