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GETTING STARTED

APP

DATA

BACKEND

OPERATIONS

SHIPPING

REFERENCE

APP

State

Keep a value in a signal and the widget that reads it redraws when it changes.

Combine signals with + * > & and the result is a signal too.

ON THIS PAGE

Create a signal with context.signal

Set, update and read a signal

Derive signals with operators

Make a model reactive

Share one object with DV.global

Observe the app lifecycle

Status

Create a signal with context.signal

Functional

Class

@DVPage(title: 'Cart')
Widget _cartPage(BuildContext context) {
  final DVSignal<int> quantity = context.signal(1);
  final DVSignal<int> price = context.signal(1200);

  // Each of these is a signal that tracks its sources.
  final total = price * quantity;
  final inStock = quantity > 0;

  return DVBox.list(<Widget>[
    // Reading .value in build redraws this page when a source changes.
    DVText('Total: ${total.value}'),
    DVText(inStock.value ? 'Ready to order' : 'Add something first'),
    DVText('Add one').modifier(
      DVModifier().onTap(() => quantity.value = quantity.value + 1),
    ),
  ]);
}

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context.signal(initial) creates a DVSignal for this widget.

Reading .value in build subscribes the widget to it.

Signals match by call order in build, like hooks. Create them unconditionally.

signal(context, value) is the same call as a top-level function.

Set, update and read a signal

quantity.value = 3; // set
quantity.update((int n) => n + 1); // set from the current value
final int now = quantity.read(); // read without subscribing

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Derive signals with operators

An operator on a signal returns a signal that tracks its sources. There is no separate computed type to learn.

final DVSignal<int> price = context.signal(1200);
final DVSignal<int> quantity = context.signal(2);
final DVSignal<bool> agreed = context.signal(false);
final DVSignal<bool> paid = context.signal(false);
final DVSignal<String> first = context.signal('Ada');

final subtotal = price * quantity; // + - * / ~/ % on numbers
final expensive = subtotal >= 10000; // < <= > >= give a bool signal
final canShip = agreed & paid; // & | ^ on bools
final greeting = first + ', welcome'; // + on strings

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num

Operators: + - * / ~/ % and < <= > >=

String

Operators: +

bool

Operators: & | ^

Read .value to redraw

A derived signal redraws a widget through its sources, so read .value in build. == and ! are not overloaded: compare .value instead.

Make a model reactive

Every generated model has model.signal(context). Pass it to the generated page to redraw on each change.

Widget liveArticle(BuildContext context, Article article) {
  final DVSignal<Article> current = article.signal(context);
  return Article.Page.signal(current);
}

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Share one object with DV.global

// Register one instance at startup.
DV.global<Cart>(Cart());

// Read it anywhere. context.global redraws the widget when it is replaced.
final Cart cart = DV.global<Cart>();
final Cart same = context.global<Cart>();

// A namespace keeps two registrations of one type apart.
DV.global<Cart>(Cart(), 'wishlist');
final Cart wishlist = DV.global<Cart>(null, 'wishlist');

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DV.global<T>(instance) registers. DV.global<T>() reads, and throws when nothing is registered.

context.global<T>() reads and redraws the widget when the object is replaced.

Use it for app-wide objects. There is no separate service container.

Observe the app lifecycle

DV.lifecycle.app.listen((DVAppLifecycle state) {
  if (state == DVAppLifecycle.backgrounded) {
    // Save a draft before the app is paused.
  }
});

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DV.lifecycle.app and DV.lifecycle.build are read-only.

context.lifecycle.page, .request and .transaction exist inside a page, a request or a transaction.

Use .listen or .changes. Reading .value in build does not redraw.

Partial

Spec section: Lifecycle Signals

Planned work and implementation limits

DV.lifecycle.build never changes at run time.

A page moves through created, ready, active, disposing and disposed. Its loading and leaving states are not emitted.

The app never reports suspended or resuming, and a request never reaches completed.

Status

Built

Spec section: State

FSL-1.1-MIT licensed. Built with Dartvel.

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