Fix hang observing user defaults on background queue

platform/ios/CHANGELOG.md

@@ -9,6 +9,7 @@ Mapbox welcomes participation and contributions from everyone.  If you’d like
 - The user dot now moves smoothly between user location updates while user location tracking is disabled. ([#1582](https://github.com/mapbox/mapbox-gl-native/pull/1582))
 - User location heading updates now resume properly when an app becomes active again. ([#4674](https://github.com/mapbox/mapbox-gl-native/pull/4674))
 - Fixed an issue preventing KVO change notifications from being generated on MGLMapView’s `userTrackingMode` key path when `-setUserTrackingMode:animated:` is called. ([#4724](https://github.com/mapbox/mapbox-gl-native/pull/4724))
+- Fixed a hang that could occur if the host application attempts to set user defaults on a background queue. ([#4745](https://github.com/mapbox/mapbox-gl-native/pull/4745))
 - Added a `-reloadStyle:` action to MGLMapView to force a reload of the current style. ([#4728](https://github.com/mapbox/mapbox-gl-native/pull/4728))
 - A more specific user agent string is now sent with style and tile requests. ([#4012](https://github.com/mapbox/mapbox-gl-native/pull/4012))
 - Mapbox Telemetry is automatically disabled while the host application is running in the iOS Simulator. ([#4726](https://github.com/mapbox/mapbox-gl-native/pull/4726))

platform/ios/src/MGLMapboxEvents.m

@@ -128,7 +128,6 @@ @interface MGLMapboxEvents () <MGLLocationManagerDelegate>
 @end
 
 @implementation MGLMapboxEvents {
-    id _userDefaultsObserver;
     NSString *_instanceID;
 }
 
@@ -199,17 +198,8 @@ - (instancetype) init {
             self.canEnableDebugLogging = YES;
         }
 
-
         // Watch for changes to telemetry settings by the user
-        __weak MGLMapboxEvents *weakSelf = self;
-        _userDefaultsObserver = [[NSNotificationCenter defaultCenter] addObserverForName:NSUserDefaultsDidChangeNotification
-                                                                                  object:nil
-                                                                                   queue:[NSOperationQueue mainQueue]
-                                                                              usingBlock:
-         ^(NSNotification *notification) {
-             MGLMapboxEvents *strongSelf = weakSelf;
-             [strongSelf pauseOrResumeMetricsCollectionIfRequired];
-         }];
+        [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(userDefaultsDidChange:) name:NSUserDefaultsDidChangeNotification object:nil];
 
         [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(pauseOrResumeMetricsCollectionIfRequired) name:UIApplicationDidEnterBackgroundNotification object:nil];
         [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(pauseOrResumeMetricsCollectionIfRequired) name:UIApplicationWillEnterForegroundNotification object:nil];
@@ -233,7 +223,7 @@ + (nullable instancetype)sharedManager {
 }
 
 - (void)dealloc {
-    [[NSNotificationCenter defaultCenter] removeObserver:_userDefaultsObserver];
+    [[NSNotificationCenter defaultCenter] removeObserver:self];
     [self pauseMetricsCollection];
 }
 
@@ -249,6 +239,12 @@ - (NSString *)instanceID {
     return _instanceID;
 }
 
+- (void)userDefaultsDidChange:(NSNotification *)notification {
+    dispatch_async(dispatch_get_main_queue(), ^{
+        [self pauseOrResumeMetricsCollectionIfRequired];
+    });
+}
+
 - (void)pauseOrResumeMetricsCollectionIfRequired {
     // Prevent blue status bar when host app has `when in use` permission only and it is not in foreground
     if ([CLLocationManager authorizationStatus] == kCLAuthorizationStatusAuthorizedWhenInUse &&