Signals

Set-able Signals

There are two types of signals. Top level signals can be assigned values. Here we create one called counter which we initially a assign a value of 4

counter = new Signal(4);

this can be changed to a five etc using Set.

counter.Set(5);

and read back using Get

counter.Get(); // prints 5

Computed Signals

The second type of signals are computed ones, these are based on other signals

isEven = SignalBuilder.DependsOn(counter).ComputedBy(counterSignal => counterSignal.Get() % 2 == 0);

isEven.Get(); // prints false as counter is 5

counter.Set(6);

isEven.Get(); // prints true as counter is 6

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The proposed Javascript version looks a bit nicer, but it's harder to work out what this signal depends on.

var isEven = SignalBuilder.ComputedBy(() => counter.Get() % 2 == 0);

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Computed signals can also be based on other computed signals

parity = SignalBuilder.DependsOn(isEven).ComputedBy(isEvenSignal => isEvenSignal.Get() ? "Even" : "Odd");

parity.Get(); // prints Even as isEven is true

counter.Set(9);

parity.Get(); // prints odd as isEven is now false

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or on multiple signals

firstName = new Signal("David");

surname = new Signal("Betteridge");

fullname = SignalBuilder.DependsOn(firstName, surname).ComputedBy((f,l) => $"{f.Get()} {l.Get()}");

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Effects can be added to signals which are automatically triggered when a value changes

counter.AddEffect((previous, current) => Console.WriteLine($"Counter changed {previous} to {current}"));

isEven.AddEffect((previous, current) => Console.WriteLine($"IsEven changed {previous} to {current}"));

parity.AddEffect((previous, current) => Console.WriteLine($"Parity changed {previous} to {current}"));

counter.Set(10);
// Counter changed 9 to 10
// IsEven changed False to True
// Parity changed Odd to Even

counter.Set(12);
// Counter changed 10 to 12

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Lazy

In the previous example not only aren't the other two effects triggered, but parity isn't even computed.

parity = SignalBuilder.DependsOn(isEven).ComputedBy(v =>
{
    Console.WriteLine("Compute parity");
    return v.Get() ? "Even" : "Odd";
});

// This changes the value for isEven so parity is computed
counter.Set(13);
parity.Get();

// This doesn't change the value for isEven so parity isn't computed
counter.Set(15);
parity.Get();

Challenge

Attempt to implement the Signals API in a language of your choosing.

1. The solution should only trigger effects if the data has really changed.
2. Signals should only be evaluated if needed.
3. Try to provide a nice API for your users.

Extension

So far we have only looked at a signals based on a primitive type (integer). In the real world we need to cater for arrays/lists of classes. For example:

record People(string Name, int Age);

var data = new Signal<List<People>>([new People("David", 48)]); 

var adults = SignalBuilder.DependsOn(data)
                          .ComputedBy(value => value.Get().Where(p => p.Age > 18).ToList());

var numberOfAdults = SignalBuilder.DependsOn(adults)
                                  .ComputedBy(value =>
                                  {
                                      Console.WriteLine("Counting adults");
                                      return value.Get().Count;
                                  });

numberOfAdults.AddEffect((oldCount, newCount) => 
    Console.WriteLine($"Number of adults changed from {oldCount} to {newCount}"));

The numberOfAdults signal only needs to be computed if adults changes.

Hint: In order to get this to work, you many have to supply your own Equality function.