ProperyType

The previous sections introduced the abstractions that RAC offers to describe signal. This section will introduce other collateral but usefull types.

PropertyType is a protocol that, when applied to a property, allows the observation of its changes. Its definition is as follows:

public protocol PropertyType {
    typealias Value

    var value: Value { get }
    var producer: SignalProducer<Value, NoError> { get }
}

The semantics of this protocol is neat:

• the getter return the current value of the property
• the producer return a producer for Signals that will send the property's current value followed by all changes over time

Starting from this protocol, RAC introduces:

• ConstantProperty, that represents a property that never change
• MutableProperty, that represents a mutable property
• PropertyOf, that represents a read-only view to a property

These types are really usefull when used in combination with the <~ operator, that binds properties together. The bind operator comes in three flavors:

/// Binds a signal to a property, updating the property's value to the latest
/// value sent by the signal.
public func <~ <P: MutablePropertyType>(property: P, signal: Signal<P.Value, NoError>) -> Disposable {}

/// Creates a signal from the given producer, which will be immediately bound to
/// the given property, updating the property's value to the latest value sent
/// by the signal.
public func <~ <P: MutablePropertyType>(property: P, producer: SignalProducer<P.Value, NoError>) -> Disposable { }

/// Binds `destinationProperty` to the latest values of `sourceProperty`.
public func <~ <Destination: MutablePropertyType, Source: PropertyType where Source.Value == Destination.Value>(destinationProperty: Destination, sourceProperty: Source) -> Disposable { }

What these operators do is to create the wires that link each property to each others, in a declarative manner. Each property is observable, through its inner SignalProducer.