A .NET library that detects when an expression changes its value
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SmartReactives Join the chat at https://gitter.im/keyboardDrummer/SmartReactives Build status

SmartReactives is an extension to Rx.NET that detects when an expression changes its value and exposes these changes as an IObservable. Detecting changes in expressions is a common problem, related to user interfaces, caching, validation, and much more.

This example demonstrates the basic functionality:

var input = Reactive.Variable(1);
var inputSquared = Reactive.Expression(() => input * input);
inputSquared.Subscribe(getSquare => Console.WriteLine("square = " + getSquare())); //Prints 'square = 1'

input.Value = 2; //Prints 'square = 4'
input.Value = 3; //Prints 'square = 9'

Reactive.Expression returns a ReactiveExpression<T> which implements IObservable<Func<T>> so we can subscribe to it. The function getSquare is simply a shortcut to ReactiveExpression<T>.Evaluate, which evaluates the function you created the reactive expression with, namely () => input * input.

To start using SmartReactives simply add the NuGet package SmartReactives to your project. Also add SmartReactives.PostSharp if you're using PostSharp. Documentation can be found on the Wiki.

If you're looking for something like SmartReactives but outside of .NET then take a look at these projects:


This section demonstrates the functionality of SmartReactives by showing a number of examples.


This example shows how to use Reactive.Cache to get a cache which automatically clears itself when it becomes stale.

var input = Reactive.Variable(2); //We define a reactive variable.
Func<int> f = () => //f is the calculation we want to cache.
    Console.WriteLine("cache miss");
    return input * input; //f depends on our reactive variable 'input'.
var cache = Reactive.Cache(f); //We base our cache on f.

Assert.AreEqual(4, cache.Get()); //Prints 'cache miss'
Assert.AreEqual(4, cache.Get()); //Cache hit.

input.Value = 3; //Cache becomes stale.

Assert.AreEqual(9, cache.Get()); //Prints 'cache miss'
Assert.AreEqual(9, cache.Get()); //Cache hit.


This example shows off how to make an IList<T> reactive by calling ToReactive on it. The reactive list is precise: if you access an index you will only get an update if that particular index changes.

var reactiveList = new List<int> { 0, 1, 2 }.ToReactive();
var elementAtIndex1 = Reactive.Expression(() => reactiveList[1]);

//Prints 'item at index 2 changed to 1'
elementAtIndex1.Subscribe(getValue => Console.WriteLine("item at index 1 changed to " + getValue())); 

reactiveList[1] = 3; //Prints 'item at index 1 changed to 3'
reactiveList[2] = 4; //Prints nothing
reactiveList.Add(5); //Prints nothing
reactiveList.Insert(0, 6); //Prints 'item at index 1 changed to 0'

Next to IList<T> there are also reactive collections for ISet<T> and IDictionary<T,U>.


This example shows that a ReactiveExpression can refer to other ReactiveExpressions. In this way you can build arbitrary graphs of reactive objects. The example demonstrates a graph in the shape of a diamond.

var input = Reactive.Variable(1);
var timesTwo = Reactive.Expression(() => input * 2);
var timesThree = Reactive.Expression(() => input * 3);
var sumOfBoth = Reactive.Expression(() => timesTwo.Evaluate() + timesThree.Evaluate());
sumOfBoth.Subscribe(getValue => Console.WriteLine("sumOfBoth = " + getValue())); //Prints 'sumOfBoth = 5'
input.Value = 2; //Prints 'sumOfBoth = 10'
input.Value = 3; //Prints 'sumOfBoth = 15'

Note that although the input has two paths in the graph to sumOfBoth, there is only one notification for sumOfBoth when input changes. SmartReactives makes sure to notify an expression only once when its values changes.


In the following example, the expression leftOrRight only depends on variable right when variable left is false, since we are using the lazy or operator ||. If we change right while left is false, then we don't get any updates from leftOrRight. In general, SmartReactives won't give you any updates for old dependencies or possible future dependencies.

var left = Reactive.Variable(false);
var right = Reactive.Variable(false);
var leftOrRight = Reactive.Expression(() => left || right);
leftOrRight.Subscribe(getValue => Console.WriteLine("leftOrRight = " + getValue())); //Prints 'leftOrRight = False'

right.Value = true; //Prints 'leftOrRight = True'
left.Value = true; //Prints 'leftOrRight = True'
right.Value = false; //Prints nothing

Note that if we only want to get updates if leftOrRight changes then we can use leftOrRight.DistinctUntilChanged().Subscribe(...).


This examples demonstrates two methods to implement a reactive property. The first method uses the class ReactiveVariable that we already know as a backing field for our reactive property. The second method applies ReactiveVariableAttribute to the property, which in combination with PostSharp does all the work.

class ReactiveProperties
    readonly ReactiveVariable<int> usingABackingField = Reactive.Variable(1);
    int UsingABackingField
        get { return usingABackingField.Value; }
        set { usingABackingField.Value = value; }

    int UsingAnAttributeAndPostSharp { get; set; } = 1;

    public void Test()
        var product = Reactive.Expression(() => UsingABackingField * UsingAnAttributeAndPostSharp);
        product.Subscribe(getProduct => Console.WriteLine("product = " + getProduct())); //Prints 'product = 1'
        UsingAnAttributeAndPostSharp = 2; //Prints 'product = 2'
        UsingABackingField = 2; //Prints 'product = 4'


The following example demonstrates using ReactiveVariableAttribute and ReactiveCacheAttribute to effortlessly setup a cache.

class CachingCalculator
    public int Input { get; set; }

    public double Square
            Console.WriteLine("cache miss");
            return Math.Pow(Input, 2);

    public static void ReactiveCache()
        var calculator = new CachingCalculator();
        calculator.Input = 2;

        Assert.AreEqual(4, calculator.Square); //Cache miss. Prints 'cache miss'
        Assert.AreEqual(4, calculator.Square); //Cache hit.

        calculator.Input = 3; //Cache becomes stale.

        Assert.AreEqual(9, calculator.Square); //Cache miss. Prints 'cache miss'
        Assert.AreEqual(9, calculator.Square); //Cache hit.


Implementing PropertyChanged for a property is a known cause for boilerplate. PostSharp allows you to remove this boilerplate using its attribute NotifyPropertyChanged. However, sometimes a property A depends on another property B. In this case we would like both properties to call PropertyChanged when B changes. The PostSharp attribute NotifyPropertyChanged won't do this, but SmartNotifyPropertyChanged will, as shown in the following example.

class Calculator : HasNotifyPropertyChanged
    public int Number { get; set; }

    public int SquareOfNumber => Number * Number;

    public static void SquareDependsOnNumber()
        var calculator = new Calculator();
        calculator.Number = 1;

        Console.WriteLine("square = " + calculator.SquareOfNumber); //Prints 'square = 1'
        calculator.PropertyChanged += (sender, eventArgs) =>
            if (eventArgs.PropertyName == nameof(SquareOfNumber))
                Console.WriteLine("square = " + calculator.SquareOfNumber);

        calculator.Number = 2; //Prints 'square = 4'
        calculator.Number = 3; //Prints 'square = 9'