Add a specialized API for SwiftUI fast path access

Author: phausler

stdlib/public/Observation/Sources/Observation/ObservationTracking.swift

@@ -46,7 +46,7 @@ public struct ObservationTracking {
     }
   }
 
-  public static func withTracking<T>(_ apply: () -> T, onChange: @Sendable @escaping () -> Void) -> T {
+  public static func withTracking<T>(_ apply: () -> T, onChange: @autoclosure () -> @Sendable () -> Void) -> T {
     var accessList: AccessList?
     let result = withUnsafeMutablePointer(to: &accessList) { ptr in
       ThreadLocal.value = UnsafeMutableRawPointer(ptr)
@@ -55,6 +55,7 @@ public struct ObservationTracking {
     }
     if let list = accessList {
       let state = ManagedCriticalState([ObjectIdentifier: Int]())
+      let onChange = onChange()
       let values = list.entries.mapValues { $0.addObserver {
         onChange()
         let values = state.withCriticalRegion { $0 }
@@ -66,4 +67,17 @@ public struct ObservationTracking {
     }
     return result
   }
+
+  public static func _installTracking(_ storage: UnsafeRawPointer, onChange: @escaping @Sendable () -> Void) {
+    let list = unsafeBitCast(storage, to: AccessList.self)
+    let state = ManagedCriticalState([ObjectIdentifier: Int]())
+    let values = list.entries.mapValues { $0.addObserver {
+      onChange()
+      let values = state.withCriticalRegion { $0 }
+      for (id, token) in values {
+        list.entries[id]?.removeObserver(token)
+      }
+    }}
+    state.withCriticalRegion { $0 = values }
+  }
 }