[ios] Fix app extension not able to find assets from unloaded bundle (#46283)

Directly use "flutter_assets" as the default path to find the asset path, this works for app extension when the bundle is unloaded.

This PR also adds integration tests for app extensions, which also tests the asset path. 

Fixes flutter/flutter#124292

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

Author: Chris Yang

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");