KVO of contentSize in C#

Question

I want to translate this code

[self.tableView addObserver:self forKeyPath:@"contentSize" options:NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld | NSKeyValueObservingOptionPrior context:NULL];

and this

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
    if ([keyPath isEqualToString:@"contentSize"]) {
        DLog(@"change = %@", change.description)

        NSValue *new = [change valueForKey:@"new"];
        NSValue *old = [change valueForKey:@"old"];

        if (new && old) {
            if (![old isEqualToValue:new]) {
                // do your stuff
            }
        }
    }
}

to C#. (taken from Get notified when UITableView has finished asking for data?)

This how I got so far:

this.TableView.AddObserver ("contentSize", NSKeyValueObservingOptions.New | NSKeyValueObservingOptions.Old | NSKeyValueObservingOptions.Prior, null);

and

public override void ObserveValue (NSString keyPath, NSObject ofObject, NSDictionary change, IntPtr context)
{
    //base.ObserveValue (keyPath, ofObject, change, context);

    if (keyPath == "contentSize") {
        NSValue newValue = (NSValue)NSValue.FromObject(change["new"]);
        NSValue oldValue = (NSValue)NSValue.FromObject(change["old"]);

        //if (newValue && oldValue) {
            if(!oldValue.IsEqualTo(newValue)){
                // do something here
            }
        //}
    }
}

The problems I have:

1. How to convert from NSObject to NSValue?
2. How can I check if NSValue has a valid value?
3. What do I take for Action<NSObservedChange> observer?
4. How do I get this working?

Answer 1

    void AddObservers()
    {
        TextView.AddObserver(this, "contentSize", NSKeyValueObservingOptions.OldNew, IntPtr.Zero);
    }

    public override void ObserveValue(NSString keyPath, NSObject ofObject, NSDictionary change, IntPtr context)
    {
        if (keyPath == "contentSize")
            OnSizeChanged(new NSObservedChange(change));
        else
            base.ObserveValue(keyPath, ofObject, change, context);
    }

    void OnSizeChanged(NSObservedChange change)
    {
        CGSize oldValue = ((NSValue)change.OldValue).CGSizeValue;
        CGSize newValue = ((NSValue)change.NewValue).CGSizeValue;

        var dy = newValue.Height - oldValue.Height;
    }

---

Look at this monotouch-samples.