Event Handlers

For every event on every control, FmgLib.MauiMarkup generates a fluent On<EventName> method — the C# equivalent of XAML's Clicked="OnCounterClicked" attributes, but attached inline, with two convenient shapes.

The Two Shapes

Using Button.Clicked as the example, the generator emits:

// 1. Classic event-handler signature
public static T OnClicked<T>(this T self, EventHandler handler) where T : Button;

// 2. Simplified action receiving the (typed!) sender only
public static T OnClicked<T>(this T self, Action<T> action) where T : Button;

Shape 2 is the everyday one: no unused object sender, EventArgs e boilerplate, and the parameter is already the concrete control type.

Method-Group Style

using FmgLib.MauiMarkup;

public class ExamPage : ContentPage
{
    int count = 0;

    public ExamPage()
    {
        this
        .Content(
            new VerticalStackLayout()
            .Children(
                new Button()
                    .Text("Click me")
                    .OnClicked(OnCounterClicked)
            )
        );
    }

    private void OnCounterClicked(Button sender)
    {
        count++;
        sender.Text = $"Clicked {count} ";
        sender.Text += count == 1 ? "time" : "times";
    }
}

Inline Lambda Style

new Button()
    .Text("Click me")
    .OnClicked(button =>
    {
        count++;
        button.Text = $"Clicked {count} ";
        button.Text += count == 1 ? "time" : "times";
    })

When You Need the Event Args

Use the classic shape — the full EventArgs are available:

new Entry()
    .OnTextChanged((sender, e) =>
    {
        Console.WriteLine($"'{e.OldTextValue}' → '{e.NewTextValue}'");
    })

new CollectionView()
    .OnSelectionChanged((sender, e) =>
    {
        var selected = e.CurrentSelection.FirstOrDefault();
        if (selected is Product p) ShowDetail(p);
    })

Common Events Cheat Sheet

Control	Fluent method	Typical use
Button	OnClicked, OnPressed, OnReleased	Actions
Entry / Editor	OnTextChanged, OnCompleted, OnFocused, OnUnfocused	Validation, search-as-you-type
CheckBox	OnCheckedChanged	Toggles
Switch	OnToggled	Settings
Slider	OnValueChanged, OnDragCompleted	Ranges
Picker	OnSelectedIndexChanged	Selection
CollectionView	OnSelectionChanged, OnScrolled, OnRemainingItemsThresholdReached	Lists, infinite scroll
RefreshView	OnRefreshing	Pull to refresh
ContentPage	OnAppearing, OnDisappearing, OnLoaded, OnUnloaded, OnNavigatedTo	Lifecycle
WebView	OnNavigating, OnNavigated	Web content

(Any event not listed follows the same On<EventName> naming.)

Page Lifecycle Inline

Because pages are BindableObjects too, lifecycle wiring can live in the same fluent chain:

public void Build() =>
    this
    .OnAppearing(async page => await ViewModel.RefreshAsync())
    .Content(/* ... */);

Events vs. Commands

Both work; pick by architecture:

// Event style — page-local logic
new Button().Text("Save").OnClicked(async b => await SaveAsync())

// Command style — MVVM
new Button()
    .Text("Save")
    .Command(e => e.Path("SaveCommand"))
    .CommandParameter(e => e.Path("."))

A pragmatic middle ground when you hold a typed view-model reference (e.g. with FmgLibContentPage<TViewModel>):

new Button().Text("Save").Command(BindingContext.SaveCommand)

Unsubscription & Hot Reload

On<Event> subscribes with +=. Two consequences:

• Rebuilding the UI creates new controls, so old subscriptions die with the old controls — no leak in the typical hot reload Build() flow.
• If you attach handlers to long-lived objects (e.g. Application.Current, a static service) inside Build(), you will stack up subscriptions on every hot reload. Attach those in the constructor instead, or unsubscribe first via InvokeOnElement.

Custom / Third-Party Events

The source generator produces the same two On… shapes for events on third-party controls:

[MauiMarkup(typeof(ZXing.Net.Maui.Controls.CameraBarcodeReaderView))]
class Markup { }

// generated: OnBarcodesDetected(handler) and OnBarcodesDetected(Action<T>)
new CameraBarcodeReaderView()
    .OnBarcodesDetected((s, e) => Process(e.Results))

Related Topics

• Gesture Recognizers — tap/pan/pointer events on arbitrary views
• Triggers — declarative reactions without code
• Hot Reload