`mouse::get_position(ctx)?` results in `EventPump` SDL error.

[dannyfritz]

I'm having trouble using mouse::get_position. Is there a certain location I can use it? Seems using it within the event loop doesn't work for me.

  fn update(&mut self, ctx: &mut Context) -> GameResult<()> {
      self.pos = mouse::get_position(ctx)?;
      Ok(())
  }

results in

thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: UnknownError("an `EventPump` instance is already alive - there can only be one `EventPump` in use at a time.")', src\libcore\result.rs:906:4
stack backtrace:
   0: std::sys_common::backtrace::_print
             at C:\projects\rust\src\libstd\sys_common\backtrace.rs:91
   1: std::panicking::default_hook::{{closure}}
             at C:\projects\rust\src\libstd\panicking.rs:380
   2: std::panicking::default_hook
             at C:\projects\rust\src\libstd\panicking.rs:397
   3: std::panicking::rust_panic_with_hook
             at C:\projects\rust\src\libstd\panicking.rs:577
   4: std::panicking::begin_panic<alloc::string::String>
             at C:\projects\rust\src\libstd\panicking.rs:538
   5: std::panicking::begin_panic_fmt
             at C:\projects\rust\src\libstd\panicking.rs:522
   6: std::panicking::rust_begin_panic
             at C:\projects\rust\src\libstd\panicking.rs:498
   7: core::panicking::panic_fmt
             at C:\projects\rust\src\libcore\panicking.rs:71
   8: core::result::unwrap_failed<ggez::error::GameError>
             at C:\projects\rust\src\libcore\macros.rs:23
   9: core::result::Result<(), ggez::error::GameError>::unwrap<(),ggez::error::GameError>
             at C:\projects\rust\src\libcore\result.rs:772
  10: mouse_trail::main
             at .\src\main.rs:35
  11: panic_unwind::__rust_maybe_catch_panic
             at C:\projects\rust\src\libpanic_unwind\lib.rs:101
  12: std::rt::lang_start
             at C:\projects\rust\src\libstd\rt.rs:51
  13: main
  14: __scrt_common_main_seh
             at f:\dd\vctools\crt\vcstartup\src\startup\exe_common.inl:283
  15: BaseThreadInitThunk

[icefoxen]

Confirmed on master and devel branches. I think I know what's going on but it's going to take some sticky lifetime stuff to fix. I suggest using the EventHandler::mouse_motion_event() callback in the mean time.

[icefoxen]

It was more sticky than anticipated! This should be fixed in devel branch though, please test.

However, I don't like it, because it requires storing the mouse position from events in event::run(). So if you roll your own event loop the mouse position no longer gets updated. I do not like making this necessary because it's super easy to miss. I dislike it more because there's already a way to get mouse position, from the MouseMotion event. Adding a workaround that violates OnceAndOnlyOnce seems dirty, even if it is for the sake of API symmetry -- the alternative is to have mouse::set_position() with no matching mouse::get_position(), which doesn't seem much better.

One possible solution would be to add a hook to feed events into the Context so it can update its own internal state based on them as necessary, which would cover both this case and the resize_viewport() call that's there anyway. Basically, you would call ctx.process_events() on each event at the start of your own event loop. That's probably the right choice...

[dannyfritz]

Thanks for the information, I'll use MouseMotion for now and look at writing an event loop at another time.

[icefoxen]

Closing this, pondering API enhancements continued in #284