Add no presenting vc auth flow support to mac catalyst external user agent

Also updates the presentExternUserAgentRequest test to work for both iOS and catalyst

Author: mdmathias

Source/AppAuth/iOS/OIDExternalUserAgentCatalyst.h

@@ -32,18 +32,27 @@ NS_ASSUME_NONNULL_BEGIN
 API_AVAILABLE(macCatalyst(13)) API_UNAVAILABLE(ios)
 @interface OIDExternalUserAgentCatalyst : NSObject<OIDExternalUserAgent>
 
-/*! @internal
-    @brief Unavailable. Please use @c initWithPresentingViewController:
+/*! @brief Create an external user-agent.
+    @discussion The specific authentication UI used depends on the iOS version and accessibility
+        options. iOS 8 uses the system browser, iOS 9-10 use @c SFSafariViewController, iOS 11 uses
+        @c SFAuthenticationSession (unless Guided Access is on which does not work) or uses
+        @c SFSafariViewController, and iOS 12+ uses @c ASWebAuthenticationSession (unless Guided
+        Access is on).
  */
-- (nonnull instancetype)init NS_UNAVAILABLE;
+- (nonnull instancetype)init;
 
-/*! @brief The designated initializer.
+/*! @brief Create an external user-agent which optionally uses a private authentication session.
+    @param prefersEphemeralSession Whether the caller prefers to use a private authentication
+        session. See @c ASWebAuthenticationSession.prefersEphemeralWebBrowserSession for more.
+ */
+- (nullable instancetype)initWithPrefersEphemeralSession:(BOOL)prefersEphemeralSession;
+
+/*! @brief Create an external user-agent with a presenting view controller.
     @param presentingViewController The view controller from which to present the
         \SFSafariViewController.
  */
 - (nullable instancetype)initWithPresentingViewController:
-    (UIViewController *)presentingViewController
-    NS_DESIGNATED_INITIALIZER;
+    (UIViewController *)presentingViewController;
 
 /*! @brief Create an external user-agent which optionally uses a private authentication session.
     @param presentingViewController The view controller from which to present the browser.

Source/AppAuth/iOS/OIDExternalUserAgentCatalyst.m

@@ -45,9 +45,22 @@ @implementation OIDExternalUserAgentCatalyst {
   ASWebAuthenticationSession *_webAuthenticationVC;
 }
 
+- (nonnull instancetype)init {
+  self = [super init];
+  return self;
+}
+
+- (nullable instancetype)initWithPrefersEphemeralSession:(BOOL)prefersEphemeralSession {
+  self = [self init];
+  if (self) {
+    _prefersEphemeralSession = prefersEphemeralSession;
+  }
+  return self;
+}
+
 - (nullable instancetype)initWithPresentingViewController:
     (UIViewController *)presentingViewController {
-  self = [super init];
+  self = [self init];
   if (self) {
     _presentingViewController = presentingViewController;
   }
@@ -71,6 +84,38 @@ - (BOOL)presentExternalUserAgentRequest:(id<OIDExternalUserAgentRequest>)request
     return NO;
   }
 
+  if (!_presentingViewController) {
+    // Find presentingViewController
+    if (@available(iOS 13.0, *)) {
+      NSSet<UIWindowScene *> *scenes =
+          (NSSet<UIWindowScene *> *)[UIApplication sharedApplication].connectedScenes;
+
+      NSMutableArray<UIWindow *> *windows = [NSMutableArray array];
+
+      for (UIWindowScene *scene in scenes) {
+        [windows addObjectsFromArray:scene.windows];
+      }
+
+      for (UIWindow *window in windows) {
+        if (window.isKeyWindow) { // False if calling before window appears
+          UIWindow *keyWindow = window;
+          _presentingViewController = keyWindow.rootViewController;
+          break;
+        }
+      }
+    } else {
+      // ≤ iOS 12.X
+      NSArray<UIWindow *> *windows = UIApplication.sharedApplication.windows;
+      NSPredicate *keyWindowPredicate = [NSPredicate predicateWithFormat:@"keyWindow == YES"];
+      UIWindow *keyWindow = [windows filteredArrayUsingPredicate:keyWindowPredicate].firstObject;
+      _presentingViewController = keyWindow.rootViewController;
+    }
+    if (!_presentingViewController) {
+      // Unable to find a presentingViewController; perhaps because no window is key and visible
+      return NO;
+    }
+  }
+
   _externalUserAgentFlowInProgress = YES;
   _session = session;
   BOOL openedUserAgent = NO;

UnitTests/AppAuth/OIDExternUserAgentIOSTests.m renamed to UnitTests/AppAuth/OIDExternUserAgentTests.m

@@ -1,4 +1,4 @@
-/*! @file OIDExternalUserAgentIOSTests.m
+/*! @file OIDExternalUserAgentTests.m
    @brief AppAuth iOS SDK
    @copyright
        Copyright 2023 The AppAuth Authors. All Rights Reserved.
@@ -16,25 +16,35 @@
        limitations under the License.
 */
 
+#import <TargetConditionals.h>
+
 #import <XCTest/XCTest.h>
 
 #if SWIFT_PACKAGE
 @import AppAuth;
 @import TestHelpers;
 #else
+#import "Source/AppAuth/iOS/OIDExternalUserAgentCatalyst.h"
 #import "Source/AppAuth/iOS/OIDExternalUserAgentIOS.h"
 #import "Source/AppAuthCore/OIDError.h"
 #import "UnitTests/TestHelpers/OIDAuthorizationRequest+TestHelper.h"
 #endif
 
-@interface OIDExternalUserAgentIOSTests : XCTestCase
+@interface OIDExternalUserAgentTests : XCTestCase
 
 @end
 
-@implementation OIDExternalUserAgentIOSTests
+@implementation OIDExternalUserAgentTests
 
 - (void)testThatPresentExternalUserAgentRequestReturnsNoWhenMissingPresentingViewController {
-  OIDExternalUserAgentIOS *userAgent = [[OIDExternalUserAgentIOS alloc] init];
+  id<OIDExternalUserAgent> userAgent;
+
+#if TARGET_OS_MACCATALYST
+  userAgent = [[OIDExternalUserAgentCatalyst alloc] init];
+#elif TARGET_OS_IOS
+  userAgent = [[OIDExternalUserAgentIOS alloc] init];
+#endif
+
   OIDAuthorizationRequest *authRequest = [OIDAuthorizationRequest testInstance];
   [OIDAuthorizationService
       presentAuthorizationRequest:authRequest