FetchSubscription.swift

import ConcurrencyExtras
import Sharing

/// A subscription associated with `@FetchAll`, `@FetchOne`, and `@Fetch` observation.
///
/// This value can be useful in associating the lifetime of observing a query to the lifetime of a
/// SwiftUI view _via_ the `task` view modifier. For example, loading a query in a view's `task`
/// will automatically cancel the observation when drilling down into a child view, and restart
/// observation when popping back to the view:
///
/// ```swift
/// .task {
///   try? await $reminders.load(Reminder.all).task
/// }
/// ```
public struct FetchSubscription: Sendable {
  let cancellable = LockIsolated<Task<Void, any Error>?>(nil)
  let onCancel: @Sendable () -> Void

  init<Value>(sharedReader: SharedReader<Value>) {
    onCancel = { sharedReader.projectedValue = SharedReader(value: sharedReader.wrappedValue) }
  }

  /// An async handle to the given fetch observation.
  ///
  /// This handle will suspend until the current task is cancelled, at which point it will terminate
  /// the observation of the associated ``FetchAll``, ``FetchOne``, or ``Fetch``.
  public var task: Void {
    get async throws {
      let task = Task {
        try await withTaskCancellationHandler {
          try await Task.never()
        } onCancel: {
          onCancel()
        }
      }
      cancellable.withValue { $0 = task }
      try await task.cancellableValue
    }
  }

  /// Cancels the database observation of the associated ``FetchAll``, ``FetchOne``, or ``Fetch``.
  public func cancel() {
    cancellable.value?.cancel()
  }
}