Bidirectional Unicall System (BUS)

With the WxEnd abstraction, we have a uniform way of communicating between different contexts, regardless of if it’s between Windows, Workers, or any combination or nested combination of them. The Bidirectional Unicall System is the layer on top of WxEnd that implements the Remote Procedure Call, or RPC.

This layer turns messaging into async function call - the implementation is basically

// this is pseudo code and greatly simplified, 
// not how the BUS is actually implemented
function callRPC() {
    return new Promise(resolve => {
        end.postMessage({
            id: 1,
            call: "foo",
            args: ["bar"]
        });
        end.onmessage = ({data}) => {
            if (data.id === 1) {
                resolve(data.returnvalue);
            }
        };
    });
}

In reality, the implementation is slightly more complicated to handle potential errors, timeouts, catching exceptions on the other end and send it back, and multiple inflight messages, and perhaps mostly importantly, muxing different protocols, which will be explained in the next chapter.