ObservedObject

Model your application code as a function of state change.

Overview

The ObservedObject package provides a convenient mechanism to observe objects as they change that is compatible with Combine pipelines. The API is very similar to ObservableObject with the key difference being subscribers are notified after the upstream object has changed, rather than before. This enables modelling pipelines as a function of an object update using its new state, in place of waiting for the values to be provided later.

The Observed property wrapper provides a similar convenience wrapper to the Published property wrapper used by ObservableObject. Annotating specific properties of an ObservedObject with this wrapper will automatically notify any subscribers after the property value has been set on the object:

class Weather {
    @Observed var temperature: Double
    init(temperature: Double) {
        self.temperature = temperature
    }
}
let weather = Weather(temperature: 20)
cancellable = weather.$temperature
    .sink() {
        print ("Temperature now: \($0)")
}

weather.temperature = 25

// Prints:
// Temperature now: 20.0
// Temperature now: 25.0

Notes

This package is intended for applications that use Combine pipelines across wider system boundaries without the desire to pollute contracts with AnyPublisher. For example, in response to the completion of a CloudKit fetch operation a local database is modified and saved. The model objects are updated by reading the contents of the database, however are typed to a protocol between module boundaries.