Nalu.Maui.Navigation

We provide a set of interfaces to react to navigation lifecycle events, and it takes care of disposing all the resources when the page is popped.

The navigation system awaits every navigation lifecycle event, so you can safely perform asynchronous operations with real async ValueTask methods instead of relying on async void ones.

Navigation.Relative()
    .Push<ContactsPageModel>()
    .Push<ContactDetailPageModel>()

Push twice in a row

There’s also a “navigation guard” feature that allows you to prevent the navigation to happen if a condition is not met. You can leverage that to ask the user to confirm leaving the page.

// Starting from: RootPageModel > ContactsPageModel > ContactDetailPageModel
Navigation.Absolute()
    .ShellContent<RootPageModel>()

// This is gonna trigger the navigation guard on ContactDetailPageModel
ValueTask<bool> CanLeaveAsync() => { ... ask the user };

Absolute navigation to root page

Leak detection

Nalu.Maui automatically detects and reports memory leaks when the debugger is attached. An alert dialog will be shown when your Page was not collected after navigating away.

In the above example the leak detection on ContactsPageModel, ContactDetailPageModel and their respective Pages triggers after the navigation completed (root page appears).

Leak detection can be enabled in MauiProgram.cs:

.UseNaluNavigation<App>(nav => nav
    .AddPages()
    .WithLeakDetectorState(NavigationLeakDetectorState.EnabledWithDebugger)
)

Implementation in detail

Dependency injection the right way

With Nalu navigation, a ServiceScope is created for each page, so you can use Scoped services in your pages and view models. Pages and view models are in fact registered as Scoped services and automatically disposed by the ServiceScope when the page is removed from the navigation stack.

Initial setup

First of all, you need to add the Nalu.Maui package to your project, then just call UseNaluNavigation in your MauiProgram:

public static class MauiProgram
{
    public static MauiApp CreateMauiApp()
    {
        var builder = MauiApp.CreateBuilder();
        builder
            .UseMauiApp<App>()
            .UseNaluNavigation<App>()

This method will scan the <App> assembly for pages and view models by naming convention MainPage => MainPageModel. You can specify a custom naming convention by passing a function that returns the view model type given the page type:

builder
    .UseMauiApp<App>()
    .UseNaluNavigation<App>(nav => nav.AddPages((pageType) => pageType.Name.Replace("Page", "ViewModel")))

Important notes:

page models needs to implement INotifyPropertyChanged interface
pages need to require the view model as constructor parameter and assign it to the BindingContext property

public partial class MainPage : ContentPage
{
    public MainPage(MainPageModel viewModel)
    {
        InitializeComponent();
        BindingContext = viewModel;
    }
}

Eventually you can specify the pages and view models manually:

builder
    .UseMauiApp<App>()
    .UseNaluNavigation<App>(nav => nav.AddPage<MainPageModel, MainPage>())

To help with testability you can also register the page model as an interface:

builder
    .UseMauiApp<App>()
    .UseNaluNavigation<App>(nav => nav.AddPage<IMainPageModel, MainPageModel, MainPage>())

Note: the automatic registration by naming convention automatically considers the page model as an interface.

Initial setup - without MVVM pattern

Nalu can be used even without MVVM pattern, just add your pages as Scoped services:

builder
    .UseNaluNavigation<App>()
    .Services
    .AddScoped<MyPage>();

You can eventually customize in one place all the back and menu icons.

builder
    .UseMauiApp<App>()
    .UseNaluNavigation<App>(nav => nav.AddPages()
        .WithMenuIcon(ImageSource.FromFile("menu.png"))
        .WithBackIcon(ImageSource.FromFile("back.png")))

Shell definition

Nalu navigation is based on Shell navigation, so you need to define your Shell in AppShell.xaml by inheriting from NaluShell. Use nalu:Navigation.PageType to specify the page type for each ShellContent.

<?xml version="1.0" encoding="utf-8"?>

<nalu:NaluShell xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
                xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
                xmlns:pages="clr-namespace:Nalu.Maui.Sample.PageModels"
                xmlns:nalu="https://nalu-development.github.com/nalu/navigation"
                x:Class="Nalu.Maui.Sample.AppShell">
    <FlyoutItem Route="main"
                FlyoutDisplayOptions="AsMultipleItems">
        <Tab Title="Pages"
             Route="pages">
            <ShellContent nalu:Navigation.PageType="pages:OnePage"
                          Title="Page One"/>
            <ShellContent nalu:Navigation.PageType="pages:TwoPage"
                          Title="Page Two"/>
        </Tab>
    </FlyoutItem>
    <ShellContent nalu:Navigation.PageType="pages:FivePage"
                  Title="Page Five"/>
</nalu:NaluShell>

In the code behind you need to set the initial shell page passing the navigation service and the initial page type to the base constructor:

public partial class App : Application
{
    public App(INavigationService navigationService)
    {
        InitializeComponent();
        MainPage = new AppShell(navigationService);
    }
}

public partial class AppShell : NaluShell
{
    public AppShell(INavigationService navigationService) : base(navigationService, typeof(OnePage))
    {
        InitializeComponent();
    }
}

Shell structure is based on Item > Section > Content hierarchy. Even when you don’t specify an Item or a Section in the Shell definition, it is automatically created for you.

For example, the following Shell definition

<nalu:NaluShell>
    <ShellContent nalu:Navigation.PageType="pages:OnePage"/>
    <ShellContent nalu:Navigation.PageType="pages:TwoPage"/>
</nalu:NaluShell>

is equivalent to

<nalu:NaluShell>
    <ShellItem>
        <ShellSection>
            <ShellContent nalu:Navigation.PageType="pages:OnePage"/>
        </ShellSection>
    </ShellItem>
    <ShellItem>
        <ShellSection>
            <ShellContent nalu:Navigation.PageType="pages:TwoPage"/>
        </ShellSection>
    </ShellItem>
</nalu:NaluShell>

That said, Nalu navigation provides the following navigation behavior when switching between ShellContents:

if the content is in the same ShellSection, navigation stack will be popped
if the content is in a different ShellSection but in the same ShellItem, the current navigation stack will be persisted
if the content is in a different ShellItem, all of the current item’s navigation stacks will be popped and the ShellContent pages will be destroyed

You can customize this behavior by providing a custom NavigationBehavior to the Navigation object.

For example you can also use the IgnoreGuards behavior to ignore the ILeavingGuard when popping a page:

await _navigationService.GoToAsync(Navigation.Relative(NavigationBehavior.IgnoreGuards).Pop());

The page view model can selectively react to navigation events by implementing the following interfaces:

IEnteringAware: defines a ValueTask OnEnteringAsync() called when the page is entering the navigation stack
IAppearingAware: defines a ValueTask OnAppearingAsync() called when the page is appearing
IDisappearingAware: defines a ValueTask OnDisappearingAsync() called when the page is disappearing
ILeavingAware: defines a ValueTask OnLeavingAsync() called when the page is leaving the navigation stack

With Nalu navigation you can also pass parameters to the target page using the IntentAware interfaces:

IEnteringAware<TIntent>: defines a ValueTask OnEnteringAsync(TIntent intent) called when the page is entering the navigation stack
IAppearingAware<TIntent>: defines a ValueTask OnAppearingAsync(TIntent intent) called when the page is appearing

Note: when an intent is passed to the view model, the OnEnteringAsync and OnAppearingAsync parameterless methods will be invoked only if the intent-specific method is not implemented. You can change this behavior by configuring Nalu with Strict behavior.

.UseNaluNavigation<App>(nav => nav
    .AddPages()
    .WithNavigationIntentBehavior(NavigationIntentBehavior.Strict)
)

Obviously nothing stops you to call the intent-less ones manually from the intent-aware one if you need to.

Leaving guards

Sometimes you want to protect a page from being popped from the navigation stack, for example when the user is editing a form. You can do that by implementing the ILeavingGuard interface which defines a ValueTask<bool> CanLeaveAsync() method from which you can eventually display a prompt to ask the user if they want to leave the page.

public class ViewModel :  ILeavingGuard
{
    public async ValueTask<bool> CanLeaveAsync()
    {
        return await ConfirmUserLeaveAsync("Are you sure you want to leave without saving?") // a method to verify the leave action
    }
}

Note: a page “appears” only when it is the target of the navigation, intermediate pages models will trigger OnAppearingAsync unless the ILeavingGuard needs to be evaluated.

First of all, you need to inject the INavigationService in your page model:

public class OnePageModel : IOnePageModel
{
    private readonly INavigationService _navigationService;

    public OnePageModel(INavigationService navigationService)
    {
        _navigationService = navigationService;
    }
}

Then you can use the GoToAsync method to navigate to a page using relative or absolute navigation:

// Add a page to the navigation stack
await _navigationService.GoToAsync(Navigation.Relative().Push<TwoPageModel>());
// Add a page to the navigation stack providing an intent
var myIntent = new MyIntent(/* ... */);
await _navigationService.GoToAsync(Navigation.Relative().Push<TwoPageModel>().WithIntent(myIntent));
// Remove the current page from the navigation stack
await _navigationService.GoToAsync(Navigation.Relative().Pop());
// Remove the current page from the navigation stack providing an intent to the previous page
var myIntent = new MyResult(/* ... */);
await _navigationService.GoToAsync(Navigation.Relative().Pop().WithIntent(myIntent))
// Pop two pages than push a new one
await _navigationService.GoToAsync(Navigation.Relative().Pop().Pop().Push<ThreePageModel>());
// Pop to the root page using absolute navigation
await _navigationService.GoToAsync(Navigation.Absolute().ShellContent<MainPageModel>());
// Switch to a different shell content and push a page there
await _navigationService.GoToAsync(Navigation.Absolute().ShellContent<OtherPageModel>().Push<OtherChildPageModel>());

Note:

if you don’t want to use MVVM pattern just use page types instead of page model types (i.e. Navigation.Relative().Push<TwoPage>()).
non-sense navigations will throw an exception, for example: pop -> push -> pop.

Nalu provides a Navigation markup extension that can be used to navigate to a page using relative or absolute navigation:

<!-- Pops the current page -->
<Button Command="{nalu:NavigateCommand}" Text="Back">
    <Button.CommandParameter>
        <nalu:RelativeNavigation>
            <nalu:NavigationPop />
        </nalu:RelativeNavigation>
    </Button.CommandParameter>
</Button>

<!-- Push a new page onto the navigation stack with intent -->
<Button Command="{nalu:NavigateCommand}" Text="Push some page">
    <Button.CommandParameter>
        <nalu:RelativeNavigation Intent="{Binding MyIntentValue}">
            <nalu:NavigationSegment Type="pages:SixPage" />
        </nalu:RelativeNavigation>
    </Button.CommandParameter>
</Button>

<!-- Navigate to main page -->
<Button Command="{nalu:NavigateCommand}" Text="Go to main page">
    <Button.CommandParameter>
        <nalu:AbsoluteNavigation>
            <nalu:NavigationSegment x:TypeArguments="pages:MainPage" />
        </nalu:AbsoluteNavigation>
    </Button.CommandParameter>
</Button>

Sometimes you need to share a service between pages, starting from a specific page down to all the nested pages.

Nalu navigation provides an INavigationServiceProvider service that can be used to provide services to nested page.

In the page model where you want to start providing the service, you need to inject the INavigationServiceProvider and call the Provide method:

public class PersonPageModel(INavigationServiceProvider navigationServiceProvider) : IPersonPageModel // which inherits from IEnteringAware<int>
{
    public ValueTask OnEnteringAsync(int personId)
    {
        var personContext = new PersonContext(personId);
        navigationServiceProvider.AddNavigationScoped<IPersonContext>(personContext);
    }
}

Then you can inject the service in the nested page models through the INavigationServiceProvider:

public class PersonDetailsPageModel(INavigationServiceProvider navigationServiceProvider) : IPersonDetailsPageModel
{
    private readonly IPersonContext _personContext = navigationServiceProvider.GetRequiredService<IPersonContext>();
}

Here’s an example of how to unit test navigation using NSubstitute:

Using record for intents is recommended to avoid having to implement an equality comparer. Suppose to have defined an intent class public record AnIntent(int Value = 0);.

// Arrange
var navigationService = Substitute.For<INavigationService>();
navigationService.GoToAsync(Arg.Any<Navigation>()).Returns(Task.FromResult(true));
var viewModel = new MyViewModel(navigationService);

// Act
await viewModel.DoSomethingAsync(5);

// Assert
var expectedNavigation = Navigation.Relative().Push<TargetPageModel>().WithIntent(new AnIntent(5));
await navigationService.Received().GoToAsync(Arg.Is<Navigation>(n => n.Matches(expectedNavigation)));

Unfortunately MAUI navigation (NavigationPage, or Shell) do not provide automatic page/view model disposal as widely explained in this issue. This is a problem because it can lead to memory/event leaks.

There are other big issues with Shell navigation:

Shell.Current.GoToAsync API is really hard to understand: can you easily tell what’s the difference between GoToAsync("Page1") / GoToAsync("/Page1") / GoToAsync("//Page1") / GoToAsync("///Page1")?
Root pages (defined as ShellContent will never be disposed, even if you navigate to a different shell item.
Have you ever wonder what’s the difference between Transient and Scoped service lifetime in MAUI?
The way to pass parameters is a bit inconvenient
There’s no way to define something and provide that value to all nested pages (like a context)
We have to rely on async void methods to perform asynchronous operations on page lifecycle events

On the other hand, Shell offers a convenient way to define the app structure including tab bar and flyout menu. Shell also supports having multiple navigation stacks alive at the same time when using a global TabBar.

Nalu navigation is based on Shell navigation, but it solves all the issues above.

Nalu