# Async Post Message
[](https://codeium.com?repo_name=khou22%2Fasync-post-message) [](https://github.com/dwyl/esta/issues) 
[](https://nodei.co/npm/async-post-message/)
The `async` / `await` for window-based communication. Fully typed and works with Typescript, React, and NextJS.
``` typescript
const resp = await window.postMessage(req)
```
## Demo
[](/docs/demo.gif)
## Motivation
I have been using the Javascript `postMessage` [[docs](https://developer.mozilla.org/en-US/docs/Web/API/Window/postMessage)] to communicate between frames, but have been frustrated that communication is not strongly guaranteed. You can send a message reliably, but there is no notion of an async call --> response. Rather, you can send a message to the other context and hope you get a response. You need to instrument listening to the correct response and yet this is still quite complex if you want to run an `async` request.
In this demo, I create a promise wrapper around the `postMessage` Javascript API and handle sending messages between contexts so that you can simply run `await asyncPromise.send('functionName', [...args])` in your client code.
## Installation
Installation with popular package managemers:
```shell
npm install async-post-message
```
```shell
yarn add async-post-message
```
```shell
bun install async-post-message
```
## Usage
### Getting Started
Define the promises types that you would like to execute across the frame contexts:
```typescript
export type MyPromises = {
getText: () => string;
multiplyByFour: (n: number) => number;
};
```
The parent process needs to be set up to handle the promise requests:
```typescript
var iframe: HTMLIFrameElement = ...;
const unsubscribe = handleWebViewRequest(
iframe.contentWindow,
async (request) => {
const { uid, functionName, args } = event.data;
switch (functionName) {
case "multiplyByFour": {
iframeRef.current.contentWindow.postMessage({
uid,
functionName: "multiplyByFour",
response: 4 * args[0],
});
break;
}
}
}
);
```
On the iFrame page (or other web view that can `postMessage`), create a new `AsyncPostMessage` singleton instance with the promise interface as the generic argument. You can then call `execute` with the function name and signature.
```typescript
const asyncPostMessage = WebViewRequester.getInstance();
// Execute the asynchronous request to the parent.
const response = await asyncPostMessage.execute("multiplyByFour", 4);
console.log(response); // 16
```
### Usage in React
You may want to use this system in React. The big change here is that you'll want to wrap things in `ref`'s and `useEffect`s.
Define the promises types that you would like to execute across the frame contexts:
```typescript
export type MyPromises = {
getText: () => string;
multiplyByFour: (n: number) => number;
induceError: () => boolean;
};
```
#### Parent Window
The parent process needs to be set up to handle the promise requests:
```tsx
const iframeRef = useRef(null);
useEffect(() => {
if (!iframeRef.current?.contentWindow) return;
const unsubscribe = handleWebViewRequest(
iframeRef.current.contentWindow,
async (request) => {
const { uid, functionName, args } = request;
switch (functionName) {
case "multiplyByFour": {
const argsTyped = args as Parameters;
const response = 4 * argsTyped[0];
return {
uid,
functionName: "multiplyByFour",
response,
};
}
}
);
return () => {
unsubscribe();
};
}, []);
```
#### iFrame Web View
On the iFrame page, create a new `AsyncPostMessage` instance with the promise interface as the generic argument.
```tsx
const asyncPostMessage = useRef(new AsyncPostMessage());
// Check to ensure it can run in an iFrame.
useEffect(() => {
if (!window) {
setError(new Error("Not an iFrame"));
}
asyncPostMessage.current = WebViewRequester.getInstance();
}, []);
```
Now to call a promise you can simply call the `send()`:
```ts
const response = await asyncPostMessage.current.send("multiplyByFour", 4);
console.log(response); // 16
```
## Development
### Building
1. `yarn install`
2. `yarn build` which will generate the `dist/` folder
3. To deploy to NPM, run `npm publish`
### Running Demo Locally
First, navigate to `/apps/www`.
1. Install dependencies: `bun install`
2. Run dev server: `bun dev`
3. Open [http://localhost:3000](http://localhost:3000) with your browser to see the result.