How to handle/cancel back navigation in Xamarin Fo

问题:

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.

回答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.

回答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();
}

回答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
}

回答4:

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.

回答5:

For anyone still fighting with this issue - basically you cannot intercept back navigation cross-platform. Having said that there are two approaches that effectively solve the problem:

1. Hide the NavigationPage back button with NavigationPage.ShowHasBackButton(this, false) and push a modal page that has a custom Back/Cancel/Close button

2. Intercept the back navigation natively for each platform. This is a good article that does it for iOS and Android: https://theconfuzedsourcecode.wordpress.com/2017/03/12/lets-override-navigation-bar-back-button-click-in-xamarin-forms/

For UWP you are on your own :)

Edit:

Well, not anymore since I did it :) It actually turned out to be pretty easy – there is just one back button and it’s supported by Forms so you just have to override ContentPage’s OnBackButtonPressed:

    protected override bool OnBackButtonPressed()
    {
        if (Device.RuntimePlatform.Equals(Device.UWP))
        {
            OnClosePageRequested();
            return true;
        }
        else
        {
            base.OnBackButtonPressed();
            return false;
        }
    }

    async void OnClosePageRequested()
    {
        var tdvm = (TaskDetailsViewModel)BindingContext;
        if (tdvm.CanSaveTask())
        {
            var result = await DisplayAlert("Wait", "You have unsaved changes! Are you sure you want to go back?", "Discard changes", "Cancel");

            if (result)
            {
                tdvm.DiscardChanges();
                await Navigation.PopAsync(true);
            }
        }
        else
        {
            await Navigation.PopAsync(true);
        }           
    }

回答6:

The trick is to implement your own navigation page that inherits from NavigationPage. It has the appropriate events Pushed, Popped and PoppedToRoot.

A sample implementation could look like this:

public class PageLifetimeSupportingNavigationPage : NavigationPage
{
    public PageLifetimeSupportingNavigationPage(Page content)
        : base(content)
    {
        Init();
    }

    private void Init()
    {
        Pushed += (sender, e) => OpenPage(e.Page);

        Popped += (sender, e) => ClosePage(e.Page);

        PoppedToRoot += (sender, e) =>
        {
            var args = e as PoppedToRootEventArgs;
            if (args == null)
                return;

            foreach (var page in args.PoppedPages.Reverse())
                ClosePage(page);
        };
    }

    private static void OpenPage(Page page)
    {
        if (page is IPageLifetime navpage)
            navpage.OnOpening();
    }

    private static void ClosePage(Page page)
    {
        if (page is IPageLifetime navpage)
            navpage.OnClosed();

        page.BindingContext = null;
    }
}

Pages would implement the following interface:

public interface IPageLifetime
{
    void OnOpening();
    void OnClosed();
}

This interface could be implemented in a base class for all pages and then delegate it's calls to it's view model.

The navigation page and could be created like this:

var navigationPage = new PageLifetimeSupportingNavigationPage(new MainPage());

MainPage would be the root page to show.

Of course you could also just use NavigationPage in the first place and subscribe to it's events without inheriting from it.

回答7:

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

base.OnBackButtonPressed() returns false on click of hardware back button. In order to prevent operation of back button or prevent navigation to previous page. the overriding function should be returned as true. On return true, it stays on the current xamarin form page and state of page is also maintained.

回答8:

Maybe this can be usefull, You need to hide the back button, and then replace with your own button:

public static UIViewController AddBackButton(this UIViewController controller, EventHandler ev){
    controller.NavigationItem.HidesBackButton = true;
    var btn = new UIBarButtonItem(UIImage.FromFile("myIcon.png"), UIBarButtonItemStyle.Plain, ev);
    UIBarButtonItem[] items = new[] { btn };
    controller.NavigationItem.LeftBarButtonItems = items;
    return controller;
}

public static UIViewController DeleteBack(this UIViewController controller)
{
    controller.NavigationItem.LeftBarButtonItems = null;
    return controller;
}

Then call them into these methods:

public override void ViewWillAppear(bool animated)
{
    base.ViewWillAppear(animated);
    this.AddBackButton(DoSomething);
    UpdateFrames();
}

public override void ViewWillDisappear(Boolean animated)
{
    this.DeleteBackButton();
}

public void DoSomething(object sender, EventArgs e)
{            
    //Do a barrel roll
}