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Event bus

A crucial piece of the scripting API is the event bus, which allows you to receive user-triggered events, e.g. a file being opened, and respond to them.

If you’ve ever written JavaScript for the web, you’ll likely find it similar to the DOM event bus.

Listening

You can listen to an event using the addEventListener function on a context, like so:

// omitting the script body, this can be done anywhere with a context
context.addEventListener("event_type", (e /*: any */, context /*: ScriptContext */) => {
    // this is the event handler
});

Now, event_type is obviously not a real event identifier, so let’s try it out with one.

context.addEventListener("preload", (e /*: PreloadEvent */) => {
    console.log("We're loading a class file: ", e.name);
});

This example listens to the preload event, which is triggered every time slicer tries to read a file as a class file (user requested file load -> 0xcafebabe header found? -> trigger preload event).

The event is mutable, so you can mutate its properties to change what gets to slicer in the end.

context.addEventListener("preload", (e /*: PreloadEvent */) => {
    e.data = e.data.subarray(4); // chop off first 4 bytes
});

Chopping off 4 bytes (the magic header) is not exactly something you’d want to do, since it’d either make slicer not read the file as a class file at all or break the disassembler down the line, but you can do it!

Un-listening

If you wanted to stop listening for an event, you can use the removeEventListener function on a context:

const handler = (e /*: any */, context /*: ScriptContext */) => {
    // this is the event handler
};


context.addEventListener("event_type", handler); // listen to the event
context.removeEventListener("event_type", handler); // stop listening to the event

Dispatching

You can also dispatch your own events onto the bus using the dispatchEvent function:

context.dispatchEvent({ type: "my_custom_event" /*, myData: ... */ });