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JSON-RPC without the JSON

Evan Brass
・2 min read

I'm new to using service workers even though I've known about them for a long time. One of the things that's bothered me is that I can't write using es6 module syntax. I've become so dependent on import / export to help herd my thoughts which has made my sw coding more difficult. Second, and the reason for writing this, I've found it an interesting challenge to define the protocol between my page and it's service worker. I started with simple messages that had a type and then some parameters to which the service worker would respond with a similar type. I had a few "create-thing" and then "thing-created" messages which made me think I should use an RPC framework.

So I looked into JSON-RPC and the format is easy and matched what I need, except since I'm using message ports, I don't have to stringify anything. My RPC thing looks like this:

function rpc_client(receive_port, send_port, local_def, remote_def) {
    let id = 0;

    // Map incoming calls from the port onto the local definition:
    receive_port.addEventListener('message', e => {
        const send_port = e.source;
        const run = (async _ => {
            const data = e.data;
            if (data.method) {
                const {params, id} = data;
                const method_name = data.method;
                if (!(local_def[method_name] instanceof Function)) {
                    const error = new Error(method_name + " isn't a function.")
                    send_port.postMessage({ id, error });
                }
                const method = local_def[method_name];
                if (params.length < method.length) {
                    console.warn(new Error(
                        'Running local RPC even though fewer parameters were supplied than the function expects.'
                    ));
                }
                try {
                    let result = method(...params);
                    if (typeof result == 'object' && result.then) {
                        result = await result;
                    }
                    send_port.postMessage({ id, result });
                } catch (error) {
                    send_port.postMessage({ id, error });
                    console.error(error);
                }
            }
        })();
        if (e.waitUntil) e.waitUntil(run);
    });

    // Create an api that reflects the remote definition:
    const api = {};
    for (const key in remote_def) {
        // Sanity check
        if (!(remote_def[key] instanceof Function)) {
            throw new Error('Remote_def can only have methods.');
        }
        const min_params = remote_def[key].length;
        api[key] = (...params) => {
            if (params.length < min_params) {
                console.warn(new Error(
                    'Running RPC even though fewer parameters were supplied than the function expects.'
                ));
            }
            const call_id = ++id;
            let resolve, reject;
            const prom = new Promise((res, rej) => {
                resolve = res;
                reject = rej;
            });
            // Listen for the response to the call:
            const handler = e => {
                const data = e.data;
                if (data && data.id == call_id) {
                    if ('error' in data) {
                        reject(data.error);
                    } else if ('result' in data) {
                        resolve(data.result);
                    }
                    receive_port.removeEventListener('message', handler);
                }
            };
            receive_port.addEventListener('message', handler);

            // Call the remote precedure:
            send_port.postMessage({
                id: call_id,
                method: key,
                params
            });

            return prom;
        };
    }
    return api;
}

It checks the number of parameters provided to each call. The definitions are just objects with methods on them. Lastly, it uses e.source to get the port to send responses to which works with service workers because the messages come in from different clients.

I haven't tested it much, I just thought I'd throw it out there. If anyone knows of tools / libraries which provide service worker rpc stuff I'd love to hear about it.

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