Add late resources example

examples/late-resources.rs

+//! Demonstrates initialization of resources in `init`.
+
+#![deny(unsafe_code)]
+#![feature(proc_macro)]
+#![no_std]
+
+extern crate cortex_m_rtfm as rtfm;
+extern crate stm32f103xx;
+
+use rtfm::{app, Threshold};
+
+app! {
+    device: stm32f103xx,
+
+    resources: {
+        // Usually, resources are initialized with a constant initializer:
+        static ON: bool = false;
+
+        // However, there are cases where this is not possible or not desired. For example, there
+        // may not be a sensible value to use, or the type may not be constructible in a constant
+        // (like `Vec`).
+        // While it is possible to use an `Option` in some cases, that requires you to properly
+        // initialize it and `.unwrap()` it at every use. It also consumes more memory.
+        //
+        // To solve this, it is possible to defer initialization of resources to `init` by omitting
+        // the initializer. Doing that will require `init` to return the values of all "late"
+        // resources.
+        static IP_ADDRESS: u32;
+    },
+
+    tasks: {
+        SYS_TICK: {
+            path: sys_tick,
+            resources: [IP_ADDRESS, ON],
+        },
+    }
+}
+
+// The signature of `init` is now required to have a specific return type.
+fn init(_p: init::Peripherals, _r: init::Resources) -> init::LateResourceValues {
+    // `init::Resources` does not contain `IP_ADDRESS`, since it is not yet initialized.
+    //_r.IP_ADDRESS;     // doesn't compile
+
+    // ...obtain value for IP_ADDRESS from EEPROM/DHCP...
+    let ip_address = 0x7f000001;
+
+    init::LateResourceValues {
+        // This struct will contain fields for all resources with omitted initializers.
+        IP_ADDRESS: ip_address,
+    }
+}
+
+fn sys_tick(_t: &mut Threshold, r: SYS_TICK::Resources) {
+    // Other tasks can access late resources like any other, since they are guaranteed to be
+    // initialized when tasks are run.
+
+    r.IP_ADDRESS;
+}
+
+fn idle() -> ! {
+    loop {
+        rtfm::wfi();
+    }
+}