Overview

Scope

Scope provides Inversion of Control using the dependency injection (DI) pattern for Dart applications.

Scope allows you to inject values into a scope and then 'use' those dependencies from any method (or constructor) called within that scope.

Scope is not a replacement for the likes of Provider. Provider does dependency injection for your BuildContext whilst Scope provides DI for your call stack.

For Java developers, Scope provides similar functionality to a thread-local variables

Authors:

• Philipp Schiffmann [e-mail]

• S. Brett Sutton

Scope is a reimagining of Philipp's zone_id package. All credit goes to Phillipp's original implementation without which Scope wouldn't exist.

Sponsored by OnePub

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Scope is available on pub.dev at:

scope | Dart PackageDart packages

This is most easily understood via an example:

import 'package:scope/scope.dart';

/// create a key to access a scoped value
final ageKey = ScopeKey<int>();

void main() {
    /// create a Scope
    Scope()
    
    /// inject a value
    ..value<int>(ageKey, 18)
    
    /// run some code within the Scope
    ..run(() => a();
}
void a() => b();

/// `use` the injected value by its key 'ageKey'
void b() => print('You are ${use(ageKey)} years old');

We create a Scope within main and call the method a() which calls b(). Both a() and b() are within the declared scope and therefore have access to the injected value ageKey.

To access an injected value you call the use method.

Inject multiple values of the same type

Scope allows you to inject multiple values of the same type

final ageKey = ScopeKey<int>();
final carKey = ScopeKey<Car>();
final otherCarKey = ScopeKey<Car>();

Scope()
..value<int>(ageKey, 18)
..value<Car>(carKey, new Car('red'))
..value<Car>(otherCarKey, new Car('blue'))
..run(() {
  print('age:  ${use(ageKey)} colour: ${use(carKey)} other: ${use(otherCarKey)}');
  // age: 18: colour: red other: blue
});

Inject Single values and Sequences

You can inject values generated from a factory method as a single value or a sequence of values.

The difference between injecting a single value and a value is that the single's factory method has access to all other values injected into the scope.

A sequence generates a new value each time the use method is called with its key.

import 'package:scope/scope.dart';
import 'package:money2/money2.dart';
final ageKey = ScopeKey<int>();
final seedKey = ScopeKey<String>();
final lotteryKey = ScopeKey<Money>();
Scope()
..value<int>(ageKey, 18)

/// call randomValue just once using ageKey as the seed.
..single<String>(seedKey, () => randomValue(use(ageKey))

/// call randomWinnings each time `use(lotteryKey)` is called.
..sequence<String>(lotteryKey, () => randomWinnings(use(seedKey))

..run(() {
  print('age:  ${use(ageKey)} you won: ${use(lotteryKey)}');
  // age: 18: you won: $2000.00
  
  Money randomWinnings(String seedValue) => ...

Nesting

Scope also allows you to nest scopes to any level.

final ageKey = ScopeKey<int>();
final carKey = ScopeKey<Car>();
Scope()
..value<int>(ageKey, 18)
..value<Car>(carKey, new Car('red'))
..run(() {
    // use values the other scope
    print('age:  ${use(ageKey)} colour: ${use(carKey)}');
  // age: 18: colour: red

  // create a nested scope 
  Scope()
  ..value<Car>(carKey, new Car('green'))
  ..run(() {
    /// use values from the inner and outer scope.
    print('age:  ${use(ageKey)} colour: ${use(carKey)}');
   // age: 18: colour: green
  });
});