问奇怪的iOS 8自动布局问题

EN

public class TableViewController : UITableViewController
{
    public override void ViewDidLoad()
    {
        base.ViewDidLoad();

        var source = new TableSource(TableView);

        var updateButton = new UIBarButtonItem("Update", UIBarButtonItemStyle.Plain, (s, args) => source.DoUpdate());
        NavigationItem.RightBarButtonItem = updateButton;

        TableView.Source = source;
        TableView.ReloadData();
    }
}

public class TableSource : UITableViewSource
{
    private readonly UITableView _tableView;
    private readonly TableCell _cell;

    public TableSource(UITableView tableView)
    {
        _tableView = tableView;
        _cell = new TableCell();
    }

    public override UITableViewCell GetCell(UITableView tableView, NSIndexPath indexPath)
    {
        return _cell;
    }

    public override float GetHeightForRow(UITableView tableView, NSIndexPath indexPath)
    {
        return 55.0f;
    }

    public override int RowsInSection(UITableView tableview, int section)
    {
        return 1;
    }

    public void DoUpdate()
    {
        _cell.DoUpdate();
        _tableView.ReloadData();
    }
}

public class TableCell : UITableViewCell
{
    private UILabel _label;

    public TableCell()
    {
        DoUpdate();
    }

    public void DoUpdate()
    {
        foreach (var subview in ContentView.Subviews)
        {
            subview.RemoveFromSuperview();
        }

        _label = new UILabel {Text = "Text here"};
        ContentView.Add(_label);

        foreach (var subview in ContentView.Subviews)
        {
            subview.TranslatesAutoresizingMaskIntoConstraints = false;
        }

        ContentView.RemoveConstraints(ContentView.Constraints);
        ContentView.AddConstraints(
            new[]
            {
                NSLayoutConstraint.Create(_label, NSLayoutAttribute.Left, NSLayoutRelation.Equal, ContentView, NSLayoutAttribute.Left, 1.0f, 15.0f),
                NSLayoutConstraint.Create(_label, NSLayoutAttribute.Width, NSLayoutRelation.Equal, null, NSLayoutAttribute.NoAttribute, 1.0f, 100.0f),
                NSLayoutConstraint.Create(_label, NSLayoutAttribute.Top, NSLayoutRelation.Equal, ContentView, NSLayoutAttribute.Top, 1.0f, 20.0f),
                NSLayoutConstraint.Create(_label, NSLayoutAttribute.Bottom, NSLayoutRelation.Equal, ContentView, NSLayoutAttribute.Bottom, 1.0f, -20.0f)
            });
    }
}