Hot Restart should dispose all previous Platform Views (macOS) (#163439)

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When using Platform Views on macOS, performing a Hot Restart throws an
exception with message "trying to create an already created view". This
is because the old Platform Views are not cleaned up. So, here we
dispose of the old Platform Views as part of the Hot Restart process.

Fixes issue: https://github.com/flutter/flutter/issues/110381

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engine/src/flutter/shell/platform/darwin/macos/framework/Source/FlutterEngine.mm

@@ -1124,6 +1124,7 @@ static void SetThreadPriority(FlutterThreadPriority priority) {
   while ((nextViewController = [viewControllerEnumerator nextObject])) {
     [nextViewController onPreEngineRestart];
   }
+  [_platformViewController reset];
 }
 
 - (void)onVSync:(uintptr_t)baton {

engine/src/flutter/shell/platform/darwin/macos/framework/Source/FlutterPlatformViewController.h

@@ -58,6 +58,11 @@
  */
 - (void)disposePlatformViews;
 
+/**
+ * Removes all platform views.
+ */
+- (void)reset;
+
 @end
 
 #endif  // FLUTTER_SHELL_PLATFORM_DARWIN_MACOS_FRAMEWORK_SOURCE_FLUTTERPLATFORMVIEWCONTROLLER_H_

engine/src/flutter/shell/platform/darwin/macos/framework/Source/FlutterPlatformViewController.mm

@@ -170,4 +170,11 @@
   _platformViewsToDispose.clear();
 }
 
+- (void)reset {
+  for (const auto& pair : _platformViews) {
+    _platformViewsToDispose.insert(pair.first);
+  }
+  [self disposePlatformViews];
+}
+
 @end

engine/src/flutter/shell/platform/darwin/macos/framework/Source/FlutterPlatformViewControllerTest.mm

@@ -141,6 +141,60 @@ TEST(FlutterPlatformViewController, TestCreateAndDispose) {
   EXPECT_TRUE(disposed);
 }
 
+TEST(FlutterPlatformViewController, TestReset) {
+  // Use id so we can access handleMethodCall method.
+  id platformViewController = [[FlutterPlatformViewController alloc] init];
+  TestFlutterPlatformViewFactory* factory = [TestFlutterPlatformViewFactory alloc];
+
+  [platformViewController registerViewFactory:factory withId:@"MockPlatformView"];
+
+  __block bool created = false;
+  FlutterResult resultOnCreate = ^(id result) {
+    // If a platform view is successfully created, the result is nil.
+    if (result == nil) {
+      created = true;
+    } else {
+      created = false;
+    }
+  };
+
+  // Create 2 views.
+  FlutterMethodCall* methodCallOnCreate0 =
+      [FlutterMethodCall methodCallWithMethodName:@"create"
+                                        arguments:@{
+                                          @"id" : @0,
+                                          @"viewType" : @"MockPlatformView"
+                                        }];
+
+  [platformViewController handleMethodCall:methodCallOnCreate0 result:resultOnCreate];
+  EXPECT_TRUE(created);
+
+  FlutterMethodCall* methodCallOnCreate1 =
+      [FlutterMethodCall methodCallWithMethodName:@"create"
+                                        arguments:@{
+                                          @"id" : @1,
+                                          @"viewType" : @"MockPlatformView"
+                                        }];
+  [platformViewController handleMethodCall:methodCallOnCreate1 result:resultOnCreate];
+  EXPECT_TRUE(created);
+
+  TestFlutterPlatformView* view = nil;
+
+  // Before the reset, the views exist.
+  view = (TestFlutterPlatformView*)[platformViewController platformViewWithID:0];
+  EXPECT_TRUE(view != nil);
+  view = (TestFlutterPlatformView*)[platformViewController platformViewWithID:1];
+  EXPECT_TRUE(view != nil);
+
+  // After a reset, the views should no longer exist.
+  [platformViewController reset];
+
+  view = (TestFlutterPlatformView*)[platformViewController platformViewWithID:0];
+  EXPECT_TRUE(view == nil);
+  view = (TestFlutterPlatformView*)[platformViewController platformViewWithID:1];
+  EXPECT_TRUE(view == nil);
+}
+
 TEST(FlutterPlatformViewController, TestDisposeOnMissingViewId) {
   // Use id so we can access handleMethodCall method.
   id platformViewController = [[FlutterPlatformViewController alloc] init];