swift-mirror/stdlib/stdlib.docc/Observation/Observation.withobservationtracking.apply.onchange.md

Observation/withObservationTracking(_:onChange:)

@Metadata { @DocumentationExtension(mergeBehavior: override) }

Tracks access to properties.

This method tracks access to any property within the apply closure, and informs the caller of value changes made to participating properties by way of the onChange closure. For example, the following code tracks changes to the name of cars, but it doesn't track changes to any other property of Car:

func render() {
    withObservationTracking {
        for car in cars {
            print(car.name)
        }
    } onChange: {
        print("Schedule renderer.")
    }
}

• Parameters:

  • apply: A closure that contains properties to track.
  • onChange: The closure invoked when the value of a property changes.

• Returns: The value that the apply closure returns if it has a return value; otherwise, there is no return value.