C# 中的 kvo(内容大小)

我想翻译这段代码

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

而这个

- (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
            }
        }
    }
}

到 C#。（摘自 UITableView 完成请求数据时收到通知？

这就是我走到这一步的方式：

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

和

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
            }
        //}
    }
}

我遇到的问题：

1. 如何从 NSObject 转换为 NSValue？
2. 如何检查 NSValue 是否具有有效值？
3. 我拿什么Action<NSObservedChange> observer？
4. 我如何让它工作？

    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;
    }