Utilize Google Maps for asynchronously geocoding addresses while ensuring their responses are received in a sequential order

I am currently working on a project that involves plotting lines between multiple addresses using Google Maps. The approach is to store the addresses in an array (in the order of the route taken), then looping over the array and geocoding them through Google Maps to obtain an array of coordinates:

function showRoute(routes) {
    var points = [];
    var geocoder = new GClientGeocoder();
    for (var i = 0; i < routes.length; i++) {
        geocoder.getLatLng(routes[i], function(point) {
            points.push(point);
            drawRoute(points, routes);
        });
    }
}

The drawRoute() function will only execute if the length of the array passed to it matches the original routes array.

While this method generally works well, it can encounter issues if there is a delay in receiving responses from the geocoder, resulting in the coordinates being out of order.

To address this issue, my initial solution was as follows:

function showRoute(routes) {
    var points = [];
    var geocoder = new GClientGeocoder();
    for (var i = 0; i < routes.length; i++) {
        geocoder.getLatLng(routes[i], function(point) {
            points[i] = point;
            drawRoute(points, routes);
        });
    }
}

However, due to the asynchronous nature of the function, the value of i within the geocode callback will always be equal to the length of the routes array, causing it to overwrite the same value repeatedly.

Does anyone have any alternative suggestions?

Answer №1

After refreshing my memory on closures by looking it up on SO, I realized the importance of understanding how they work before attempting to answer any related questions. Every time I encounter closures, I find myself revisiting the concept for clarity.

To ensure that the index variable is correctly set when the outer function returns (in this case, showRoute), I concluded that wrapping getLatLng in another function which accepts the index as a parameter would be most effective:

function handlePoint(geocoder, routes, i, points) {
    geocoder.getLatLng(routes[i], function(point) {
        points[i] = point;
        drawRoute(points, routes);
    });
}

Thus, the modified loop within showRoute now appears as follows:

var geocoder = new GClientGeocoder();
for (var i = 0; i < wp.length; i++) 
     handlePoint(geocoder, routes, i, points);

I created a simple demonstration to showcase this solution in action. The source code can be accessed here.

Answer №2

If you want to execute each geocode one by one and ensure the correct order, you can set up a callback function to trigger the next geocode operation.

For example:

let map = null;
let geocoder = null;
let nextIndex = 0;
let points = [];

const addresses = [
  "1521 1st Ave, Seattle, WA",
  "2222 2nd Ave, Seattle, WA",
  "14 Mercer St, Seattle, WA"
];

function initialize() {
  if (GBrowserIsCompatible()) {
    map = new GMap2(document.getElementById("map_canvas"));
    map.setCenter(new GLatLng(47.61630, -122.34546), 13);
    map.setUIToDefault();
    geocoder = new GClientGeocoder();
    geocodeAll();
  }
}

function geocodeAll() {
  if (nextIndex < addresses.length) {
    // Geocode the next address.
    geocoder.getLatLng(addresses[nextIndex], callBack);
  } else {
    doneCallback();  // All geocoding operations completed.
  }
  nextIndex += 1;
}

function callBack(point) {
  points.push(point);
  geocodeAll();
}

// Callback function when all geocodes are done.
function doneCallback() {
  // Add a polyline to connect the geocoded points.
  const poly = new GPolyline(points);
  map.addOverlay(poly);

  // Add markers for each geocoded point.
  for (let i = 0; i < points.length; ++i) {
    map.addOverlay(new GMarker(points[i]));
  }
}

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