c# - 何时何地调用 RemoveObserver

Question

我有一个 UITextView 子类，我在其中添加了一个 NSNotificationCenter 观察器。但是我在哪里再次删除观察者呢？

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# 中的何处执行相同操作

据我了解我应该调用的文档

_textDidChangeNotification.Dispose()

我试过

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

但它从未被调用。

完整的类，根据要求:

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, 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();
    }       
}

Answer 1

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 ViewDidDisappear 就像在 Xamarin 示例代码中一样 here

更新我现在明白你的意思了，我怀疑有什么东西阻止了自定义 View 被垃圾收集。你看过这个blog post吗？这可能会有所帮助。

也来自这个sample code看起来您正在正确调用处置，但它们取消了 ViewDidUnload 上的自定义 View here :