[ios] Fix app extension not able to find assets from unloaded bundle

shell/platform/darwin/common/framework/Source/FlutterNSBundleUtils.mm

@@ -9,6 +9,7 @@
 FLUTTER_ASSERT_ARC
 
 const NSString* kDefaultAssetPath = @"Frameworks/App.framework/flutter_assets";
+static NSString* GetFlutterAssetPathFromBundle(NSBundle* bundle);
 
 NSBundle* FLTFrameworkBundleInternal(NSString* flutterFrameworkBundleID, NSURL* searchURL) {
   NSDirectoryEnumerator<NSURL*>* frameworkEnumerator = [NSFileManager.defaultManager
@@ -29,7 +30,7 @@
 }
 
 NSBundle* FLTGetApplicationBundle() {
-  NSBundle* mainBundle = [NSBundle mainBundle];
+  NSBundle* mainBundle = NSBundle.mainBundle;
   // App extension bundle is in <AppName>.app/PlugIns/Extension.appex.
   if ([mainBundle.bundleURL.pathExtension isEqualToString:@"appex"]) {
     // Up two levels.
@@ -48,7 +49,7 @@
     flutterFrameworkBundle = [NSBundle bundleWithIdentifier:flutterFrameworkBundleID];
   }
   if (flutterFrameworkBundle == nil) {
-    flutterFrameworkBundle = [NSBundle mainBundle];
+    flutterFrameworkBundle = NSBundle.mainBundle;
   }
   return flutterFrameworkBundle;
 }
@@ -58,13 +59,23 @@
 }
 
 NSString* FLTAssetsPathFromBundle(NSBundle* bundle) {
+  NSString* flutterAssetsPath = GetFlutterAssetPathFromBundle(bundle);
+  if (flutterAssetsPath.length == 0) {
+    flutterAssetsPath = GetFlutterAssetPathFromBundle(NSBundle.mainBundle);
+  }
+  return flutterAssetsPath;
+}
+
+static NSString* GetFlutterAssetPathFromBundle(NSBundle* bundle) {
   NSString* flutterAssetsPath = FLTAssetPath(bundle);
   // Use the raw path solution so that asset path can be returned from unloaded bundles.
   // See https://github.com/flutter/engine/pull/46073
-  NSString* assetsPath = [bundle pathForResource:flutterAssetsPath ofType:@""];
-
+  NSString* assetsPath = [bundle pathForResource:flutterAssetsPath ofType:nil];
   if (assetsPath.length == 0) {
-    assetsPath = [[NSBundle mainBundle] pathForResource:flutterAssetsPath ofType:@""];
+    // In app extension, using full relative path (kDefaultAssetPath)
+    // returns nil when the app bundle is not loaded. Try to use
+    // the sub folder name, which can successfully return a valid path.
+    assetsPath = [bundle pathForResource:@"flutter_assets" ofType:nil];
   }
   return assetsPath;
 }

shell/platform/darwin/ios/framework/Source/FlutterDartProjectTest.mm

@@ -93,7 +93,7 @@ - (void)testFLTAssetsURLFromBundle {
     id mockBundle = OCMClassMock([NSBundle class]);
     OCMStub([mockBundle objectForInfoDictionaryKey:@"FLTAssetsPath"]).andReturn(@"foo/assets");
     NSString* resultAssetsPath = @"path/to/foo/assets";
-    OCMStub([mockBundle pathForResource:@"foo/assets" ofType:@""]).andReturn(resultAssetsPath);
+    OCMStub([mockBundle pathForResource:@"foo/assets" ofType:nil]).andReturn(resultAssetsPath);
     NSString* path = FLTAssetsPathFromBundle(mockBundle);
     XCTAssertEqualObjects(path, @"path/to/foo/assets");
   }
@@ -102,9 +102,9 @@ - (void)testFLTAssetsURLFromBundle {
     id mockBundle = OCMClassMock([NSBundle class]);
     id mockMainBundle = OCMPartialMock([NSBundle mainBundle]);
     NSString* resultAssetsPath = @"path/to/foo/assets";
-    OCMStub([mockBundle pathForResource:@"Frameworks/App.framework/flutter_assets" ofType:@""])
+    OCMStub([mockBundle pathForResource:@"Frameworks/App.framework/flutter_assets" ofType:nil])
         .andReturn(nil);
-    OCMStub([mockMainBundle pathForResource:@"Frameworks/App.framework/flutter_assets" ofType:@""])
+    OCMStub([mockMainBundle pathForResource:@"Frameworks/App.framework/flutter_assets" ofType:nil])
         .andReturn(resultAssetsPath);
     NSString* path = FLTAssetsPathFromBundle(mockBundle);
     XCTAssertEqualObjects(path, @"path/to/foo/assets");