ContentPageRenderer.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Reflection;
using Xamarin.Forms;
using Xamarin.Forms.Platform.iOS;
using UIKit;
using MyAppName.iOS.Renderers;

[assembly: ExportRenderer(typeof(ContentPage), typeof(ContentPageRenderer))]
namespace MyAppName.iOS.Renderers
{
  /// <summary>
  /// Custom renderer for ContentPage that allows for action buttons to be on the left or the right hand side (ex: a modal with cancel and done buttons)
  /// ToolbarItems need to have Priority set to 0 to show on the left, 1 to show on the right (ref: https://gist.github.com/alexlau811/f1fff9e726333e6b4a2f)
  /// </summary>
  public class ContentPageRenderer : PageRenderer
  {
    public override void ViewWillAppear(bool animated)
    {
      base.ViewWillAppear(animated);

      var contentPage = this.Element as ContentPage;
      if (contentPage == null || NavigationController == null)
        return;

      var itemsInfo = contentPage.ToolbarItems;

      var navigationItem = this.NavigationController.TopViewController.NavigationItem;
      var leftNativeButtons = (navigationItem.LeftBarButtonItems ?? new UIBarButtonItem[] { }).ToList();
      var rightNativeButtons = (navigationItem.RightBarButtonItems ?? new UIBarButtonItem[] { }).ToList();

      var newLeftButtons = new UIBarButtonItem[] { }.ToList();
      var newRightButtons = new UIBarButtonItem[] { }.ToList();

      rightNativeButtons.ForEach(nativeItem =>
      {
        // [Hack] Get Xamarin private field "item"
        var field = nativeItem.GetType().GetField("_item", BindingFlags.NonPublic | BindingFlags.Instance);
        if (field == null)
          return;

        var info = field.GetValue(nativeItem) as ToolbarItem;
        if (info == null)
          return;

        if (info.Priority == 0)
          newLeftButtons.Add(nativeItem);
        else
          newRightButtons.Add(nativeItem);
      });
      
      leftNativeButtons.ForEach(nativeItem =>
      {
        newLeftButtons.Add(nativeItem);
      });      

      navigationItem.RightBarButtonItems = newRightButtons.ToArray();
      navigationItem.LeftBarButtonItems = newLeftButtons.ToArray();
    }
  }
}

This seems broken on the latest version of Xamarin.Forms (3.2.0.839982), Xamarin.iOS (12.0.0.15). This was working for me before upgrading. It appears that it can't find the private field "item" anymore.