How to handle/cancel back navigation in Xamarin Forms

Question

I tried to use the back navigation by overriding OnBackButtonPressed, but somehow it wasn't get called at all. I am using the ContentPage and the latest 1.4.2 release.

Solution 1

Alright, after many hours I figured this one out. There are three parts to it.

#1 Handling the hardware back button on android. This one is easy, override OnBackButtonPressed. Remember, this is for a hardware back button and android only. It will not handle the navigation bar back button. As you can see, I was trying to back through a browser before backing out of the page, but you can put whatever logic you need in.

  protected override bool OnBackButtonPressed()
    {
        if (_browser.CanGoBack)
        {
            _browser.GoBack();
            return true;
        }
        else
        {
            //await Navigation.PopAsync(true);
            base.OnBackButtonPressed();
            return true;
        }
    }

#2 iOS navigation back button. This one was really tricky, if you look around the web you'll find a couple examples of replacing the back button with a new custom button, but it's almost impossible to get it to look like your other pages. In this case I made a transparent button that sits on top of the normal button.

[assembly: ExportRenderer(typeof(MyAdvantagePage), typeof

(MyAdvantagePageRenderer))]
namespace Advantage.MyAdvantage.MobileApp.iOS.Renderers
{
    public class MyAdvantagePageRenderer : Xamarin.Forms.Platform.iOS.PageRenderer
    {
        public override void ViewWillAppear(bool animated)
        {
            base.ViewWillAppear(animated);

            if (((MyAdvantagePage)Element).EnableBackButtonOverride)
            {
                SetCustomBackButton();
            }
        }
        private void SetCustomBackButton()
        {
            UIButton btn = new UIButton();
            btn.Frame = new CGRect(0, 0, 50, 40);
            btn.BackgroundColor = UIColor.Clear;

            btn.TouchDown += (sender, e) =>
            {
                // Whatever your custom back button click handling
                if (((MyAdvantagePage)Element)?.
                CustomBackButtonAction != null)
                {
                    ((MyAdvantagePage)Element)?.
                       CustomBackButtonAction.Invoke();
                }
            };
            NavigationController.NavigationBar.AddSubview(btn);
        }
    }
}

Android, is tricky. In older versions and future versions of Forms once fixed, you can simply override the OnOptionsItemselected like this

       public override bool OnOptionsItemSelected(IMenuItem item)
    {
        // check if the current item id 
        // is equals to the back button id
        if (item.ItemId == 16908332)
        {
            // retrieve the current xamarin forms page instance
            var currentpage = (MyAdvantagePage)
            Xamarin.Forms.Application.
            Current.MainPage.Navigation.
            NavigationStack.LastOrDefault();

            // check if the page has subscribed to 
            // the custom back button event
            if (currentpage?.CustomBackButtonAction != null)
            {
                // invoke the Custom back button action
                currentpage?.CustomBackButtonAction.Invoke();
                // and disable the default back button action
                return false;
            }

            // if its not subscribed then go ahead 
            // with the default back button action
            return base.OnOptionsItemSelected(item);
        }
        else
        {
            // since its not the back button 
            //click, pass the event to the base
            return base.OnOptionsItemSelected(item);
        }
    }

However, if you are using FormsAppCompatActivity, then you need to add onto your OnCreate in MainActivity this to set your toolbar:

Android.Support.V7.Widget.Toolbar toolbar = this.FindViewById<Android.Support.V7.Widget.Toolbar>(Resource.Id.toolbar);
            SetSupportActionBar(toolbar);

But wait! If you have too old a version of .Forms or too new version, a bug will come up where toolbar is null. If this happens, the hacked together way I got it to work to make a deadline is like this. In OnCreate in MainActivity:

        MobileApp.Pages.Articles.ArticleDetail.androdAction = () =>
        {
            Android.Support.V7.Widget.Toolbar toolbar = this.FindViewById<Android.Support.V7.Widget.Toolbar>(Resource.Id.toolbar);
            SetSupportActionBar(toolbar);
        };

ArticleDetail is a Page, and androidAction is an Action that I run on OnAppearing if the Platform is Android on my page. By this point in your app, toolbar will no longer be null.

Couple more steps, the iOS render we made above uses properties that you need to add to whatever page you are making the renderer for. I was making it for my MyAdvantagePage class that I made, which implements ContentPage . So in my MyAdvantagePage class I added

public Action CustomBackButtonAction { get; set; }

        public static readonly BindableProperty EnableBackButtonOverrideProperty =
               BindableProperty.Create(
               nameof(EnableBackButtonOverride),
               typeof(bool),
               typeof(MyAdvantagePage),
               false);

        /// <summary>
        /// Gets or Sets Custom Back button overriding state
        /// </summary>
        public bool EnableBackButtonOverride
        {
            get
            {
                return (bool)GetValue(EnableBackButtonOverrideProperty);
            }
            set
            {
                SetValue(EnableBackButtonOverrideProperty, value);
            }
        }

Now that that is all done, on any of my MyAdvantagePage I can add this

:


 this.EnableBackButtonOverride = true;
            this.CustomBackButtonAction = async () =>
            {
                if (_browser.CanGoBack)
                {
                    _browser.GoBack();
                }
                else
                {
                    await Navigation.PopAsync(true);
                }
            };

That should be everything to get it to work on Android hardware back, and navigation back for both android and iOS.

Solution 2

You are right, in your page class override OnBackButtonPressed and return true if you want to prevent navigation. It works fine for me and I have the same version.

protected override bool OnBackButtonPressed()
{
    if (Condition)
        return true;
    return base.OnBackButtonPressed();
}

Solution 3

Depending on what exactly you are looking for (I would not recommend using this if you simply want to cancel back button navigation), OnDisappearing may be another option:

protected override void OnDisappearing()
{
       //back button logic here
}

Solution 4

Additional to Kyle Answer Set

Inside YOURPAGE

public static Action SetToolbar;

YOURPAGE OnAppearing

if (Device.RuntimePlatform == Device.Android)
{
    SetToolbar.Invoke();
}

MainActivity

YOURPAGE.SetToolbar = () =>
{
    Android.Support.V7.Widget.Toolbar toolbar = 
        this.FindViewById<Android.Support.V7.Widget.Toolbar>(Resource.Id.toolbar);
    SetSupportActionBar(toolbar);
};

Solution 5

OnBackButtonPressed() this will be called when a hardware back button is pressed as in android. This will not work on the software back button press as in ios.

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