Watch features

[james-pre]

Originally jvilk/BrowserFS#163

By @amir-arad

[james-pre]

Closing this since it is part of #24

[james-pre]

Reopening since this will be done separately from #24, due to the internal API changes that will be needed.

[james-pre]

Noting my thoughts on how the watch features can be implemented, keeping in mind that the goal is to implement the various watch functions:

File changes must be detected and an event dispatched to the listener passed to a watch function.

One easy way to do this would be to have FileSystem extend EventEmitter. Then, some events could be defined.

Perhaps:

interface FileSystemEvent {
	path: string;
	// more can be added
}

abstract class FileSystem extends EventEmitter<Record<'change' | 'rename' | 'create' | 'delete', FileSystemEvent>> {
	//...
}

// Then, an implementation can emit the event

class SomeFS extends FileSystem {
	//...
	async rename(path, /*...*/) {
		this.emit('rename', { path });
		//...
	}
	//...
}

// Finally, the watch functions can listen for these events

interface Watcher {
	path: string
	listener: WatchListener
}

const watchers: Set<Watcher>

function watch(path, listener) {
	watchers.add({ path, listener })
}

// And they will be caught

function watchListener(eventType, event) {
	for(const { path, listener } of watchers) {
		if(event.path != path) { // or some other way to match it
			continue;
		}
		
		listener({
			eventType,
			filename: path
		});
	}
}

function mount(point, fs) {
	//...
	fs.on('change', (event) => watchListener('change', event));
	fs.on('rename', (event) => watchListener('change', event));
	//...
}

[james-pre]

Note the approach I drafted above was not used, instead the watch features were implemented in the emulation layer.