Perception

Swift's Observation tools, back-ported to more platforms.

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This library was created by Brandon Williams and Stephen Celis, who host the Point-Free video series which explores advanced Swift language concepts.

Overview

The Perception library back-ports @Observable, withObservationTracking, and Observations all the way back to iOS 13, macOS 10.15, tvOS 13 and watchOS 6. This means you can take advantage of all of Swift's observation tools today, even if you can't drop support for older Apple platforms. Using this library's tools works almost exactly as using the official tools, with one small exception.

To begin, mark a class as being observable by using the @Perceptible macro instead of the @Observable macro:

@Perceptible
class FeatureModel {
  var count = 0
}

Then you can hold onto a perceptible model in your view using a regular let property:

struct FeatureView: View {
  let model: FeatureModel

  // ...
}

And in the view's body you must wrap your content using the WithPerceptionTracking view in order for observation to be correctly hooked up:

struct FeatureView: View {
  let model: FeatureModel

  var body: some View {
    WithPerceptionTracking {
      Form {
        Text("\(model.count)")
        Button("Increment") { model.count += 1 }
      }
    }
  }
}

It's unfortunate to have to wrap your view's content in WithPerceptionTracking, but if you forget then you will helpfully get a runtime warning letting you know that observation is not set up correctly:

🟣 Runtime Warning: Perceptible state was accessed from a view but is not being tracked.

Finally, the Observations async sequence has been back-ported as Perceptions, which can be used to observe changes to perceptible and observable objects over time:

let counts = Perceptions { model.count }
for await count in counts {
  print("Count changed: \(count)")
}

Bindable

SwiftUI's @Bindable property wrapper has also been back-ported to support perceptible objects. You can simply qualify the property wrapper with the Perception module:

struct FeatureView: View {
  @Perception.Bindable var model: FeatureModel

  // ...
}

Environment

SwiftUI's @Environment property wrapper and environment view modifier's support for observation has also been back-ported to support perceptible objects using the exact same APIs:

struct FeatureView: View {
  @Environment(Settings.self) var settings

  // ...
}

// In some parent view:
.environment(settings)

Community

If you want to discuss this library or have a question about how to use it to solve a particular problem, there are a number of places you can discuss with fellow Point-Free enthusiasts:

• For long-form discussions, we recommend the discussions tab of this repo.
• For casual chat, we recommend the Point-Free Community Slack.

Documentation

The latest documentation for the Perception APIs is available here.

License

This library is released under the MIT license. See LICENSE for details.