Optimize search for the default bundle

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

@@ -32,6 +32,50 @@
 
 static const char* kApplicationKernelSnapshotFileName = "kernel_blob.bin";
 
+// Finds a bundle with the named `bundleID` within `searchURL`.
+//
+// Returns `nil` if the bundle cannot be found or if errors are encountered.
+NSBundle* FLTFrameworkBundleInternal(NSString* bundleID, NSURL* searchURL) {
+  NSDirectoryEnumerator<NSURL*>* frameworkEnumerator = [NSFileManager.defaultManager
+                 enumeratorAtURL:searchURL
+      includingPropertiesForKeys:nil
+                         options:NSDirectoryEnumerationSkipsSubdirectoryDescendants |
+                                 NSDirectoryEnumerationSkipsHiddenFiles
+                    // Skip directories where errors are encountered.
+                    errorHandler:nil];
+
+  for (NSURL* candidate in frameworkEnumerator) {
+    NSBundle* bundle = [NSBundle bundleWithURL:candidate];
+    if ([bundle.bundleIdentifier isEqualToString:bundleID]) {
+      return bundle;
+    }
+  }
+  return nil;
+}
+
+// Finds a bundle with the named `bundleID`.
+//
+// `+[NSBundle bundleWithIdentifier:]` is slow, and can take in the order of
+// tens of milliseconds in a minimal flutter app, and closer to 100 milliseconds
+// in a medium sized Flutter app on an iPhone 13. It is likely that the slowness
+// comes from having to traverse and load all bundles known to the process.
+// Using `+[NSBundle allframeworks]` and filtering also suffers from the same
+// problem.
+//
+// This implementation is an optimization to first limit the search space to
+// `+[NSBundle privateFrameworksURL]` of the main bundle, which is usually where
+// frameworks used by this file are placed. If the desired bundle cannot be
+// found here, the implementation falls back to
+// `+[NSBundle bundleWithIdentifier:]`.
+NS_INLINE NSBundle* FLTFrameworkBundleWithIdentifier(NSString* bundleID) {
+  NSBundle* bundle = FLTFrameworkBundleInternal(bundleID, NSBundle.mainBundle.privateFrameworksURL);
+  if (bundle != nil) {
+    return bundle;
+  }
+  // Fallback to slow implementation.
+  return [NSBundle bundleWithIdentifier:bundleID];
+}
+
 flutter::Settings FLTDefaultSettingsForBundle(NSBundle* bundle) {
   auto command_line = flutter::CommandLineFromNSProcessInfo();
 
@@ -46,7 +90,7 @@
 
   bool hasExplicitBundle = bundle != nil;
   if (bundle == nil) {
-    bundle = [NSBundle bundleWithIdentifier:[FlutterDartProject defaultBundleIdentifier]];
+    bundle = FLTFrameworkBundleWithIdentifier([FlutterDartProject defaultBundleIdentifier]);
   }
   if (bundle == nil) {
     bundle = mainBundle;
@@ -309,7 +353,7 @@ - (instancetype)initWithSettings:(const flutter::Settings&)settings {
 
 + (NSString*)flutterAssetsName:(NSBundle*)bundle {
   if (bundle == nil) {
-    bundle = [NSBundle bundleWithIdentifier:[FlutterDartProject defaultBundleIdentifier]];
+    bundle = FLTFrameworkBundleWithIdentifier([FlutterDartProject defaultBundleIdentifier]);
   }
   if (bundle == nil) {
     bundle = [NSBundle mainBundle];

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

@@ -60,6 +60,19 @@ - (void)testLeakDartVMSettingsAreCorrectlyParsed {
   XCTAssertEqual(settings.leak_vm, NO);
 }
 
+- (void)testFLTFrameworkBundleInternalWhenBundleIsNotPresent {
+  NSBundle* found =
+      FLTFrameworkBundleInternal(@"doesNotExist", NSBundle.mainBundle.privateFrameworksURL);
+  XCTAssertNil(found);
+}
+
+- (void)testFLTFrameworkBundleInternalWhenBundleIsPresent {
+  NSString* presentBundleID = @"io.flutter.flutter";
+  NSBundle* found =
+      FLTFrameworkBundleInternal(presentBundleID, NSBundle.mainBundle.privateFrameworksURL);
+  XCTAssertNotNil(found);
+}
+
 - (void)testEnableImpellerSettingIsCorrectlyParsed {
   // The FLTEnableImpeller's value is defined in Info.plist
   NSBundle* mainBundle = [NSBundle mainBundle];

shell/platform/darwin/ios/framework/Source/FlutterDartProject_Internal.h

@@ -12,6 +12,8 @@
 
 NS_ASSUME_NONNULL_BEGIN
 
+NSBundle* FLTFrameworkBundleInternal(NSString* bundleID, NSURL* searchURL);
+
 flutter::Settings FLTDefaultSettingsForBundle(NSBundle* bundle = nil);
 
 @interface FlutterDartProject ()