How to received promises from backend in Firebase httpscallable().call()? - ios

Question

I've been playing around with stripe and would like to learn how to get ephemeral keys in the following way:
Back-end:
//Stripe API requirement for payment
exports.stripeEphemeralKey = functions.https.onCall((data, context) => {
const uid = context.auth.uid;
//Get values
admin.database().ref().child("users").child(uid)
.on("value", (snapshot) =>{
//Get user data
let user = snapshot.val()
//Log data
console.log("Create ephemeral key for:")
console.log(user)
//Create ephemeral key
stripe.ephemeralKeys.create(
{customer: user.customerid },
{stripe_version: '2018-11-08'}
)
.then((key) => {
console.log(key)
console.log("Succesful path. Ephemeral created.")
return "Testing"
})
.catch((err) => {
console.log("Unsuccesful path. Ephemeral not created.")
console.log(err)
return {
valid: false,
data: "Error creating Stripe key"
}
})
})
})
Client-side:
functions.httpsCallable("stripeEphemeralKey").call(["text": "Testing"]) { (result, error) in
print(result?.data)
}
I have tested this code by replacing the body of the stripeEphemeralKey with a simple "Testing" string and that returns just fine. But with the code above I just get Optional() back.
For testing I added lots of console logs. Firebase logs show the execution path gets to the "Succesful path. Ephemeral created." log, and furthermore I can actually see the ephemeral key I get back from stripe.
So, what is the proper correct way to get the ephemeral key in Swift for iOS using the onCall Firebase function?
The backend does what it should, but I can't seem to get the answer back.
Thank you.

Answer 1

The backend does not actually do what it should do. You're doing at least two things wrong here.
First, your callable function needs to return a promise that resolves with the value that you want to send to the client. Right now, your function callback isn't returning anything at all, which means the client won't receive anything. You have return values inside promise handlers, but you need a top-level return statement.
Second, you're using on() to read data from Realtime Database, which attaches a listener that persists until it's removed. This is almost certainly never what you want to do in a Cloud Function. Instead, use once() to get a single snapshot of the data you want to read, and act on that.

Answer 2

For my own reference, and those who might find this helpful:
//Stripe API requirement for payment
exports.stripeEphemeralKey = functions.https.onCall((data, context) => {
const uid = context.auth.uid;
return admin.database().ref().child("users").child(uid)
.once("value", (snapshot) =>{
console.log(snapshot.val() )
})
.then( (snap) => {
const customer = snap.val()
//Log data
console.log("Create ephemeral key for:")
console.log(customer)
//Create ephemeral key
return stripe.ephemeralKeys.create(
{customer: customer.customerid },
{stripe_version: '2018-11-08'}
)
.then((key) => {
console.log(key)
console.log("Succesful path. Ephemeral created.")
return {
valid: true,
data: key
}
})
.catch((err) => {
console.log("Unsuccesful path. Ephemeral not created.")
console.log(err)
return {
valid: false,
data: "Error creating Stripe key"
}
})
})
.catch( err => {
console.log("Unsuccesful path. Ephemeral not created.")
console.log(err)
return {
valid: false,
data: "Error gettting customerid"
}
})
})
The key seems to be to chain the initial database request with .then(), and do our our work chaining the returns uninterrupted as we use functions that return promises. In particular, placing my work-code inside the callback on the original admin.database().ref().once() function did not work for me.
I am new to this kind of programming, so someone who knows about this might the why better.