How to Get User Location

How can I get user's location without asking for the permission or if user permitted once so need not to ask ever again?

3 ways to do this in this answer:

Get a GPS precise location asking the user to allow access to its browser's API
Get an approximate location (country, city, area) using an external GeoIP service
Get an approximate location (country, city, area) using CDN service

Ask the user to allow access to its browser's API

You can get the location of the client using HTML5 features. This will get you the exact location of the user if it is done from a device which has a GPS, or an approximate location. Once the user approved to share his location, you'll get it without any more approval.
This solution uses Geolocation.getCurrentPosition(). It is required for most cases to do this under HTTPS protocol.

If you are in a hurry, here is a CodePen with this solution: https://codepen.io/sebastienfi/pen/pqNxEa

<!DOCTYPE html>
<html>
<body>

<p id="demo">Click the button to get your coordinates:</p>

<button onclick="getLocation()">Try It</button>

<script>
var x = document.getElementById("demo");

function getLocation() {
    if (navigator.geolocation) {
        navigator.geolocation.getCurrentPosition(
            // Success function
            showPosition, 
            // Error function
            null, 
            // Options. See MDN for details.
            {
               enableHighAccuracy: true,
               timeout: 5000,
               maximumAge: 0
            });
    } else { 
        x.innerHTML = "Geolocation is not supported by this browser.";
    }
}

function showPosition(position) {
    x.innerHTML="Latitude: " + position.coords.latitude + 
    "<br>Longitude: " + position.coords.longitude;  
}
</script>

</body>
</html>

Handling Errors and Rejections
The second parameter of the getCurrentPosition() method is used to handle errors. It specifies a function to run if it fails to get the user's location:

function showError(error) {
    switch(error.code) {
        case error.PERMISSION_DENIED:
            x.innerHTML = "User denied the request for Geolocation."
            break;
        case error.POSITION_UNAVAILABLE:
            x.innerHTML = "Location information is unavailable."
            break;
        case error.TIMEOUT:
            x.innerHTML = "The request to get user location timed out."
            break;
        case error.UNKNOWN_ERROR:
            x.innerHTML = "An unknown error occurred."
            break;
    }
}

Displaying the Result in a Map

function showPosition(position) {
    var latlon = position.coords.latitude + "," + position.coords.longitude;

    var img_url = "http://maps.googleapis.com/maps/api/staticmap?center=
    "+latlon+"&zoom=14&size=400x300&sensor=false";

    document.getElementById("mapholder").innerHTML = "<img src='"+img_url+"'>";
}

GeoIP

If the solution above doesn't work in your scenario, you can obtain an approximate position using IP's location. You will not necessarily get the exact location of the user, but may end up with the location of the nearest Internet node in the user's connection point area, which may be close enough for 99% of the use cases (country, city, area).

As this is not precise but doesn't require the user's approval, this may also meet your requirements.

Find below 2 ways to do that. I would recommend doing this using the CDN solution because it is faster and yes, speed matters a lot.

Get an approximate location (country, city, area) using an external GeoIP service

Many external services allows you to obtain the GeoIP location.
Here is an example with Google.

To get your Google API Key, go here: https://developers.google.com/loader/signup?csw=1

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html>
    <head>
        <title>Get web visitor's location</title>
        <meta name="robots" value="none" />
    </head>
    <body>
    <div id="yourinfo"></div>
    <script type="text/javascript" src="http://www.google.com/jsapi?key=<YOUR_GOOGLE_API_KEY>"></script>
    <script type="text/javascript">
        if(google.loader.ClientLocation)
        {
            visitor_lat = google.loader.ClientLocation.latitude;
            visitor_lon = google.loader.ClientLocation.longitude;
            visitor_city = google.loader.ClientLocation.address.city;
            visitor_region = google.loader.ClientLocation.address.region;
            visitor_country = google.loader.ClientLocation.address.country;
            visitor_countrycode = google.loader.ClientLocation.address.country_code;
            document.getElementById('yourinfo').innerHTML = '<p>Lat/Lon: ' + visitor_lat + ' / ' + visitor_lon + '</p><p>Location: ' + visitor_city + ', ' + visitor_region + ', ' + visitor_country + ' (' + visitor_countrycode + ')</p>';
        }
        else
        {
            document.getElementById('yourinfo').innerHTML = '<p>Whoops!</p>';
        }
    </script>
    </body>
</html>

Get an approximate location (country, city, area) using CDN service

An alternative to using external services is provided by most of the CDN. The advantage of this solution is that no extra HTTP request is required, so it is faster. If you already have a CDN, this is the way to go. You can use CloudFlare, CloudFront, etc.

In this scenario, you will most likely end up with the location as a parameter of the HTTP request's header, or even directly in the window object so you can get it with Javascript. Read the CDN's documentation to know more.

Edited on mid December 2018 to add more options.

What is the simplest and most robust way to get the user's current location on Android?

Here's what I do:

First of all I check what providers are enabled. Some may be disabled on the device, some may be disabled in application manifest.
If any provider is available I start location listeners and timeout timer. It's 20 seconds in my example, may not be enough for GPS so you can enlarge it.
If I get update from location listener I use the provided value. I stop listeners and timer.
If I don't get any updates and timer elapses I have to use last known values.
I grab last known values from available providers and choose the most recent of them.

Here's how I use my class:

LocationResult locationResult = new LocationResult(){
    @Override
    public void gotLocation(Location location){
        //Got the location!
    }
};
MyLocation myLocation = new MyLocation();
myLocation.getLocation(this, locationResult);

And here's MyLocation class:

import java.util.Timer;
import java.util.TimerTask;
import android.content.Context;
import android.location.Location;
import android.location.LocationListener;
import android.location.LocationManager;
import android.os.Bundle;

public class MyLocation {
    Timer timer1;
    LocationManager lm;
    LocationResult locationResult;
    boolean gps_enabled=false;
    boolean network_enabled=false;

    public boolean getLocation(Context context, LocationResult result)
    {
        //I use LocationResult callback class to pass location value from MyLocation to user code.
        locationResult=result;
        if(lm==null)
            lm = (LocationManager) context.getSystemService(Context.LOCATION_SERVICE);

        //exceptions will be thrown if provider is not permitted.
        try{gps_enabled=lm.isProviderEnabled(LocationManager.GPS_PROVIDER);}catch(Exception ex){}
        try{network_enabled=lm.isProviderEnabled(LocationManager.NETWORK_PROVIDER);}catch(Exception ex){}

        //don't start listeners if no provider is enabled
        if(!gps_enabled && !network_enabled)
            return false;

        if(gps_enabled)
            lm.requestLocationUpdates(LocationManager.GPS_PROVIDER, 0, 0, locationListenerGps);
        if(network_enabled)
            lm.requestLocationUpdates(LocationManager.NETWORK_PROVIDER, 0, 0, locationListenerNetwork);
        timer1=new Timer();
        timer1.schedule(new GetLastLocation(), 20000);
        return true;
    }

    LocationListener locationListenerGps = new LocationListener() {
        public void onLocationChanged(Location location) {
            timer1.cancel();
            locationResult.gotLocation(location);
            lm.removeUpdates(this);
            lm.removeUpdates(locationListenerNetwork);
        }
        public void onProviderDisabled(String provider) {}
        public void onProviderEnabled(String provider) {}
        public void onStatusChanged(String provider, int status, Bundle extras) {}
    };

    LocationListener locationListenerNetwork = new LocationListener() {
        public void onLocationChanged(Location location) {
            timer1.cancel();
            locationResult.gotLocation(location);
            lm.removeUpdates(this);
            lm.removeUpdates(locationListenerGps);
        }
        public void onProviderDisabled(String provider) {}
        public void onProviderEnabled(String provider) {}
        public void onStatusChanged(String provider, int status, Bundle extras) {}
    };

    class GetLastLocation extends TimerTask {
        @Override
        public void run() {
             lm.removeUpdates(locationListenerGps);
             lm.removeUpdates(locationListenerNetwork);

             Location net_loc=null, gps_loc=null;
             if(gps_enabled)
                 gps_loc=lm.getLastKnownLocation(LocationManager.GPS_PROVIDER);
             if(network_enabled)
                 net_loc=lm.getLastKnownLocation(LocationManager.NETWORK_PROVIDER);

             //if there are both values use the latest one
             if(gps_loc!=null && net_loc!=null){
                 if(gps_loc.getTime()>net_loc.getTime())
                     locationResult.gotLocation(gps_loc);
                 else
                     locationResult.gotLocation(net_loc);
                 return;
             }

             if(gps_loc!=null){
                 locationResult.gotLocation(gps_loc);
                 return;
             }
             if(net_loc!=null){
                 locationResult.gotLocation(net_loc);
                 return;
             }
             locationResult.gotLocation(null);
        }
    }

    public static abstract class LocationResult{
        public abstract void gotLocation(Location location);
    }
}

Somebody may also want to modify my logic. For example if you get update from Network provider don't stop listeners but continue waiting. GPS gives more accurate data so it's worth waiting for it. If timer elapses and you've got update from Network but not from GPS then you can use value provided from Network.

One more approach is to use LocationClient http://developer.android.com/training/location/retrieve-current.html. But it requires Google Play Services apk to be installed on user device.

Get user's location and then show direction with one end point as the user's location using GeolocateControl mapbox

You can retrieve the user's current location each time it is updated by using the geolocate control's 'geolocate' event. For example:

geolocate.on('geolocate', function(position) {
  console.log(`latitude: ${position.coords.latitude}, longitude: ${position.coords.longitude}`);
});

Once you've retrieved the latitude and longitude as shown above, you can pass these coordinates to the Mapbox Directions API along with the coordinates of the "fixed point on the map" to generate a route between the user's current location and said fixed point.

Unable to get user's location in Flutter

I Understand your problem. In my project I am using https://pub.dev/packages/location package. Steps to be followed for getting location.

Future<void> getfieldlocation() async {
    _serviceEnabled = await location.serviceEnabled();
    if (!_serviceEnabled) {
      _serviceEnabled = await location.requestService();
      if (!_serviceEnabled) {
        return;
      }
    }

    _permissionGranted = await location.hasPermission();
    if (_permissionGranted == PermissionStatus.denied) {
      _permissionGranted = await location.requestPermission();
      if (_permissionGranted != PermissionStatus.granted) {
        return;
      }
    }
    _locationData = await location.getLocation();
    setState(() {
      fieldLatitude = _locationData.latitude;
      fieldLogitude = _locationData.longitude;
      print(fieldLatitude);print(fieldLogitude);
    });
  }

I have called this in initstate() function which only calls at start.
If you want to call it continuously
Use setduration function.

If it works don't forget to contribute,or if having any doubts give a reply.
Have a Good Day : )