maui-shell-navigation

# .NET MAUI Shell Navigation

Implement page navigation in .NET MAUI apps using Shell. Shell provides URI-based navigation, a flyout menu, tab bars, and a four-level visual hierarchy — all configured declaratively in XAML.

## When to Use

- Setting up top-level app navigation with tabs or a flyout menu
- Navigating between pages programmatically with `GoToAsync`
- Passing data between pages via query parameters or object parameters
- Registering detail-page routes for push navigation
- Guarding navigation with confirmation dialogs (e.g., unsaved changes)
- Customizing back button behavior per page

## When Not to Use

- Deep linking from external URLs or app links — see [.NET MAUI deep linking docs](https://learn.microsoft.com/dotnet/maui/fundamentals/app-links)
- Data binding on navigation target pages — use `maui-data-binding`
- Dependency injection for pages and view models — use `maui-dependency-injection`
- Apps using `NavigationPage` without Shell (different navigation API)

## Inputs

- A .NET MAUI project with `AppShell.xaml` as the root shell
- Pages (`ContentPage`) to navigate between
- Route names for detail pages not in the visual hierarchy

## Shell Visual Hierarchy

Shell uses a four-level hierarchy. Each level wraps the one below it:

```
Shell
 ├── FlyoutItem / TabBar          (top-level grouping)
 │    ├── Tab                     (bottom-tab grouping)
 │    │    ├── ShellContent        (page slot → ContentPage)
 │    │    └── ShellContent        (multiple = top tabs)
 │    └── Tab
 └── FlyoutItem / TabBar
```

- **FlyoutItem** — appears in the flyout menu; contains `Tab` children
- **TabBar** — bottom tab bar with no flyout entry
- **Tab** — groups `ShellContent`; multiple children produce top tabs
- **ShellContent** — each points to a `ContentPage`

### Implicit Conversion

You can omit intermediate wrappers. Shell auto-wraps:

| You write                    | Shell creates                         |
|------------------------------|---------------------------------------|
| `ShellContent` only          | `FlyoutItem > Tab > ShellContent`     |
| `Tab` only                   | `FlyoutItem > Tab`                    |
| `ShellContent` in `TabBar`   | `TabBar > Tab > ShellContent`         |

## Workflow: Set Up AppShell

1. Define `AppShell.xaml` inheriting from `Shell`
2. Add `FlyoutItem` or `TabBar` elements for top-level navigation
3. Add `Tab` elements for bottom tabs; nest multiple `ShellContent` for top tabs
4. **Always use `ContentTemplate`** with `DataTemplate` so pages load on demand
5. Register detail-page routes in the `AppShell` constructor

```xml
<Shell xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
       xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
       xmlns:views="clr-namespace:MyApp.Views"
       x:Class="MyApp.AppShell"
       FlyoutBehavior="Flyout">

    <FlyoutItem Title="Animals" Icon="animals.png">
        <Tab Title="Cats">
            <ShellContent Title="Domestic"
                          ContentTemplate="{DataTemplate views:DomesticCatsPage}" />
            <ShellContent Title="Wild"
                          ContentTemplate="{DataTemplate views:WildCatsPage}" />
        </Tab>
        <Tab Title="Dogs" Icon="dogs.png">
            <ShellContent ContentTemplate="{DataTemplate views:DogsPage}" />
        </Tab>
    </FlyoutItem>

    <TabBar>
        <ShellContent Title="Home" Icon="home.png"
                      ContentTemplate="{DataTemplate views:HomePage}" />
        <ShellContent Title="Settings" Icon="settings.png"
                      ContentTemplate="{DataTemplate views:SettingsPage}" />
    </TabBar>
</Shell>
```

```csharp
// AppShell.xaml.cs
public partial class AppShell : Shell
{
    public AppShell()
    {
        InitializeComponent();
        Routing.RegisterRoute("animaldetails", typeof(AnimalDetailsPage));
        Routing.RegisterRoute("editanimal", typeof(EditAnimalPage));
    }
}
```

## Workflow: Navigate with GoToAsync

All programmatic navigation uses `Shell.Current.GoToAsync`. Always `await` the call.

### Route Prefixes

| Prefix | Meaning                                     |
|--------|---------------------------------------------|
| `//`   | Absolute route from Shell root              |
| (none) | Relative; pushes onto the current nav stack |
| `..`   | Go back one level                           |
| `../`  | Go back then navigate forward               |

### Navigation Examples

```csharp
// 1. Absolute — switch to a specific hierarchy location
await Shell.Current.GoToAsync("//animals/cats/domestic");

// 2. Relative — push a registered detail page
await Shell.Current.GoToAsync("animaldetails");

// 3. With query string parameters
await Shell.Current.GoToAsync($"animaldetails?id={animal.Id}");

// 4. Go back one page
await Shell.Current.GoToAsync("..");

// 5. Go back two pages
await Shell.Current.GoToAsync("../..");

// 6. Go back one page, then push a different page
await Shell.Current.GoToAsync("../editanimal");
```

## Workflow: Pass Data Between Pages

### Option 1: IQueryAttributable (Preferred)

Implement on ViewModels to receive all parameters in one call:

```csharp
public class AnimalDetailsViewModel : ObservableObject, IQueryAttributable
{
    public void ApplyQueryAttributes(IDictionary<string, object> query)
    {
        if (query.TryGetValue("id", out var id))
            AnimalId = id.ToString();
    }
}
```

### Option 2: QueryProperty Attribute

Apply directly on the page class:

```csharp
[QueryProperty(nameof(AnimalId), "id")]
public partial class AnimalDetailsPage : ContentPage
{
    public string AnimalId { get; set; }
}
```

### Option 3: Complex Objects via ShellNavigationQueryParameters

Pass objects without serializing to strings:

```csharp
var parameters = new ShellNavigationQueryParameters
{
    { "animal", selectedAnimal }
};
await Shell.Current.GoToAsync("animaldetails", parameters);
```

Receive via `IQueryAttributable`:

```csharp
public void ApplyQueryAttributes(I