Runtime warn for nested observe calls (#2996)

* Runtime warning when we detect nested observe.

* finesse

* wip

* add some tests

---------

Co-authored-by: Brandon Williams <[email protected]>

Sources/ComposableArchitecture/UIKit/NSObject+Observation.swift

@@ -139,6 +139,16 @@
  /// ```
  @discardableResult
  public func observe(_ apply: @escaping () -> Void) -> ObservationToken {
+ if ObserveLocals.isApplying {
+ runtimeWarn(
+ """
+ An "observe" was called from another "observe" closure, which can lead to \
+ over-observation and unintended side effects.
+
+ Avoid nested closures by moving child observation into their own lifecycle methods.
+ """
+ )
+ }
  let token = ObservationToken()
  self.tokens.insert(token)
  @Sendable func onChange() {
@@ -149,7 +159,9 @@
  Task { @MainActor in
  guard !token.isCancelled
  else { return }
- onChange()
+ ObserveLocals.$isApplying.withValue(true) {
+ onChange()
+ }
  }
  }
  }
@@ -191,3 +203,7 @@
  }
  }
 #endif
+
+private enum ObserveLocals {
+ @TaskLocal static var isApplying = false
+}

Tests/ComposableArchitectureTests/ObserveTests.swift

@@ -14,9 +14,13 @@
  model.count += 1
  try await Task.sleep(nanoseconds: 1_000_000)
  XCTAssertEqual(counts, [0, 1])
+
+ model.otherCount += 1
+ try await Task.sleep(nanoseconds: 1_000_000)
+ XCTAssertEqual(counts, [0, 1])
+
  _ = observation
  }
-
  func testCancellation() async throws {
  let model = Model()
  var counts: [Int] = []
@@ -30,10 +34,48 @@
  XCTAssertEqual(counts, [0])
  _ = observation
  }
+
+ @MainActor
+ func testNestedObservation() async throws {
+ XCTExpectFailure {
+ $0.compactDescription == """
+ An "observe" was called from another "observe" closure, which can lead to \
+ over-observation and unintended side effects.
+
+ Avoid nested closures by moving child observation into their own lifecycle methods.
+ """
+ }
+
+ let model = Model()
+ var counts: [Int] = []
+ var innerObservation: Any!
+ let observation = observe { [weak self] in
+ guard let self else { return }
+ counts.append(model.count)
+ innerObservation = observe {
+ _ = model.otherCount
+ }
+ }
+ defer {
+ _ = observation
+ _ = innerObservation
+ }
+
+ XCTAssertEqual(counts, [0])
+
+ model.count += 1
+ try await Task.sleep(nanoseconds: 1_000_000)
+ XCTAssertEqual(counts, [0, 1])
+
+ model.otherCount += 1
+ try await Task.sleep(nanoseconds: 1_000_000)
+ XCTAssertEqual(counts, [0, 1, 1])
+ }
  }
 
  @Perceptible
  class Model {
  var count = 0
+ var otherCount = 0
  }
 #endif