ios: Check we're on the main thread before opening URL

Cargo.toml

@@ -36,9 +36,9 @@ jni = "0.21"
 ndk-context = "0.1"
 
 [target.'cfg(any(target_os = "ios", target_os = "tvos", target_os = "visionos"))'.dependencies]
-block2 = "0.5.0"
-objc2 = "0.5.1"
-objc2-foundation = { version = "0.2.0", default-features = false, features = [
+block2 = "0.6.0"
+objc2 = "0.6.0"
+objc2-foundation = { version = "0.3.0", default-features = false, features = [
     "std",
     "NSDictionary",
     "NSString",

src/ios.rs

@@ -1,12 +1,20 @@
 use crate::{Browser, BrowserOptions, Error, ErrorKind, Result, TargetType};
 use block2::Block;
-use objc2::rc::Id;
+use objc2::rc::Retained;
 use objc2::runtime::Bool;
-use objc2::{class, msg_send, msg_send_id};
+use objc2::{class, msg_send, MainThreadMarker};
 use objc2_foundation::{NSDictionary, NSObject, NSString, NSURL};
 
-fn app() -> Option<Id<NSObject>> {
-    unsafe { msg_send_id![class!(UIApplication), sharedApplication] }
+fn app(mtm: MainThreadMarker) -> Option<Retained<NSObject>> {
+    let _ = mtm;
+    // SAFETY: The signature is correct, and we hold `MainThreadMarker`, so we
+    // know we're on the main thread where it's safe to access the shared
+    // UIApplication.
+    //
+    // NOTE: `sharedApplication` is declared as returning non-NULL, but it
+    // will only do so inside `UIApplicationMain`; if called outside, the
+    // shared application is NULL.
+    unsafe { msg_send![class!(UIApplication), sharedApplication] }
 }
 
 fn open_url(
@@ -29,12 +37,19 @@ pub(super) fn open_browser_internal(
     // ensure we're opening only http/https urls, failing otherwise
     let url = target.get_http_url()?;
 
+    let mtm = MainThreadMarker::new().ok_or_else(|| {
+        Error::new(
+            ErrorKind::Other,
+            "Cannot open URL from a thread that is not the main thread",
+        )
+    })?;
+
     // always return true for a dry run
     if options.dry_run {
         return Ok(());
     }
 
-    let app = app().ok_or(Error::new(
+    let app = app(mtm).ok_or(Error::new(
         ErrorKind::Other,
         "UIApplication is null, can't open url",
     ))?;