Get coordinates from photo with JavaScript

When you take a photo with your mobile phone, photos will be geotagged. This information could be used for to show on a map where the photo is taken. This post will show you how to read this information with HTML5 and JavaScript.

To help me with this I using to external librarys written by Jacob Seidelin, binaryajax and exif-js.

To read the input file I using FileReader API in HTML5.

The coordinates returned from the EXIF data is in WGS84 degrees/minutes/seconds, for to show them on a map they need to be converted to WGS84 decimal.

//Get the photo from the input form
var input = document.getElementById('Files');
var files = input.files;
for(var i = 0; i < files.length; i++)
{
            var file = files[0];
            var reader = new FileReader; // use HTML5 file reader to get the file
 
            reader.onloadend = function () {
                // get EXIF data
                var exif = EXIF.readFromBinaryFile(new BinaryFile(this.result));
 
                var lat = exif.GPSLatitude;
                var lon = exif.GPSLongitude;
 
                //Convert coordinates to WGS84 decimal
                var latRef = exif.GPSLatitudeRef || "N";  
                var lonRef = exif.GPSLongitudeRef || "W";  
                lat = (lat[0] + lat[1]/60 + lat[2]/3600) * (latRef == "N" ? 1 : -1);  
                lon = (lon[0] + lon[1]/60 + lon[2]/3600) * (lonRef == "W" ? -1 : 1); 
 
               //Send the coordinates to your map
                Map.AddMarker(lat,lon);
            } 
            reader.readAsBinaryString(file);   
}

Real time communication with signalR

If you need realtime communication and don’t want to use long polling ajax calls for example for an game or a chat and also want fallback if WebSockets not is supported, signalR is a good library. This is a short tutorial on how to use signalR in an ASP.NET MVC application.

If you want to use a WebSocket connection if it is supported by the client you need to use IIS8 and enable the WebSocket Protocol, read this post for a step by step guide.

If websocket is not supported, signalR has fallback to server side events and ajax long polling, so web browsers without support for websockets can also use signalR.

Add signalR libraries

The easiest way to add signalR to your web application is to use NuGet. It will add the assemblies and javascript libraries needed to the web project.

Install-Package Microsoft.AspNet.SignalR

Server side

First you need to set “aspnet:UseTaskFriendlySynchronizationContext” to true in settings in your Web.config if you want to use websocket.

  <appSettings>
    <add key="aspnet:UseTaskFriendlySynchronizationContext" value="true"/>
  </appSettings>

Next step is to create a new class that inherits from PersistentConnection

public class MyConnection : PersistentConnection
{
}

In this new class you can override the methods OnConnected, OnReceived and OnDisconnected.

OnConnected
This method is called when a client connecting. Here can you for an example add users to users online lists.

OnReceived
This method is called when a message from the client is received, you send it further to your buisness logic or an other user (or users).

OnDisconnected
This method is called when a client disconnecting. Here can you for an example remove users from users online lists.

In my example I have a player object that handles logic for a player. To keep the player object alive I add a static dictionary with the connection id as key.

public class MyConnection : PersistentConnection
{
        private static Dictionary<string, Player> _players;
 
        protected override System.Threading.Tasks.Task OnConnected(IRequest request, string connectionId)
        {
            if (_players == null)
            {
                _players = new Dictionary<string, Player>;();
            }
 
            var player = new Player();
 
            _players.Add(connectionId, player);
 
            player.SomethingHappend += (sender, args) =&gt; Connection.Send(connectionId, "Hello!");
 
            return base.OnConnected(request, connectionId);
        }
 
        protected override System.Threading.Tasks.Task OnReceived(IRequest request, string connectionId, string data)
        {
            var player = _players[connectionId];
            player.DoStuff(data);
 
            return base.OnReceived(request, connectionId, data);
        }
 
        protected override System.Threading.Tasks.Task OnDisconnected(IRequest request, string connectionId)
        {
            var player = _players[connectionId];
 
            player.Dispose();
 
            return base.OnDisconnected(request, connectionId);
}
        }

Before you can start using your signalR server you need to map a route in Global.asax.cs. Add the route to the Application_Start method before other route mappings.

protected void Application_Start()
{
            RouteTable.Routes.MapConnection<MyConnection>("connection", "/my");
}

The url to the signalR server will now be http://pageurl/connection/my.

Connection.Send(connectionId,message) sending a message to a specific user using the connection id. It make’s it possible to have communications between two clients.

If you want to send a message to all connected users you should use Connection.Broadcast(message).

Client side

Start to add a reference to the signalR javascript library.

Connect
To connect to the signalR server use the relative url that you created with the mapping in Global.asax.cs.
If you want to know when the connection has successfully established use callback on the start function.
Use the stateChanged event if you want to know when states changes on the connection. For example you can use it for to tell users that connection has been lost.

The connection has four states:

  • Connecting
  • Connected
  • Reconnecting
  • Disconnected
var myConnection = $.connection('/connection/my');
myConnection.start(callback);
        myConnection.stateChanged(function (change) {
            if (change.newState === $.signalR.connectionState.connected) {
 
            }
        });

Send message
To send a message use the send(message) method on the connection object.

myConnection.Send(message);

Received

myConnection.received(function (message) {
     //Handle the message here.
});

That is all. It is not harder than that to use real time communication in your web application.

Browser mode in Internet Explorer

We building a web version of WordRoom, today when I tested the game on Surface RT it doesn’t look like expected. I tested in desktop and I had same issue there as I had in the Windows store app of Internet Explorer. But in desktop mode I could use Internet Explorer developer tools. It shows that Internet Explorer was in IE7-mode. Because we use a lot of HTML5 features, we want to run it in IE10-mode from start.

I searched the web for solution and I found a meta tag that fixed my issue.

<meta http-equiv="x-ua-compatible" content="IE=10" />

If you want to read more about this, read this page, http://msdn.microsoft.com/en-us/library/jj676915%28v=vs.85%29.aspx

Authentication with Azure Mobile Services

When developed the multiplayer part of WordRoom we discussed to use different authentication providers. But it was a lot of code to integrate each provider. And we wanted more than one. Instead we developed our own solution.

But just ask yourself, do you want to create an account everywhere with different username and passwords. I don’t. When we discovered Azure Mobile Services it was an obvious choice. Mobile Service gives you authentication with Microsoft-, Google-, Facebook and Twitter account and it’s for free. You only need to connect it to a database server. But we already had a database server in Azure. So i next version of WordRoom, you can authenticate with your Microsoft-, Google-, Facebook- or Twitter account.

With just a few rows of code you can authenticate the user with one of the authentication providers.
That Mobile Service is for five different platforms is a huge strength. You can use it for Windows Store applications, Windows Phone, Android, iOS and HTML5/JavaScript. If you use Xamarin Studio for developing your Android and iOS apps, there is a third part library.

In a Windows Store application you just add the SDK, some rows in App.xaml.cs and one row where you want to authenticate the user.

user = await App.MobileService
                .LoginAsync(MobileServiceAuthenticationProvider.Facebook);

Read more about Mobile Services