Fix deadlock running tests in Xcode 16

Fixes #56

STULabel/STUMainScreenProperties.h

@@ -14,6 +14,11 @@ STU_DISABLE_CLANG_WARNING("-Wunguarded-availability")
 STU_REENABLE_CLANG_WARNING
 };
 
+/// This function is normally executed automatically when the library is loaded from an Objective C @c load initialization method.
+/// If this leads to issues, you can define an @c STULabel_NoMainScreenPropertiesInitializationOnLoad environment variable
+/// and then call this function explicity from the main thread before using any of the STULabel functionality.
+void stu_initializeMainScreenProperties(void);
+
 /// Returns the value of @c UIScreen.main.fixedCoordinateSpace.bounds.size.
 /// Thread-safe.
 CGSize stu_mainScreenPortraitSize(void);

STULabel/STUMainScreenProperties.m

@@ -57,15 +57,12 @@ static void updateMainScreenProperties(void) {
 #undef store
 }
 
-@interface UIScreen (STUMainScreenProperties)
-+ (void)load;
-@end
-@implementation UIScreen (STUMainScreenProperties)
-+ (void)load {
-  // We can't do this initialization lazily, because UIScreen must only be accessed on the
-  // main thread. (Using `dispatch_sync(dispatch_get_main_queue(), ...)` would lead to a
-  // deadlock when the main thread is waiting for the thread in which stu_mainScreen... is called
-  // for the first time.)
+void stu_initializeMainScreenProperties(void) {
+  static bool initialized = false;
+  if (initialized) {
+      return;
+  }
+  initialized = true;
   updateMainScreenProperties();
 #if !STU_MAIN_SCREEN_PROPERTIES_ARE_CONSTANT
   NSNotificationCenter * const notificationCenter = NSNotificationCenter.defaultCenter;
@@ -81,6 +78,32 @@ + (void)load {
                               usingBlock:updateMainScreenPropertiesBlock];
 #endif
 }
+
+@interface UIScreen (STUMainScreenProperties)
++ (void)load;
+@end
+@implementation UIScreen (STUMainScreenProperties)
++ (void)load {
+  // We can't do this initialization lazily, because UIScreen must only be accessed on the
+  // main thread. (Using `dispatch_sync(dispatch_get_main_queue(), ...)` would lead to a
+  // deadlock when the main thread is waiting for the thread in which stu_mainScreen... is called
+  // for the first time.)
+  // However, when executing this `load` method when a test bundle is loaded,
+  // calling stu_initializeMainScreenProperties synchronously leads to a deadlock in UIScreen.mainScreen,
+  // so for that special case we just invoke stu_initializeMainScreenProperties asynchronously,
+  // relying on the main screen properties not beeing used before stu_initializeMainScreenProperties has run.
+  NSDictionary* env = NSProcessInfo.processInfo.environment;
+  if ([env valueForKey:@"STULabel_NoMainScreenPropertiesInitializationOnLoad"]) {
+    return;
+  }
+  if ([env valueForKey:@"XCTestBundlePath"]) {
+    dispatch_async(dispatch_get_main_queue(), ^{
+      stu_initializeMainScreenProperties();
+    });
+    return;
+  }
+  stu_initializeMainScreenProperties();
+}
 @end
 
 #if STU_MAIN_SCREEN_PROPERTIES_ARE_CONSTANT