Make FlutterSceneLifeCycleProvider.sceneLifeCycleDelegate readonly (flutter#177240)

And add documentation that says it should not change because the
delegate implementation is stateful.

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Author: LongCatIsLooong

dev/integration_tests/ios_add2app_uiscene/native/SceneDelegate-FlutterSceneLifeCycleProvider.swift

@@ -8,8 +8,7 @@ import UIKit
 class SceneDelegate: UIResponder, UIWindowSceneDelegate,
   FlutterSceneLifeCycleProvider
 {
-  var sceneLifeCycleDelegate: FlutterPluginSceneLifeCycleDelegate =
-    FlutterPluginSceneLifeCycleDelegate()
+  let sceneLifeCycleDelegate = FlutterPluginSceneLifeCycleDelegate()
 
   var window: UIWindow?
 

dev/integration_tests/ios_add2app_uiscene/native/SwiftUIApp-FlutterSceneLifeCycleProvider.swift

@@ -38,8 +38,7 @@ class SceneDelegate: UIResponder, UIWindowSceneDelegate,
   FlutterSceneLifeCycleProvider
 {
 
-  var sceneLifeCycleDelegate: FlutterPluginSceneLifeCycleDelegate =
-    FlutterPluginSceneLifeCycleDelegate()
+  let sceneLifeCycleDelegate = FlutterPluginSceneLifeCycleDelegate()
 
   var window: UIWindow?
 

engine/src/flutter/shell/platform/darwin/ios/framework/Headers/FlutterSceneLifeCycle.h

@@ -207,11 +207,25 @@ API_AVAILABLE(ios(13.0))
  * `FlutterPluginSceneLifeCycleDelegate` that can be used to forward scene life-cycle events to
  * Flutter plugins.
  *
- * This is typically implemented by the app's `SceneDelegate`.
+ * This is typically implemented by the app's `SceneDelegate`, as a `FlutterSceneLifeCycleProvider`
+ * is associated with one and only one `UIScene`.
  */
 API_AVAILABLE(ios(13.0))
 @protocol FlutterSceneLifeCycleProvider
-@property(nonatomic, strong) FlutterPluginSceneLifeCycleDelegate* sceneLifeCycleDelegate;
+
+/**
+ * The `FlutterPluginSceneLifeCycleDelegate` instance for forwarding `UIScene` events
+ * to plugins associated with this `UIScene`.
+ *
+ * The implementer of this protocol is responsible for creating the
+ * `FlutterPluginSceneLifeCycleDelegate` object, as well as forwarding `UIScene` events
+ * to plugins by calling the corresponding methods defined on
+ * `FlutterPluginSceneLifeCycleDelegate`.
+ *
+ * The `FlutterPluginSceneLifeCycleDelegate` implementation is stateful. For this reason,
+ * this property getter should typically always return the same object.
+ */
+@property(nonatomic, readonly) FlutterPluginSceneLifeCycleDelegate* sceneLifeCycleDelegate;
 @end
 
 NS_ASSUME_NONNULL_END

engine/src/flutter/shell/platform/darwin/ios/framework/Source/FlutterSceneLifeCycleTest.mm

@@ -21,7 +21,7 @@
 // This class is meant to mimic the SceneDelegate class generated by SwiftUI that does not conform
 // FlutterSceneLifeCycleProvider but actually does.
 @interface FlutterSwiftUIAppSceneDelegate : NSObject
-@property(nonatomic, strong) FlutterPluginSceneLifeCycleDelegate* sceneLifeCycleDelegate;
+@property(nonatomic, readonly) FlutterPluginSceneLifeCycleDelegate* sceneLifeCycleDelegate;
 @end
 
 @implementation FlutterSwiftUIAppSceneDelegate
@@ -36,6 +36,16 @@ - (instancetype)init {
 }
 @end
 
+// This class is not referenced in test because it's a test itself. The implementation
+// verifies that sceneLifeCycleDelegate is a readonly property.
+@interface TestSceneLifecycleProvider : NSObject <FlutterSceneLifeCycleProvider>
+@end
+@implementation TestSceneLifecycleProvider
+- (FlutterPluginSceneLifeCycleDelegate*)sceneLifeCycleDelegate {
+  return [[FlutterPluginSceneLifeCycleDelegate alloc] init];
+}
+@end
+
 @interface FlutterSceneLifecycleTest : XCTestCase
 @end