objc2-media-player: invalid message send to -[_MPMusicPlayerMediaItemProxy title]: method not found

[valentinegb]

The following code:

unsafe {
    let player = MPMusicPlayerController::systemMusicPlayer();
    let now_playing_item = player.nowPlayingItem();

    match now_playing_item {
        Some(now_playing_item) => {
            let title = now_playing_item
                .title()
                .map(|ns_string| ns_string.to_string())
                .unwrap_or("a song".to_string());
            let artist = now_playing_item
                .artist()
                .map(|ns_string| ns_string.to_string())
                .unwrap_or("an unknown artist".to_string());

            println!("Now playing {title} by {artist}");
        }
        None => println!("There is nothing playing now"),
    }
}

produces the following output:

thread 'main' panicked at /Users/valentinebriese/.cargo/registry/src/index.crates.io-6f17d22bba15001f/objc2-media-player-0.2.2/src/generated/MPMediaItem.rs:232:1:
invalid message send to -[_MPMusicPlayerMediaItemProxy title]: method not found
stack backtrace:
   0:        0x10057be2c - std::backtrace_rs::backtrace::libunwind::trace::he4f0a5f56afe8e37
                               at /rustc/129f3b9964af4d4a709d1383930ade12dfe7c081/library/std/src/../../backtrace/src/backtrace/libunwind.rs:105:5
   1:        0x10057be2c - std::backtrace_rs::backtrace::trace_unsynchronized::habb302958e80f800
                               at /rustc/129f3b9964af4d4a709d1383930ade12dfe7c081/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5
   2:        0x10057be2c - std::sys_common::backtrace::_print_fmt::h9819d35e2a5cda77
                               at /rustc/129f3b9964af4d4a709d1383930ade12dfe7c081/library/std/src/sys_common/backtrace.rs:68:5
   3:        0x10057be2c - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::h1f3776e0b5c7517d
                               at /rustc/129f3b9964af4d4a709d1383930ade12dfe7c081/library/std/src/sys_common/backtrace.rs:44:22
   4:        0x100591bf4 - core::fmt::rt::Argument::fmt::h626862aa6242248a
                               at /rustc/129f3b9964af4d4a709d1383930ade12dfe7c081/library/core/src/fmt/rt.rs:165:63
   5:        0x100591bf4 - core::fmt::write::heedef092c8c0962e
                               at /rustc/129f3b9964af4d4a709d1383930ade12dfe7c081/library/core/src/fmt/mod.rs:1157:21
   6:        0x10057a02c - std::io::Write::write_fmt::h7178e8e2ea928914
                               at /rustc/129f3b9964af4d4a709d1383930ade12dfe7c081/library/std/src/io/mod.rs:1832:15
   7:        0x10057bc84 - std::sys_common::backtrace::_print::ha0f584bc7bfb9d2b
                               at /rustc/129f3b9964af4d4a709d1383930ade12dfe7c081/library/std/src/sys_common/backtrace.rs:47:5
   8:        0x10057bc84 - std::sys_common::backtrace::print::h417292deb95532ed
                               at /rustc/129f3b9964af4d4a709d1383930ade12dfe7c081/library/std/src/sys_common/backtrace.rs:34:9
   9:        0x10057cde0 - std::panicking::default_hook::{{closure}}::h0cb68f1228c4613a
  10:        0x10057cad4 - std::panicking::default_hook::h24535936bc1f51de
                               at /rustc/129f3b9964af4d4a709d1383930ade12dfe7c081/library/std/src/panicking.rs:298:9
  11:        0x10057d698 - std::panicking::rust_panic_with_hook::h5db4d2345b297bed
                               at /rustc/129f3b9964af4d4a709d1383930ade12dfe7c081/library/std/src/panicking.rs:795:13
  12:        0x10057d0c8 - std::panicking::begin_panic_handler::{{closure}}::h3fd558f09a0d5492
                               at /rustc/129f3b9964af4d4a709d1383930ade12dfe7c081/library/std/src/panicking.rs:664:13
  13:        0x10057c2b4 - std::sys_common::backtrace::__rust_end_short_backtrace::hfc76eebe1ce501b2
                               at /rustc/129f3b9964af4d4a709d1383930ade12dfe7c081/library/std/src/sys_common/backtrace.rs:171:18
  14:        0x10057ce38 - rust_begin_unwind
                               at /rustc/129f3b9964af4d4a709d1383930ade12dfe7c081/library/std/src/panicking.rs:652:5
  15:        0x10059d47c - core::panicking::panic_fmt::hc2b459a5bd3dce66
                               at /rustc/129f3b9964af4d4a709d1383930ade12dfe7c081/library/core/src/panicking.rs:72:14
  16:        0x10055581c - objc2::runtime::message_receiver::panic_verify::he9a1f6a7746f7dc5
                               at /Users/valentinebriese/.cargo/registry/src/index.crates.io-6f17d22bba15001f/objc2-0.5.2/src/runtime/message_receiver.rs:346:5
  17:        0x100555604 - objc2::runtime::message_receiver::msg_send_check_class::h8ce8854db5297ee6
                               at /Users/valentinebriese/.cargo/registry/src/index.crates.io-6f17d22bba15001f/objc2-0.5.2/src/runtime/message_receiver.rs:334:5
  18:        0x1005554e4 - objc2::runtime::message_receiver::msg_send_check::h2c95653d1055f9c6
                               at /Users/valentinebriese/.cargo/registry/src/index.crates.io-6f17d22bba15001f/objc2-0.5.2/src/runtime/message_receiver.rs:311:5
  19:        0x10054a238 - objc2::runtime::message_receiver::MessageReceiver::send_message::hcbf565f7dd70acd0
                               at /Users/valentinebriese/.cargo/registry/src/index.crates.io-6f17d22bba15001f/objc2-0.5.2/src/runtime/message_receiver.rs:426:13
  20:        0x10054a10c - objc2::__macro_helpers::msg_send::MsgSend::send_message::h377ae34697ba117c
                               at /Users/valentinebriese/.cargo/registry/src/index.crates.io-6f17d22bba15001f/objc2-0.5.2/src/__macro_helpers/msg_send.rs:27:31
  21:        0x100548b70 - <objc2::__macro_helpers::method_family::RetainSemantics<5_u8> as objc2::__macro_helpers::msg_send_id::MsgSendId<T,core::option::Option<objc2::rc::id::Retained<U>>>>::send_message_id::h50cf3642fa03ff6c
                               at /Users/valentinebriese/.cargo/registry/src/index.crates.io-6f17d22bba15001f/objc2-0.5.2/src/__macro_helpers/msg_send_id.rs:359:28
  22:        0x100549df4 - objc2_media_player::generated::__MPMediaItem::MPMediaItem::title::hb51bb52498dfa77b
                               at /Users/valentinebriese/.cargo/registry/src/index.crates.io-6f17d22bba15001f/objc2-0.5.2/src/macros/extern_methods.rs:247:14
  23:        0x10035cff4 - media_player_rich_presence::main::h33d9b7bb285513d1
                               at /Users/valentinebriese/Developer/media-player-rich-presence/src/main.rs:13:29
  24:        0x10035e0fc - core::ops::function::FnOnce::call_once::ha9f785aeb04fc58b
                               at /rustc/129f3b9964af4d4a709d1383930ade12dfe7c081/library/core/src/ops/function.rs:250:5
  25:        0x10035e068 - std::sys_common::backtrace::__rust_begin_short_backtrace::h0e805b2a2dc04d23
                               at /rustc/129f3b9964af4d4a709d1383930ade12dfe7c081/library/std/src/sys_common/backtrace.rs:155:18
  26:        0x10035dcfc - std::rt::lang_start::{{closure}}::h88f7cff5978608f0
                               at /rustc/129f3b9964af4d4a709d1383930ade12dfe7c081/library/std/src/rt.rs:159:18
  27:        0x100575d74 - core::ops::function::impls::<impl core::ops::function::FnOnce<A> for &F>::call_once::h2f86a413382a979d
                               at /rustc/129f3b9964af4d4a709d1383930ade12dfe7c081/library/core/src/ops/function.rs:284:13
  28:        0x100575d74 - std::panicking::try::do_call::hd40c9eb4d233b111
                               at /rustc/129f3b9964af4d4a709d1383930ade12dfe7c081/library/std/src/panicking.rs:559:40
  29:        0x100575d74 - std::panicking::try::h13ac68ffa70c387b
                               at /rustc/129f3b9964af4d4a709d1383930ade12dfe7c081/library/std/src/panicking.rs:523:19
  30:        0x100575d74 - std::panic::catch_unwind::habea7b6fc986e966
                               at /rustc/129f3b9964af4d4a709d1383930ade12dfe7c081/library/std/src/panic.rs:149:14
  31:        0x100575d74 - std::rt::lang_start_internal::{{closure}}::h6b16436250c3cf62
                               at /rustc/129f3b9964af4d4a709d1383930ade12dfe7c081/library/std/src/rt.rs:141:48
  32:        0x100575d74 - std::panicking::try::do_call::h9970b928a0b20951
                               at /rustc/129f3b9964af4d4a709d1383930ade12dfe7c081/library/std/src/panicking.rs:559:40
  33:        0x100575d74 - std::panicking::try::h4dfbe3cb4cc8f253
                               at /rustc/129f3b9964af4d4a709d1383930ade12dfe7c081/library/std/src/panicking.rs:523:19
  34:        0x100575d74 - std::panic::catch_unwind::hf6a5e1e8ce5a10f5
                               at /rustc/129f3b9964af4d4a709d1383930ade12dfe7c081/library/std/src/panic.rs:149:14
  35:        0x100575d74 - std::rt::lang_start_internal::hecc68fef83c8f44d
                               at /rustc/129f3b9964af4d4a709d1383930ade12dfe7c081/library/std/src/rt.rs:141:20
  36:        0x10035dcc8 - std::rt::lang_start::hac1ec87399390c88
                               at /rustc/129f3b9964af4d4a709d1383930ade12dfe7c081/library/std/src/rt.rs:158:17
  37:        0x10035d308 - _main

I am using objc2-media-player v0.2.2 with the features MPMediaEntity, MPMediaItem, and MPMusicPlayerController on macOS 15.1 Beta (24B5024e).

[madsmtm]

Thanks for the report - I remember seeing this asked on Matrix as well, the issue is that our debug assertion check is overly zealous about when methods exist. I think we need to use the Objective-C method instead of class_getInstanceMethod, but can't remember the details rn.

You can use something like the following to work around this issue in the meantime, apologies for the hazzle:

[profile.dev.package.objc2]
debug-assertions = false

[lucasfernog]

the method check is also a problem for methods that are only available in newer macOS releases - crashing the debug app when running on older operating systems (see #11501 #11496)
can we move the check behind a feature flag perhaps? or completely remove it?

[madsmtm]

the method check is also a problem for methods that are only available in newer macOS releases - crashing the debug app when running on older operating systems

While true, that's kinda the expected behaviour, even if this objc2 issue didn't exist, the application is gonna crash anyhow.

You must check the OS version and do something else when the API isn't available (can be done in the next release of objc2 with e.g. if objc2::available!(macos = 11.3) { ... }, otherwise you can query NSProcessInfo for the information).

See also the discussion on the wry issues: tauri-apps/wry#1401, tauri-apps/wry#1402.

can we move the check behind a feature flag perhaps? or completely remove it?

Both good options, not sure yet what I'll do (but I'll definitely do something to fix this before the next release).

[lucasfernog]

@madsmtm that's true for API calls, but not for delegate methods (in this case you would just register a function for a class with a selector that simply do not exist and is not called by the operating system)
to conditionally register the method in this case we'd need to support the version check in the method macros

[madsmtm]

Oh yeah, you're right!