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An easy and safe DI (Dependency Injection) solution for Dart.

Introduction

Pot is a single-type DI container holding an object of a particular type.

A pot is usually assigned to a global variable. Each pot has a Singleton factory function that is triggered to create an object as needed. It is possible to replace the factory or discard the object in a pot at your preferred timing, which is useful for testing as well as for implementing app features.

Advantages

  • Easy
    • Straightforward because it is specialised for DI, without other features.
    • Simple API that you can be sure how to use.
    • A pot as a global variable is easy to handle; auto-completion works in IDEs.
  • Safe
    • Not dependent on types.
      • No runtime error basically. The object always exists when it is accessed as long as the pot has a valid factory.
    • Not dependent on strings either.
      • Pot does not provide a dangerous way to access the content like referencing it by type name.

Policy

This package will not adopt new features easily so that it'll be kept simple to use. The focus will be more on enhancing stability and robustness.

Examples

  • Pottery
    • A package that helps you use pots in Flutter by allowing to limit the scope of pots in the widget tree.
    • Whether to use this is up to you. It is just an additional utility.

Usage

Create a pot with a so-called Singleton factory that instantiates an object.

final counterPot = Pot(() => Counter(0));

Now you can use the pot in whatever file importing the above declaration.

Note that the created pot should be assigned to a global variable unless there is a special reason against it.

Getting the object

Calling the call() method triggers the factory to create an object on the fly if no object has been created yet or one has already been discarded. Otherwise, the existing object is returned.

call() is a special function in Dart that allows a class instance to be called like a function, so instead of counterPot.call(), you can write it as follows:

void main() {
  final counter = counterPot();
  ...
}

Creating an object

Use create() if you want to instantiate an object without obtaining it.

This is practically the same as call(), except that create() does not return the created object while call() does.

void main() {
  counterPot.create();
  ...
}

Discarding the object

You can discard the object with several methods like reset(). The resources are saved if objects are properly discarded when they become unnecessary.

Even if an object is discarded, the pot itself is not discarded. A new object is created when it is needed again, so no worry that the object may be no longer accessible.

If a callback function is passed to the disposer argument of the constructor, it is triggered when the object in the pot is discarded. Use it for doing a clean-up related to the object.

final counterPot = Pot(
  () => Counter(0),
  disposer: (counter) => counter.dispose(),
);

void main() {
  final counter = counterPot();
  counter.increment();
  ...

  // Discards the Counter object and triggers the disposer function.
  counterPot.reset();
}

replace(), Pot.popScope(), Pot.resetAllInScope() and Pot.resetAll() also discard existing object(s). These will be explained in later sections of this document.

Advanced usage

Replacing factory and object

Pots created by Pot.replaceable() have the replace() method. It replaces the object factory, which was set in the constructor of Pot, and the object held in a pot. Otherwise, the replace() method is not available.

final userPot = Pot.replaceable(() => User.none());

Future<User> signIn() async {
  final userId = await Auth.signIn(...);
  userPot.replace(() => User(id: userId));
  return userPot();
}

Note that the replace() method discards an existing object, triggering the disposer, but only if an object has already been created. It behaves differently depending on whether the object exists. See the document of replace() for details on the behaviour.

Creating a pot with no factory

Pot.pending() is an alternative to Pot.replaceable(), useful if the object is unnecessary or the factory is unavailable until some point.

Note that a PotNotReadyException occurs if the pot is used before a valid factory is set with replace().

final userPot = Pot.pending<User>();
// final user = userPot(); // PotNotReadyException

...

userPot.replace(() => User(id: userId));
final user = userPot();

It is also possible to remove the existing factory by resetAsPending() to switch the state of the pot to pending.

Replacements for testing

If replacements are only necessary in tests, avoid using Pot.replaceable for safety. Instead, enable the use of replaceForTesting() by setting Pot.forTesting to true.

final counterPot = Pot(() => Counter(0));

void main() {
  Pot.forTesting = true;

  test('Some test', () {
    counterPot.replaceForTesting(() => Counter(100));
    ...
  });
}

Listening for events

The static method listen() allows you to listen for events related to pots. See the document of PotEventKind for event types, such as instantiated and reset.

final removeListener = Pot.listen((event) {
  ...
});

// Don't forget to stop listening when it is no longer necessary.
removeListener();

Note:

  • Events of changes in the objects held in pots are not emitted automatically.
    • Call notifyObjectUpdate() to manually emit those events if necessary.
  • There is no guarantee that the event data format remains unchanged in the future. Use the method and the data passed to the callback function only for debugging purposes.

Scoping

Pot or Pottery

The scoping feature of this package is for Dart in general, not designed for Flutter. Consider using Pottery instead. It is a utility wrapping this pot package for use in Flutter. It limits the lifespan of pots according to the lifecycle of widgets, which is more natural in Flutter and less error-prone.

What is scoping

A "scope" in this package is a notion related to the lifespan of an object held in a pot. It is given a sequential number starting from 0. Adding a scope increments the index number of the current scope, and removing one decrements it.

For example, if a scope is added when the current index number is 0, the number turns 1. If an object is created then, it gets bound to the current scope 1. It means the object exists while the current scope is 1 or newer, so it is discarded and the disposer is triggered when the scope 1 is removed. The current index number goes back to 0.

final counterPot = Pot(() => Counter());

void main() {
  print(Pot.currentScope);     // 0

  Pot.pushScope();
  print(Pot.currentScope);     // 1

  // The Counter object is created here, and it gets bound to scope 1.
  final counter = counterPot();
  print(counterPot.hasObject); // true

  // The scope 1 is removed, causing the object to be discarded.
  Pot.popScope();
  print(Pot.currentScope);     // 0
  print(counterPot.hasObject); // false
}

If multiple objects are bound to the current scope, you can discard all of them by just calling Pot.popScope().

Combining replace() with scoping

If an object is used only from some point onwards, you can make use of Pot.popScope() and replace().

Declare a pot with Pot.pending() initially, and replace the factory with the actual one after adding a scope. It allows a factory to be set only at a specific scope, and enables the object to be discarded by removal of the scope.

// A dummy factory for the moment, which only throws an exception if called.
final userPot = Pot.pending<User>();

...

// A new scope is added, and the dummy factory is replaced with the actual one.
Pot.pushScope();
todoPot.replace(() => User(id, name));

// The User object is created and gets bound to the current scope.
final user = userPot();

...

// The scope is removed and the object is discarded.
// It is better to replace the factory so that it throws if called
// unexpectedly after this.
Pot.popScope();
userPot.replace(() => throw PotNotReadyException());

Resetting objects in the current scope

Pot.resetAllInScope() discards all the objects bound to the current scope, but the scope is not removed.

The behaviour of a reset of each object is the same as reset(); the disposer is triggered in the same way.

Resetting objects in all scopes

Pot.resetAll() discards all the objects that are bound to any scope. This is useful to reset all for testing.

By default, this method does not remove scopes themselves. If you want both objects and scopes to be reset, call it with keepScopes: false. It may be used for clearing the state to make the app behave as if it has restarted.

Caveats

DON'T declare a pot locally

All pots should usually be declared globally. It is possible to declare pots locally as long as their resources are properly discarded, but it is almost meaningless to use it like the code below. It isn't much different from having the MyService object directly as a property of MyClass.

Example code (Click to open) 
void main() {
  final myClass = MyClass();
  ...
  myClass.dispose();
}

class MyClass {
  final servicePot = Pot(() => MyService());

  // Use reset() in a disposing method like this
  // and make sure to call it at some point.
  void dispose() {
    servicePot.reset();
  }

  void someMethod() {
    final service = servicePot();
    ...
  }
}

This brings the danger that its partial data remains until the end of the program because the data related to scoping is stored globally even if the pot is assigned to a local variable, and it is not automatically discarded when the variable goes out of use. It therefore must be discarded manually with reset() or other methods that have the same effect.

Libraries

pot