GeoFire geoQuery returning data - angularjs

Question

I'm newbie with Firebase + GeoFire and I'm having trouble with geoFire query function.
I want to add in an array the results from geoQuery function and return it in a function. But the data I have manipulating inside geoQuery.on method seems out of scope or not available or due to promises, I dont know... the fact is outside the geoquery.on method the variable sellers is empty.
How can I return results from geoQuery and save it into a return variable
//Set seller position in firebase db
var setPosition = function() {
navigator.geolocation.getCurrentPosition(setPositionSuccess, error, options);
//navigator.geolocation.watchPosition(setPositionSuccess, positionError, { enableHighAccuracy:true })
};
//Get sellers near buyer position
var getSellersForCurrentPosition = function() {
navigator.geolocation.getCurrentPosition(getPositionSuccess, error, options);
//navigator.geolocation.watchPosition(positionSuccess, positionError, { enableHighAccuracy:true })
};
//Callback function from html 5 geo api
function getPositionSuccess(pos) {
var crd = pos.coords;
var currentPosition = [crd.latitude, crd.longitude];
// Query radius
var radiusInKm = 2;
var firebaseRef = new Firebase(FBURL + "/geofire/sellers/");
var geoFire = new GeoFire(firebaseRef);
var geoQuery = geoFire.query({
center: currentPosition,
radius: radiusInKm
});
var sellers = [];
var oneSeller = {};
var onKeyEnteredRegistration = geoQuery.on("key_entered", function(key, location, distance) {
oneSeller = {
id: key,
distance: distance,
location: location
};
sellers.push(oneSeller);
});
var onReadyRegistration = geoQuery.on("ready", function() {
geoQuery.cancel();
});
return sellers;
}
By the way, how accurate is html5 geolocation? Is it different between desktop browser and mobile browser?

Answer 1

Geofire monitors the sellers in the range you indicate. Any time a seller enters/exits the range, it fires a key_entered or key_exited event. These events can happen at any time after you start the query. In JavaScript terms this is often described as: the callbacks happen asynchronously.
A simple event flow, might explain what happens best:
you call getPositionSuccess()
you start a Geoquery to monitor the sellers that are in range: geoFire.query()
no sellers are immediately in range, so your callback doesn't fire
the getPositionSuccess() function is done and exits
a seller comes in range
GeoFire fires the key_entered event and your callback runs
but getPositionSuccess() has already exited, so how can it return a value?
Even if you were to wait for the first seller to come into range before returning (not possible in a browser, but it is possible in other languages/environments), how will you return the value when a second seller comes in range?
For this reason, you have to deal with asynchronous data differently. Typically you do this by moving the code that would call the getPositionSuccess() function into the function.
Say you are now trying to do this:
var sellers = getPositionSuccess(pos);
sellers.forEach(function(seller) {
addSellerToMap(seller);
});
To handle the asynchronous nature of the events, you'd move this code into getPositionSuccess:
//Callback function from html 5 geo api
function getPositionSuccess(pos) {
var crd = pos.coords;
var currentPosition = [crd.latitude, crd.longitude];
// Query radius
var radiusInKm = 2;
var firebaseRef = new Firebase(FBURL + "/geofire/sellers/");
var geoFire = new GeoFire(firebaseRef);
var geoQuery = geoFire.query({
center: currentPosition,
radius: radiusInKm
});
var oneSeller = {};
geoQuery.on("key_entered", function(key, location, distance) {
oneSeller = {
id: key,
distance: distance,
location: location
};
addSellerToMap(oneSeller);
});
}
I understand that in your use-case your sellers won't move, so it may be more intuitive to think of them as a static list. But even in this case, the results are loaded from a remote database and it will take some time before that data is loaded. The modern web loads data asynchronously and all your code will have to deal with it in a way similar to what I outlined above.