[ios] Display map on connected displays

Whenever an external display is connected, show a map synchronized with the main map on the main display.

platform/ios/app/MBXViewController.m

@@ -205,6 +205,7 @@ @interface MBXViewController () <UITableViewDelegate,
 @property (nonatomic) BOOL frameTimeGraphEnabled;
 @property (nonatomic) BOOL shouldLimitCameraChanges;
 @property (nonatomic) BOOL randomWalk;
+@property (nonatomic) NSMutableArray<UIWindow *> *helperWindows;
 
 @end
 
@@ -289,6 +290,32 @@ - (void)viewDidLoad
         }
     }
     [self.mapView addGestureRecognizer:singleTap];
+
+    // Display a secondary map on any connected external display.
+    // https://developer.apple.com/documentation/uikit/windows_and_screens/displaying_content_on_a_connected_screen?language=objc
+    self.helperWindows = [NSMutableArray array];
+    [[NSNotificationCenter defaultCenter] addObserverForName:UIScreenDidConnectNotification object:nil queue:nil usingBlock:^(NSNotification * _Nonnull note) {
+        UIScreen *helperScreen = note.object;
+        UIWindow *helperWindow = [[UIWindow alloc] initWithFrame:helperScreen.bounds];
+        helperWindow.screen = helperScreen;
+        UIViewController *helperViewController = [[UIViewController alloc] init];
+        MGLMapView *helperMapView = [[MGLMapView alloc] initWithFrame:helperWindow.bounds styleURL:MGLStyle.satelliteStreetsStyleURL];
+        helperMapView.autoresizingMask = UIViewAutoresizingFlexibleWidth | UIViewAutoresizingFlexibleHeight;
+        helperMapView.camera = self.mapView.camera;
+        helperMapView.compassView.hidden = YES;
+        helperViewController.view = helperMapView;
+        helperWindow.rootViewController = helperViewController;
+        helperWindow.hidden = NO;
+        [self.helperWindows addObject:helperWindow];
+    }];
+    [[NSNotificationCenter defaultCenter] addObserverForName:UIScreenDidDisconnectNotification object:nil queue:nil usingBlock:^(NSNotification * _Nonnull note) {
+        UIScreen *helperScreen = note.object;
+        for (UIWindow *window in self.helperWindows) {
+            if (window.screen == helperScreen) {
+                [self.helperWindows removeObject:window];
+            }
+        }
+    }];
 }
 
 - (void)saveState:(__unused NSNotification *)notification
@@ -2214,6 +2241,7 @@ - (BOOL)mapView:(MGLMapView *)mapView shouldChangeFromCamera:(MGLMapCamera *)old
 - (void)mapViewRegionIsChanging:(MGLMapView *)mapView
 {
     [self updateHUD];
+    [self updateHelperMapViews];
 }
 
 - (void)mapView:(MGLMapView *)mapView regionDidChangeWithReason:(MGLCameraChangeReason)reason animated:(BOOL)animated
@@ -2223,12 +2251,20 @@ - (void)mapView:(MGLMapView *)mapView regionDidChangeWithReason:(MGLCameraChange
     }
 
     [self updateHUD];
+    [self updateHelperMapViews];
 }
 
 - (void)mapView:(MGLMapView *)mapView didUpdateUserLocation:(MGLUserLocation *)userLocation {
     [self updateHUD];
 }
 
+- (void)updateHelperMapViews {
+    for (UIWindow *window in self.helperWindows) {
+        MGLMapView *mapView = (MGLMapView *)window.rootViewController.view;
+        mapView.camera = self.mapView.camera;
+    }
+}
+
 - (void)updateHUD {
     if (!self.reuseQueueStatsEnabled && !self.mapInfoHUDEnabled) return;