Application Class

• The App class is instantiated and loaded by the boostrap code for each platform.
• It is the model of the application as a whole.
• Inherits default behaviors from Microsoft.Maui.Controls.Application.
• Creates an instance of AppShell and assigns it to the MainPage property.
• Handles lifecycle events (like when the app is sent to the background).

AppShell Class

An AppShell:

• Describes the hierarchy of an app
• Provides a common navigation experience via URIs
• Includes an integrated search handler

An AppShell derives from the Shell class and contains three main hierarchical objects:

1. FlyoutItem or TabBar
   1. A FlyoutItem is used when the app requires a flyout
   2. A TabBar is the bottom tab bar and used when a flyout is not used
   3. Each FlyoutItem or TabBar is a child of the AppShell.
2. Tab — represented grouped content; navigable from the bottom tabs.
   1. Each Tab is a child of either a FlyoutItem or a TabBar.
3. ShellContent — The ContentPage object for each tab
   1. Each ShellContent is a child of a Tab.
   2. When more than one ShellContent exist in a Tab, the objects are navigable by the tabs on top of the page.

ContentPage

ContentPage can contain only a single child, either a view or a layout with child views. ContentPage.Content property is set to this child.

Pages

• The root of the UI hierarchy inside of a Shell.
• An example is MainPage, which derives from ContentPage.

Page Types

• ContentPage — simply displays content.
  • A content page can only contain one item (like a Control) unless you also have a Layout.
• TabbedPage — the root page used for tab navigation.
  • Contains child page objects (one for each tab).
• FlyoutPage — contains a list of items; when you select that item, a view displaying the details for that item appears.

Views

• A view enables the developer to retrieve and present data in a specific manner.
• A ContentPage displays a view.
• A ContentView displays items as-is; if you shrink the view, items may disappear.
• A ScrollView displays items in a scrolling window.
• A CarouselView allows users to swipe through a collection of items.
• A CollectionView can retrieve data from a data source and present each item using a template.

Controls and Layouts

• Controls — buttons, labels, text boxes.
• Layout — defines the rules by which controls are displayed.
  • Technically, a layout is also a control, so it can be added to a view.
    

    

Common Controls

• VerticalStackLayout and HorizontalStackLayout

  • Or, StackLayout, with a StackOrientation property that can be set to Horizontal or Vertical.
  • This is how screens can change orientation dynamically from portrait to landscape:
    

    

• AbsoluteLayout — set exact coordinates for controls.

• FlexLayout — like StackLayout, but allows wrapping child controls if they don’t fit in a single row or column.

  • Includes options for alignment to different screen sizes:
    

    

• Grid — lays out controls according to a column and row location the developer defines:
  

  

Controls: Properties & Events

• All controls have properties to alter visualization.
  • Initial values for these properties are defined in XAML.
  • Many proeprties can be modified in C# code.
• Controls have events to provide interactive behavior.

Margin, Padding, and Spacing Properties

• Each control has a Margin property to give space between controls.
• Each layout has a Padding property to give space between children and the border.
• VerticalStackLayout and HorizonalStackLayout have a Spacing property to give space between children of the layout.
  • This spacing is additive to Margin.

C# & XAML

• Entire pages, views, and layouts can be written in C#.
• Here are two such examples, one written in XAML and one in C#:

XAML Code

<ContentPage xmlns="http://schemas.microsoft.com/_net/2021/maui"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             x:Class="Phoneword.MainPage">

    <ScrollView>
        <VerticalStackLayout>
            <Label Text="Current count: 0"
                   Grid.Row="0"
                   FontSize="18"
                   FontAttributes="Bold"
                   x:Name="CounterLabel"
                   HorizontalOptions="Center" />

            <Button Text="Click me"
                    Grid.Row="1"
                    Clicked="OnCounterClicked"
                    HorizontalOptions="Center" />
        </VerticalStackLayout>
    </ScrollView>
</ContentPage>

Equivalent C# Code

public partial class TestPage : ContentPage
{
    int count = 0;
    // Named Label - declared as a member of the class
    Label counterLabel;

    public TestPage()
    {
        var myScrollView = new ScrollView();

        var myStackLayout = new VerticalStackLayout();
        myScrollView.Content = myStackLayout;

        counterLabel = new Label
        {
            Text = "Current count: 0",
            FontSize = 18,
            FontAttributes = FontAttributes.Bold,
            HorizontalOptions = LayoutOptions.Center
        };
        myStackLayout.Children.Add(counterLabel);
        var myButton = new Button
        {
            Text = "Click me",
            HorizontalOptions = LayoutOptions.Center
        };
        myStackLayout.Children.Add(myButton);

        myButton.Clicked += OnCounterClicked;

        this.Content = myScrollView;
    }

    private void OnCounterClicked(object sender, EventArgs e)
    {
        count++;
        counterLabel.Text = $"Current count: {count}";

        SemanticScreenReader.Announce(counterLabel.Text);
    }
}

Then, in AppShell.xaml:

<ShellContent Title="Home" ContentTemplate="{DataTemplate local:TestPage}" Route="TestPage" />