generator and async generator support

fixes developit#38
First thing to note about this is that I broke the microbundle build.
I'll attempt to fix the generator function to not use things microbundle
doesn't expect, but I'm not sure how far I'll get
with that. Which that's also something tying up this pull request:
developit/greenlet#50

One other thing to note about this is that I decided on
returning a promise of the asyncIterator from the generator function.
This is due to the fact that the function from the user only ever gets
evaluated as an executable function on the worker side.

Author: johnsonjo4531

src/index.js

@@ -36,11 +36,30 @@ export default function workerize(code, options) {
 		URL.revokeObjectURL(url);
 		term.call(worker);
 	};
-	worker.call = (method, params) => new Promise( (resolve, reject) => {
+	worker.call = (method, params, genStatus=0, genId=undefined) => new Promise( (resolve, reject) => {
 		let id = `rpc${++counter}`;
 		callbacks[id] = [resolve, reject];
-		worker.postMessage({ type: 'RPC', id, method, params });
-	});
+		worker.postMessage({ type: 'RPC', id, genId, method, genStatus, params });
+	}).then(d => !d.hasOwnProperty("genId") ? d : (async function* workerAsyncGenPassthrough () {
+				const genId = d.genId;
+				try {
+					let result = {done: false};
+					let value;
+					while (!result.done) {
+						result = await worker.call(method, [value], 0, genId);
+						if (result.done) { break; }
+						value = yield result.value;
+					}
+					return result.value;
+				}
+				catch (err) {
+					await worker.call(method, ['' + err], 2, genId);
+					throw err;
+				}
+				finally {
+					await worker.call(method, [undefined], 1, genId);
+				}
+			})());
 	worker.rpcMethods = {};
 	setup(worker, worker.rpcMethods, callbacks);
 	worker.expose = methodName => {
@@ -51,10 +70,13 @@ export default function workerize(code, options) {
 	for (i in exports) if (!(i in worker)) worker.expose(i);
 	return worker;
 }
-
 function setup(ctx, rpcMethods, callbacks) {
+	let gencounter = 0;
+	let GENS = {};
 	ctx.addEventListener('message', ({ data }) => {
 		let id = data.id;
+		let genId = data.genId;
+		let genStatus = data.genStatus;
 		if (data.type!=='RPC' || id==null) return;
 		if (data.method) {
 			let method = rpcMethods[data.method];
@@ -63,15 +85,31 @@ function setup(ctx, rpcMethods, callbacks) {
 			}
 			else {
 				Promise.resolve()
-					.then( () => method.apply(null, data.params) )
-					.then( result => { ctx.postMessage({ type: 'RPC', id, result }); })
+					// Either use a generator or call a method.
+					.then( () => !GENS[genId] ? method.apply(null, data.params) :  GENS[genId][genStatus](data.params[0]))
+					.then( result => {
+						if (method.constructor.name === 'AsyncGeneratorFunction' || method.constructor.name === 'GeneratorFunction') {
+							if (!GENS[genId]) {
+								GENS[++gencounter] = [result.next.bind(result), result.return.bind(result), result.throw.bind(result)];
+								// return an initial message of success.
+								// genId should only be sent to the main thread when initializing the generator
+								return ctx.postMessage({ type: 'RPC', id, genId: gencounter, result:  { value: undefined, done: false } });
+							}
+						}
+						ctx.postMessage({ type: 'RPC', id, result });
+						if (result.done) {
+							GENS[genId] = null;
+						}
+					})
 					.catch( err => { ctx.postMessage({ type: 'RPC', id, error: ''+err }); });
 			}
 		}
 		else {
 			let callback = callbacks[id];
 			if (callback==null) throw Error(`Unknown callback ${id}`);
 			delete callbacks[id];
+			// genId should only be sent to the main thread when initializing the generator
+			if(data.genId) { data.result.genId = data.genId; }
 			if (data.error) callback[1](Error(data.error));
 			else callback[0](data.result);
 		}