NanoVar

A variable that can be subscribed to so the subscribers get notified when changes occur.

Usage

Basic usage

Create a NanoVar instance with a given initial value:

final counter = NanoVar(0);

Subscribe to changes made to the value of the NanoVar instance:

final unsubscribe = counter.subscribe((int oldValue, int newValue) {
    print("The counter changed from $oldValue to $newValue");
});

Assign a new value to the NanoVar instance:

counter.value = 1;

The callback declared above is now called with the assigned value and the previous value.

Call unsubscribe to make the NanoVar instance stop calling the callback:

unsubscribe();

Limit to read-only

NanoVar instances can be casted to the type NanoRead, which contains the same functionality for accessing values as NanoVar yet cannot be altered:

final NanoRead<int> readOnlyCounter = counter;

// This line will cause a compilation error.
readOnlyCounter.value = 1;

Functional features

It is possible to modify the behavior of NanoVar instances as they can behave like functors, applicative functors and monads.

Functor

Call map on an existing NanoVar instance with a function that can transform any value held by the NanoVar instance to another value:

final stringCounter = counter.map((int value) {
    return value.toString();
});

The returned NanoRead instance stringCounter is updated whenever a change is made to counter and stringCounter can be used in the same way as counter for the purpose of retrieving values. For instance, stringCounter can be subscribed to:

final unsubscribe = counter.subscribe((String oldValue, String newValue) {
    print("The counter changed from $oldValue to $newValue");
});

Applicative functor

Call liftA2 on an existing NanoVar instance with another NanoVar instance and a function that can transform any value pair held by the NanoVar instances to a third value:

final doubleCounter = NanoVar(0.0);

final stringCounter = counter.liftA2((int firstValue, double secondValue) {
    return (firstValue + secondValue).toString();
}, doubleCounter);

The returned NanoRead instance stringCounter is updated whenever a change is made to counter or doubleCounter.

Monad

Call bind on an existing NanoVar instance with a function that can transform any value held by the NanoVar instance to another NanoVar instance:

final stringCounter = counter.bind((int value) {
    return NanoVar(value.toString());
});

The returned NanoRead instance stringCounter is updated whenever a change is made to counter or the NanoVar instance most recently returned by the callback provided to bind.

Future observing

Suppose this asynchronous function exists:

Future<DetailsModel> loadDetails(int id) async {
    return await callEndpoint("/details/$id");
}

Call loadDetails and then call nanoRead on the returned Future instance to get a NanoRead instance that updates whenever the Future instance is completed:

final futureNanoRead = loadDetails(1).nanoRead;

void getStatus(status) {
    return status.status(
        uncompleted: () {
            return "futureNanoRead has not yet completed";
        },
        success: (value) {
            return "futureNanoRead has completed with the value $value";
        },
        fail: (error, stackTrace) {
            return "futureNanoRead has completed with the error $error " +
                "and the stack trace $stackTrace";
        },
    );
}

// Prints "futureNanoRead has not yet completed".
print(getStatus(futureNanoRead.value));

// Eventually prints "futureNanoRead has completed with the value..."
// if loadCount() succeeds or "futureNanoRead has completed with the error..."
// if loadCount() fails.
final unsubscribe = futureNanoRead.subscribe((oldStatus, newStatus) {
    print(getStatus(newStatus));
});

A possible use case is to combine this feature with bind so that loadDetails can be called each time counter is changed and the current loading status for the most recent value of counter can be managed using a NanoRead instance:

final detailsStatus = counter.bind((int value) {
    return loadDetails(value).nanoRead;
});

Motivation

This library was created to be used in Flutter NanoVar, which provides a lightweight method of state management in Flutter. However, NanoVar can be used on its own as well.