Header only reactive C++ library. Thread-safe, memory-safe. Concsists from:

• Event (C# like Event, but thread-safe, does not block while event occurs, subscribtion/unsbuscription possible right from the observer )
• ObservableProperty
• ReactiveProperty

Helpers observe and bind.

There is also non thread safe version. And you can mix them safely.

Througthput. 400'000 reactive updates took 60ms on my Intel i7. Should be more than enough to build your UI model/view interaction. See test/BenchmarkReactivity.h to know your mileage.

Usage

Add src folder to your compiler's INCLUDE path.

#include <reactive/ObservableProperty.h>
#include <reactive/ReactiveProperty.h>
#include <reactive/bind.h>

using namespace reactive;

ObservableProperty<int> x = 1;
ObservableProperty<int> y = 2;

ReactiveProperty<int> sum;
sum.set([](int x, int y){ return x+y; }, x, y);

sum += [](int sum){
    cout << "new sum is " << sum << endl;
};

struct MyWidget{
    void setText(const std::string& msg){
        cout << msg << endl;
    }
};
auto widget = std::make_shared<MyWidget>();     // need shared_ptr to keep track about widget aliveness in multithreaded environment

// non-intrusive bind
bind(widget, [](auto& widget, int sum, int x, int y){
    widget->setText(std::to_string(sum) + " = " + std::to_string(x) + " + " + std::to_string(y));
}, sum, x, y);

x = 10;
widget.reset();     // safe to kill, bind will take care about auto unsubscribing
y = 20;

Event

Event is basis of reactivity. It let us know that something changed.

#include <reactive/Event.h>
using namespace reactive;

Event<int, int> mouseMove;

mouseMove += [](int x, int y){
    std::cout << "mouse position " << x << ":" << y << std::endl;
};

Delegate<int, int> delegate;
delegate = [&](int x, int y){    
    if (y == 100){        
        mouseMove -= delegate;        // it is possible to unsubscribe/subscrive right from the event;
    }
    std::cout << "delegate can be unsubscribed. Position " << x << ":" << y << std::endl;
};
mouseMove += delegate;      // delegate's function copied to event queue
mouseMove -= delegate;


// Delegate is shortcut for this:
DelegateTag tag;
mouseMove.subscribe(tag, [&, tag](int x, int y){
    if (x == 100){        
        mouseMove -= tag;   // You can store tag ( delegate.tag() ) for latter unsubscription
    }
    std::cout << "Event can be unsubscribed by Delegate tag too. Position " << x << ":" << y << std::endl;
});
mouseMove -= tag;

// call event
mouseMove(10,6);

reactive/Event.h Synopsis:

void operator()()
void operator+=(Closure&&)
void operator+=(const Delegate&)
void operator-=(const Delegate&)
void operator-=(const DelegateTag&)
void subscribe(const DelegateTag&, Closure&&)

Implementation details:

Event use "deferred" container (see details/utils/DeferredForwardContainer.h), erase/emplace queued in separate std::vector, and applied before foreach(). Thus, foreach() have minimal interference with container modification.

• subscription/unsubscription occurs before next event() call.
• Subscription/unsubscription never blocked by event call();
• event call() does not block another event call(), if there is no subscription's/unsubscription's from previous call. Otherwise block till changes to event queue applied.
• Event queue is unordered.

Delegate

struct Delegate{
    std::function
    DelegateTag m_tag

    auto& tag() { return m_tag; }
}

DelegateTag

struct DelegateTag{
    unsigned log long int uuid;
}

ObservableProperty

ObservableProperty = Value + Event

using namespace reactive;

struct Vec2{
    int x,y;
    Vec2() nothrow {}
    Vec2(int x, int y)
        :x(x)
        ,y(y)
    {}
}

ObservableProperty<Vec2> vec {10, 20};

vec += [](const Vec2& vec){
    std::cout << "new vec " << vec.x << ", " << vec.y << std::endl;
};

vec = {3,4};
// Output: new vec 3,4

{
    auto vec_ptr = vec.write_lock();
    vec_ptr->x = 2;
    vec_ptr->y = 5;
}
// Output: new vec 2,5

{
    auto vec_ptr = vec.lock();
    std::cout << "current vec " << vec_ptr->x << ", " << vec_ptr->y << std::endl;
}
// Output: current vec 2,5

std::thread t([](vec_weak = vec.weak_ptr()){
    ObservableProperty<Vec2> property(vec_weak);
    if (!property) return;

    Vec2 vec = property.getCopy();
    std::cout << "current vec " << vec.x << ", " << vec.y << std::endl;
});

If possible, on set, new value compares with previous, and event triggers only if values are not the same.

reactive/ObservableProperty.h Synopsis

constructors
ObservableProperty(Args&&...) in place object construct
ObservableProperty(const ObservableProperty&) will copy only value
ObservableProperty(ObservableProperty&&)
ObservableProperty(const WeakPtr&) may be invalid after construction. Check with bool()
ObservableProperty(const SharedPtr&)
pointer
WeakPtr weak_ptr() const
const SharedPtr& shared_ptr() const
operator bool() const
event manipulation
void operator+=(Closure&&) const
void operator+=(const Delegate&) const
void operator-=(const Delegate&) const
void operator-=(const DelegateTag&) const
void subscribe(const DelegateTag&, Closure&&) const
void pulse() const
accessors
ReadLock lock() const
T getCopy() const
mutators
WriteLock write_lock()
void operator=(const T& value)
void operator=(T&& value)
ObservableProperty& operator=(const ObservableProperty&) will copy only value

---

lock()/write_lock() lock object(with mutex, if applicable, see below), and provides pointer like object access. WriteLock will trigger event on destruction (aka update).

---

ReadLock/WriteLock Synopsis (ReadLock with const modifier):
T& get()
operator T&()
T* operator->()
T& operator*()
void unlock() unlocks underlying mutex(if applicable) and call event loop with new value(for WriteLock)
void silent(bool be_silent = true) does not call event on WriteLock destruction

ObservableProperty may be configured with additional parameter ObservableProperty<T, blocking_mode>.

Where blocking_mode can be:

• default_blocking (by default).

  if (T is trivially copyable && size <= 128)  nonblocking_atomic
  if (T is copyable && size <= 128) nonblocking
  else blocking

• blocking use upgrade_mutex. ReadLock use shared_lock. WriteLock use unique_lock. On setting new value, mutex locks with shared_lock, event called with value reference.
• nonblocking use SpinLock. ReadLock copy value, does not use lock. WriteLock use unique_lock. On setting new value, event called with value copy (no locks).
• nonblocking_atomic use std::atomic<T>. ReadLock copy value, does not use lock. WriteLock work with value copy, then atomically update property's value with it. On setting new value, event called with value copy (no locks).

All in all, blocking never copy value, but lock internal mutex each time when you work with it. For small objects it is faster to copy, than lock, that's why blocking not used as default.

Thoeretically, hardware supported std::atomic with nonblocking_atomic should be the fastest. Keep in mind, that mostly, atomics are lockless for sizeof(T) <= 8.

Most of the time you will be happy with default. But, for containers, blocking mode preferable:

ObservableProperty< std::vector<int>, blocking >

Because all other modes, will make temporary copy of the vector.

Implementation details:

ObservableProperty internally holds shared_ptr. This needed to track alivness(and postpone destruction) in multithreaded environment.

Observable property consists from value and event. Event internally holds queue of observers, in heap allocated memory (std::vector) anyway. So shared_ptr construction overhead is not that big.

template<class T> struct ObservableProperty{
    struct Data{
        T value;
        Event<const T&> event;        
    };
    std::shared_ptr<Data> data;
}

ReactiveProperty

Same as ObservableProperty + Can listen multiple ObservableProperties/ReactiveProperties and update value reactively.

using namespace reactive;

ObservableProperty<int> x = 1;
ObservableProperty<int> y = 3;

struct Vec2{
    int x,y;
    Vec2(int x, int y) :x(x) ,y(y){}
}
ReactiveProperty<Vec2> vec2 {0, 0};

vec2.set([](int x, int y){ return Vec2{x*x, y*y}; }, x, y);

vec2 += [](const Vec2& vec2){
    cout << "vec2 is " << vec2.x << ", " << vec2.y << endl;
};

x = 10;
// Output: vec2 is 100, 9

y = 2;
// Output: vec2 is 100, 4

vec2.update([](Vec2& vec2, int x, int y){ vec2.y = x+y; }, x, y);    // unsubscribe, and modify value
// Output: vec2 is 100, 12

x = 12;
// Output: vec2 is 100, 14

vec2 = {3,4};                       // unsubscribe, set value
// Output: vec2 is 3, 4

x = -2; // will not triger any changes in vec2

Synopsis

same as ObservableProperty, except all mutators, first unsubscrbe previous listeners.

set<blocking_mode = default_blockign>(Closure&& closure, ObservableProperty/ReactiveProperty&...)
observe properties, and call closure with values of properties (const Ts&...), result of closure set as current ReactiveProperty value. See observe.

template<class T>
struct ReactiveProperty{
   T value;
   void set(Closure&& closure, Properties&&... properties){
       unsubscribe_previous();

       observe([closure, &value]( auto& ... values){
           value = closure(values...);
       }, properties);
   }
}

update<blocking_mode = default_blockign>(Closure&&, ObservableProperty/ReactiveProperty&...)
observe properties, and call closure with first parameter as current value reference, and other as values of properties (const Ts&...). See observe.

template<class T>
struct ReactiveProperty{
    T value;
    void update(Closure&& closure, Properties&&... properties){
        unsubscribe_previous();

        observe([closure, &value]( auto& ... values){
            closure(value, values...);
        }, properties);
    }
}

void operator=(const ObservableProperty/ReactiveProperty& property) listen for property changes, and update self value with new one.

Observe

Allow observe multiple properties.

using namespace reactive;

ObservableProperty<int> x = 1;
ObservableProperty<int> y = 2;

auto unsubscribe = observe([](int x, int y){
    std::cout << x << ", " << y << std::endl;
}, x, y);

y = 4;
// Output: 1, 4

x = 5;
// Output: 5, 4

unsubscribe();
x = 8;
// trigger nothing


observe_w_unsubscribe([](auto unsubscribe, int x, int y){
    if (x == 100){
        unsubscribe();
        return;
    }
    
    std::cout << x << ", " << y << std::endl;
}, x, y);

x = 6;
// Output: 6, 4

x = 100;
y = 200;
// unsubscribed, trigger nothing

observe/observe_w_unsubscribe have optional blocking_mode template parameter:

template<class blocking_mode = default_blocking, class Closure, class ...Observables>
auto observe(Closure&&, Observables&...)

• If blocking_mode == blocking, closure called with observables.lock()... If someone of observables dies, observe auto-unsubscribes.
• Otherwise, values stored in local tuple, and each time observables changes, tuple updates. Closure called with copy of that tuple. Thus, ommiting potential mutex lock on observables.lock()... If someone of observables dies, closure will be called with last known value of dead observable. Thus, it stop listen only when all observables dies.

default_blocking will try to use non-blocking mode when possible.

Bind

bind designed for non-intrusive binding ObservableProperties/ReactivePropeties to non-aware class. Class must be in std::shared_ptr. bind take care of observables and object lifetimes.

using namespace reactive;

ObservableProperty<int> len = 2;
ObservableProperty<int> y{ 100 };

class Box {
    int m_len = 0;
public:

    auto len(int x) {
        m_len = x;
    }

    void show() {
        std::cout << m_len << std::endl;
    }
};

std::shared_ptr<Box> box = std::make_shared<Box>();

len = 40;

bind(box, [](auto box, int len) {
    box->len(len);
    box->show();
}, len);

bind_w_unsubscribe(box, [](auto unsubscibe, auto box, int len) {
    if (len > 100) unsubscibe();
    box->len(len);
    box->show();
}, len);

len = 50;
len = 101;
len = 60;

bind Stores object's weak_ptr, listen for observables. If object dies, unsubscribe self.
bind_w_unsubscribe do the same - but you can manually unsubscribe.

Synopsis

template<class blocking_mode = default_blocking, class Obj, class Closure, class ...Observables>
auto bind(const std::shared_ptr<Obj>& obj, Closure&& closure, const Observables&... observables)
// return unsubscriber

template<class blocking_mode = default_blocking, class Obj, class Closure, class ...Observables>
auto bind_w_unsubscribe(const std::shared_ptr<Obj>& obj, Closure&& closure, const Observables&... observables)
// return unsubscriber

non_thread_safe

Non thread safe version lies in reactive/non_thread_safe namespace and folder. The only difference, apart being not thread safe, is existance of operator->() and operator*(), which allow value access without lock()/getCopy()

You can mix thread-safe with non-thread-safe version:

#include <reactive/ObservableProperty>
#include <reactive/non_thread_safe/ObservableProperty>
#include <reactive/ReactiveProperty>

using namespace reactive;

ObservableProperty<int> i1{1};


struct MyWidget{    
    non_thread_safe::ObservableProperty<int> i2{2};

    vodi show(){
        std::cout << *i2 << std::endl;  // operator*() exists in non_thread_safe version
    }
};
MyWidget widget;

ReactiveProperty<int> sum;

// nonblocking stores copy of values when their event triggers, 
// thus it is safe to mix threaded and non-threaded properties in this mode (value must be copyable)
// and only in nonblocking mode
sum.set<nonblocking>([&](int i1, int i2){   // you may ommit <nonblocking>, it will be set by default for this case
    return i1 + i2;
}, i1, i2);

See test/BenchmarkReactivity.h for performance comparsion. Huge (10-20 times) difference in gcc 6.3 compiled version, and almost the same speed in all versions under VS2017.

---

Compiler support

Library should compiles with any standard compatible c++14/17 compiler.
Tested with clang, gcc 6.3, vs 2017 c++

VS2015-2017 , GCC < 7

Objects in ObservableProperty, ReactiveProperty must be, also, no-throw default constructable in order to work in nonblocking_atomic mode https://developercommunity.visualstudio.com/content/problem/69560/stdatomic-load-does-not-work-with-non-default-cons.html

in default_blocking mode, trivially constructable objects, but without no-throw default constructor, will work in nonblocking mode.