On subscription run a "callback" function once - ionic-framework

Question

I'm working with cordova's BLE (bluetooth low energy)
After I subscribe to notifications of BLE (which returns Observable), I want to send some message to the ble device, what is the best way to perform this, basically I need to run a function once after the subscription is made so that once device responds back to me, the code in the subscription is run.
ble.startNotification(deviceId, uuid1, uuid2).subscribe(bufferData=> {
//do something with bufferData
})
now after this, I want to run something like a callback,
.then(()=> {
//send message to device (only once), after the message is sent, the device will respond back and the `do something with bufferData` code will be run
})
I could easily do a setTimeout and send a message to the device after few seconds, and of course it works, but I want to do it cleanly, after I'm sure the subscription happened (subscription to the Observable of course)

Answer 1

You can wrap existing method using create operator and add custom code that will be executed on every new subscription.
See the example:
// emulate cordova with "dummy" bluetooth interface
const BLE = {
startNotification: () => Rx.Observable.interval(1000)
}
const wrappedBLE = (...params) =>
Rx.Observable.create(observer => {
// constructor fn will be executed on every new subscribtion
const disposable = BLE.startNotification(...params).subscribe(observer);
// place code to send notification here, instead of console log
console.log('New subscriber for BLE with params: ', params);
return disposable;
});
wrappedBLE("param1", "param2", "param3")
.subscribe(e => console.log("received notification: ", e));
<script src="https://unpkg.com/rxjs#5.4.3/bundles/Rx.min.js"></script>