Add example for updating state in serial message callback (#375)

* Add example for updating state in serial message callback

* Use take instead of manually setting to None

---------

Co-authored-by: Alexander Koz <[email protected]>

api/system/Cargo.toml

@@ -50,6 +50,11 @@ required-features = ["sys/entry-point", "sys/lang-items"]
 name = "set-serial-message-callback"
 crate-type = ["dylib", "staticlib"]
 path = "examples/set-serial-message-callback.rs"
+
+[[example]]
+name = "update-state-in-serial-message-callback"
+crate-type = ["dylib", "staticlib"]
+path = "examples/update-state-in-serial-message-callback.rs"
 required-features = ["sys/entry-point", "sys/lang-items"]
 
 [package.metadata.playdate]

api/system/examples/README.md

@@ -11,6 +11,8 @@ cargo playdate run -p=playdate-system --example=handler-boxed --features=sys/lan
 cargo playdate run -p=playdate-system --example=handler-pinned --features=sys/lang-items,sys/entry-point
 
 cargo playdate run -p=playdate-system --example=set-serial-message-callback --features=sys/lang-items,sys/entry-point
+
+cargo playdate run -p=playdate-system --example=update-state-in-serial-message-callback --features=sys/lang-items,sys/entry-point
 ```
 
 More information how to use [cargo-playdate][] in help: `cargo playdate --help`.

api/system/examples/update-state-in-serial-message-callback.rs

@@ -0,0 +1,81 @@
+#![no_std]
+extern crate alloc;
+
+#[macro_use]
+extern crate sys;
+extern crate playdate_system as system;
+
+use alloc::string::String;
+use core::ptr::NonNull;
+
+use sys::EventLoopCtrl;
+use sys::ffi::*;
+use system::System;
+use system::event::SystemEventExt as _;
+use system::prelude::*;
+
+struct State {
+	initialized: bool,
+	latest_message: Option<String>,
+}
+
+impl State {
+	fn new() -> Self {
+		Self { initialized: false,
+		       latest_message: None }
+	}
+
+	/// System event handler
+	fn event(&'static mut self, event: SystemEvent) -> EventLoopCtrl {
+		match event {
+			SystemEvent::Init => {
+				System::Default().set_serial_message_callback(Some(|msg| {
+					                 self.latest_message = Some(msg);
+				                 }));
+
+				// Verify that `set_serial_message_callback` doesn't prevent us from
+				// updating other parts of `State`
+				self.initialized = true;
+
+				println!("Game init complete");
+			},
+			_ => {},
+		}
+		EventLoopCtrl::Continue
+	}
+}
+
+impl Update for State {
+	fn update(&mut self) -> UpdateCtrl {
+		if let Some(latest_message) = self.latest_message.take() {
+			println!("Latest message: {:#?}", latest_message);
+		}
+		UpdateCtrl::Continue
+	}
+}
+
+/// Entry point
+#[no_mangle]
+pub fn event_handler(_api: NonNull<PlaydateAPI>, event: SystemEvent, _sim_key_code: u32) -> EventLoopCtrl {
+	pub static mut STATE: Option<State> = None;
+
+	if unsafe { STATE.is_none() } {
+		let state = State::new();
+		unsafe { STATE = Some(state) }
+	}
+
+	// We call `set_update_handler` here because it requires (mutably) borrowing
+	// all of `State`.
+	//
+	// If we were to do this inside of `event` it would prevent us from being able
+	// to (mutably) borrow any of `State`, which means we couldn't update any
+	// state inside of our `set_serial_message_callback` closure.
+	if event == SystemEvent::Init {
+		unsafe { STATE.as_mut().expect("impossible").set_update_handler() }
+	}
+
+	unsafe { STATE.as_mut().expect("impossible").event(event) }
+}
+
+// Needed for debug build
+ll_symbols!();