Reactive programming

In computing, reactive programming is a declarative programming paradigm concerned with data streams and the propagation of change. With this paradigm, it is possible to express static (e.g., arrays) or dynamic (e.g., event emitters) data streams with ease, and also communicate that an inferred dependency within the associated execution model exists, which facilitates the automatic propagation of the changed data flow.

For example, in imperative programming, a := b + c would mean that a is being assigned the result of b + c at the time the expression is evaluated, and later, the values of b and c can be changed with no effect on the value of a. In contrast, in reactive programming, the value of a is automatically updated whenever the values of b or c change, without the program having to explicitly re-state the statement a := b + c to re-assign the value of a.

int b = 1;
int c = 2;
int a = b + c;
b = 10;
System.out.println(a); // 3 (not 12 because "=" is not a reactive assignment operator)

// now imagine a special operator "$=" exists that changes the value of a variable (executes code on the right side of the operator and assigns result to left side variable) whenever explicitly initialized, and when referenced variables (on the right side of the operator) are changed
int b = 1;
int c = 2;
int a $= b + c;
b = 10;
System.out.println(a); // 12

Another example is a hardware description language such as Verilog, where reactive programming enables changes to be modeled as they propagate through circuits.

Reactive programming has been proposed as a way to simplify creating interactive user interfaces and near-real-time computing animation.

For example, in a model–view–controller (MVC) architecture, reactive programming can facilitate changes in an underlying model being reflected automatically in an associated view.