Add notify API and add test case for ViewGraphGeometryObservers (#699)

Author: Kyle-Ye

Sources/OpenSwiftUICore/View/Graph/ViewGraphGeometryObservers.swift

@@ -77,6 +77,22 @@ package struct ViewGraphGeometryObservers<Measurer> where Measurer: ViewGraphGeo
         return result
     }
 
+    // [?]
+    mutating func notify() {
+        let keys = store.keys
+        for proposal in keys {
+            guard var observer = store[proposal] else { continue }
+            guard case let .pending(oldSize, pending: newSize) = observer.storage else {
+                continue
+            }
+            if oldSize != newSize {
+                observer.callback(oldSize, newSize)
+            }
+            observer.storage = .value(newSize)
+            store[proposal] = observer
+        }
+    }
+
     /// Adds an observer for a specific layout proposal.
     ///
     /// - Parameters:
@@ -202,7 +218,7 @@ package struct ViewGraphGeometryObservers<Measurer> where Measurer: ViewGraphGeo
                     return true
                 case let .pending(value, _):
                     guard size != value else {
-                        self = .value(size)
+                        self = .value(size) // ?
                         return false
                     }
                     self = .pending(value, pending: size)

Tests/OpenSwiftUICoreTests/View/Graph/ViewGraphGeometryObserversTests.swift

@@ -10,12 +10,14 @@ private struct TestMeasurer: ViewGraphGeometryMeasurer {
     typealias Proposal = CGSize
     typealias Size = CGFloat
 
+    static var mockValue: CGFloat?
+
     static func measure(given proposal: CGSize, in graph: ViewGraph) -> CGFloat {
-        max(proposal.width, proposal.height)
+        mockValue ?? max(proposal.width, proposal.height)
     }
 
     static func measure(proposal: CGSize, layoutComputer: LayoutComputer, insets: EdgeInsets) -> CGFloat {
-        max(proposal.width, proposal.height)
+        mockValue ?? max(proposal.width, proposal.height)
     }
 
     static var invalidValue: CGFloat = .nan
@@ -28,14 +30,19 @@ struct ViewGraphGeometryObserversTests {
     @MainActor
     @Test
     func observeCallback() async throws {
-        // TODO: when the callback got called.
-        await confirmation(expectedCount: 0) { confirm in
+        await confirmation(expectedCount: 1) { confirm in
             var observers = Observers()
-            observers.addObserver(for: CGSize(width: 10, height: 20)) { _, _ in
+            observers.addObserver(for: CGSize(width: 10, height: 20)) { oldSize, newSize in
                 confirm()
+                #expect(oldSize.isApproximatelyEqual(to: 20.0))
+                #expect(newSize.isApproximatelyEqual(to: 30.0))
             }
             let emptyViewGraph = ViewGraph(rootViewType: EmptyView.self)
             _ = observers.needsUpdate(graph: emptyViewGraph)
+            TestMeasurer.mockValue = 30.0
+            defer { TestMeasurer.mockValue = nil }
+            _ = observers.needsUpdate(graph: emptyViewGraph)
+            observers.notify()
         }
     }
     #endif