Add share to selection controls (flutter#44554)

In native iOS, users are able to select text and initiate a share menu, which provides several standard services, such as copy, sharing to social media, direct ability to send to various contacts through messaging apps, etc. 

https://github.com/flutter/engine/assets/36148254/d0af7034-31fd-412e-8636-a06bbff54765

This PR is the engine portion of the changes that will allow Share to be implemented
This PR addresses flutter/flutter#107578
More details are available in this [design doc](https://github.com/flutter/engine/pull/flutter.dev/go/add-missing-features-to-selection-controls)

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

@@ -123,11 +123,23 @@ - (void)handleMethodCall:(FlutterMethodCall*)call result:(FlutterResult)result {
   } else if ([method isEqualToString:@"LookUp.invoke"]) {
     [self showLookUpViewController:args];
     result(nil);
+  } else if ([method isEqualToString:@"Share.invoke"]) {
+    [self showShareViewController:args];
+    result(nil);
   } else {
     result(FlutterMethodNotImplemented);
   }
 }
 
+- (void)showShareViewController:(NSString*)content {
+  UIViewController* engineViewController = [_engine.get() viewController];
+  NSArray* itemsToShare = @[ content ];
+  UIActivityViewController* activityViewController =
+      [[[UIActivityViewController alloc] initWithActivityItems:itemsToShare
+                                         applicationActivities:nil] autorelease];
+  [engineViewController presentViewController:activityViewController animated:YES completion:nil];
+}
+
 - (void)searchWeb:(NSString*)searchTerm {
   NSString* escapedText = [searchTerm
       stringByAddingPercentEncodingWithAllowedCharacters:[NSCharacterSet
@@ -338,7 +350,6 @@ - (BOOL)isLiveTextInputAvailable {
 
 - (void)showLookUpViewController:(NSString*)term {
   UIViewController* engineViewController = [_engine.get() viewController];
-  FML_DCHECK(![engineViewController presentingViewController]);
   UIReferenceLibraryViewController* referenceLibraryViewController =
       [[[UIReferenceLibraryViewController alloc] initWithTerm:term] autorelease];
   [engineViewController presentViewController:referenceLibraryViewController

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

@@ -19,6 +19,7 @@ @interface FlutterPlatformPlugin ()
 - (BOOL)isLiveTextInputAvailable;
 - (void)searchWeb:(NSString*)searchTerm;
 - (void)showLookUpViewController:(NSString*)term;
+- (void)showShareViewController:(NSString*)content;
 @end
 
 @interface UIViewController ()
@@ -122,6 +123,36 @@ - (void)testLookUpCallInitiated {
   [self waitForExpectationsWithTimeout:2 handler:nil];
 }
 
+- (void)testShareScreenInvoked {
+  FlutterEngine* engine = [[[FlutterEngine alloc] initWithName:@"test" project:nil] autorelease];
+  [engine runWithEntrypoint:nil];
+  std::unique_ptr<fml::WeakPtrFactory<FlutterEngine>> _weakFactory =
+      std::make_unique<fml::WeakPtrFactory<FlutterEngine>>(engine);
+
+  XCTestExpectation* presentExpectation =
+      [self expectationWithDescription:@"Share view controller presented"];
+
+  FlutterViewController* engineViewController =
+      [[[FlutterViewController alloc] initWithEngine:engine nibName:nil bundle:nil] autorelease];
+  FlutterViewController* mockEngineViewController = OCMPartialMock(engineViewController);
+
+  FlutterPlatformPlugin* plugin =
+      [[[FlutterPlatformPlugin alloc] initWithEngine:_weakFactory->GetWeakPtr()] autorelease];
+  FlutterPlatformPlugin* mockPlugin = OCMPartialMock(plugin);
+
+  FlutterMethodCall* methodCall = [FlutterMethodCall methodCallWithMethodName:@"Share.invoke"
+                                                                    arguments:@"Test"];
+  FlutterResult result = ^(id result) {
+    OCMVerify([mockEngineViewController
+        presentViewController:[OCMArg isKindOfClass:[UIActivityViewController class]]
+                     animated:YES
+                   completion:nil]);
+    [presentExpectation fulfill];
+  };
+  [mockPlugin handleMethodCall:methodCall result:result];
+  [self waitForExpectationsWithTimeout:1 handler:nil];
+}
+
 - (void)testClipboardHasCorrectStrings {
   [UIPasteboard generalPasteboard].string = nil;
   FlutterEngine* engine = [[[FlutterEngine alloc] initWithName:@"test" project:nil] autorelease];