Add example, rename Ctx fn, make things pub(crate), misc changes

examples/callback.rs

@@ -0,0 +1,46 @@
+/// This example demonstrates the use of callbacks: calling functions (Host and Wasm)
+/// passed to us from the Wasm via hostcall
+use wasmer_runtime::{compile_with, compiler_for_backend, func, imports, Backend, Ctx};
+use wasmer_runtime_core::{structures::TypedIndex, types::TableIndex};
+
+static WASM: &'static str = "examples/callback-guest.wasm";
+
+/// This function matches our arbitrarily decided callback signature
+/// in this example we'll only call functions that take no arguments and return one value
+fn host_callback(_ctx: &mut Ctx) -> u32 {
+    55
+}
+
+fn call_guest_fn(ctx: &mut Ctx, guest_fn: u32) -> u32 {
+    // We get a TableIndex from our raw value passed in
+    let guest_fn_typed = TableIndex::new(guest_fn as usize);
+    // and use it to call the corresponding function
+    let result = ctx.call_with_table_index(guest_fn_typed, &[]).unwrap();
+
+    println!("Guest fn {} returned {:?}", guest_fn, result);
+
+    0
+}
+
+fn main() {
+    let wasm_bytes =
+        std::fs::read(WASM).expect(&format!("Could not read in WASM plugin at {}", WASM));
+
+    let imports = imports! {
+        "env" => {
+            "call_guest_fn" => func!(call_guest_fn),
+            "call_guest_fn2" => func!(call_guest_fn),
+            "host_callback" => func!(host_callback),
+        },
+    };
+
+    let compiler = compiler_for_backend(Backend::default()).unwrap();
+    let module = compile_with(&wasm_bytes[..], compiler.as_ref()).unwrap();
+    let instance = module
+        .instantiate(&imports)
+        .expect("failed to instantiate wasm module");
+
+    let entry_point = instance.func::<(u32, u32), u32>("main").unwrap();
+
+    entry_point.call(0, 0).expect("START");
+}