Avoid refetching and recompiling the WASM.

Author: microbit-matt-hillsdon

src/board/index.ts

@@ -167,6 +167,7 @@ export class Board {
       fs: this.fs,
       conversions,
       noInitialRun: true,
+      instantiateWasm,
     });
     const module = new ModuleWrapper(wrapped);
     this.audio.initializeCallbacks({
@@ -675,3 +676,34 @@ function isFileSystem(
     ([k, v]) => typeof k === "string" && v instanceof Uint8Array
   );
 }
+
+const fetchWasm = async () => {
+  const response = await fetch("/build/firmware.wasm");
+  if (!response.ok) {
+    throw new Error(response.statusText);
+  }
+  return response.arrayBuffer();
+};
+
+const compileWasm = async () => {
+  // Can't use streaming in Safari 14 but would be nice to feature detect.
+  return WebAssembly.compile(new Uint8Array(await fetchWasm()));
+};
+
+let compiledWasmPromise: Promise<WebAssembly.Module> = compileWasm();
+
+const instantiateWasm = function (imports: any, successCallback: any) {
+  // No easy way to communicate failure here so hard to add retries.
+  compiledWasmPromise
+    .then(async (wasmModule) => {
+      const instance = await WebAssembly.instantiate(wasmModule, imports);
+      console.log("Woah!");
+      successCallback(instance);
+    })
+    .catch((e) => {
+      console.error("Failed to instantiate WASM");
+      console.error(e);
+    });
+  // Result via callback.
+  return {};
+};