Getting started

The library has a few classes and a set of macros and function overloads:

subject<void(Args …)>

Provides a way to notify subscribed observers when an event occurs.

value<T>

Holds value types and provides a way to notify subscribed observers when the value changes.

OBSERVABLE_PROPERTIES and observable_property

Values nested inside an enclosing class with setters accessible only from inside that class.

observe()

A group of overloaded functions that can create observable values that are updated when their argument expression changes.

Getting started with subjects

Subjects are the simplest observable objects. They provide a way to get notified when some event occurs.

#include <iostream>
#include <string>
#include <observable/observable.hpp>

using namespace std;
using namespace observable;

// Will print "Hello {name}" each time you enter a name.
// Type exit to stop.

int main()
{
    auto name_entered = subject<void(string)> { };

    // Subscribe to name_entered notifications.
    name_entered.subscribe([](auto const & name) {
                               cout << "Hello "s
                                    << name
                                    << "!"s
                                    << endl;
                           });

    while(cin)
    {
        cout << "What's your name?"s << endl;

        // Read the name.
        auto input_name = string { };
        cin >> input_name;

        if(input_name.empty() || input_name == "exit"s)
            break;

        // Notify all name_entered observers.
        name_entered.notify(input_name);
    }

    return 0;
}

Getting started with values

Values are wrappers around value-types. You can subscribe to changes to the contained value-type.

The observe() functions can be used to generate values associated with an expression. For example: auto result = observe((a + b) / 2.0). The result value will recompute the average, each time either the a or b value changes.

#include <iostream>
#include <string>
#include <observable/observable.hpp>

using namespace std;
using namespace observable;

// Will print "Hello {name}" each time you enter a name.
// Type exit to stop.

int main()
{
    auto name = value<string> { };

    // Recompute the greeting each time name changes and
    // fire change notifications.
    auto greeting = observe("Hello "s + name + "!"s);

    // Subscribe to greeting changes.
    greeting.subscribe([](auto const & hello) {
                           cout << hello << endl;
                       });

    while(cin)
    {
        cout << "What's your name?"s << endl;

        // Read the name.
        auto input_name = string { };
        cin >> input_name;

        if(input_name.empty() || input_name == "exit"s)
            break;

        // Update the name value.
        name = input_name;
    }

    return 0;
}

Getting started with observable properties

Properties are just values that are nested inside a class. This makes all setters inaccessible from outside the enclosing class.

#include <iostream>
#include <string>
#include <observable/observable.hpp>

using namespace std;
using namespace observable;

// Will print "Hello {name}" each time you enter a name.
// Type exit to stop.

// Greet people using names read from stdin.
class Greeter
{
    OBSERVABLE_PROPERTIES(Greeter)

public:
    // Current name.
    observable_property<string> name;

    // Current greeting.
    observable_property<string> greeting = observe(
                                            "Hello "s + name + "!"s
                                           );

public:
    // Read names from stdin until the user quits.
    void read_names()
    {
        while(cin)
        {
            cout << "What's your name?"s << endl;

            auto input_name = string { };
            cin >> input_name;

            if(input_name.empty() || input_name == "exit"s)
                break;

            name = input_name;
        }
    }
};

int main()
{
    Greeter greeter;

    // Print the greetings.
    greeter.greeting.subscribe([](auto const & hello) {
                                   cout << hello << endl;
                               });

    // Properties cannot be set from outside the class. The
    // line below will not compile:
    // greeter.name = input_name;

    greeter.read_names();
    return 0;
}