So I hacked together a small api, Days Since, what it does is returns the days passed since an inputted date. It's a small Rails site listening to one route (year/month/day) and then calculates the offset compared to the current date. For fastest way of hosting I put it on my Heroku account, the free version is more than enough for the small load it will generate. The index page looks rather boring since I lack all forms of UI skills :) Source can be found on github.

I've been working on a project involving sockets, threads and other fun stuff and thought it might be something worth writing about. First something that is good to know if you want to connect to your computer from your emulator the IP address to use is 10.0.2.2. First a quick refresh on the activity life cycle in android. The important part here is what happens at onPause Other applications need memory in my experience that doesn't happen on your emulator but when the end user loads it on his/her phone it's usually full of applications causing all kinds of problems if you assume that the application will remain active after it leaves focus.

 

So in this activity I've implemented onResume and onPause (I've omitted onCreate in the sample), the way this sample works is that it connects to the server when the activity is activated and when it is put into background by launching another activity or returning to the launcher the socket is closed. In onResume I create a socket using the ip of my computer hosting the emulator and port 12345, then i get the input and output stream from the socket and use them to create a BufferedReader and a BufferedWriter. Next step is to create a Thread that will wait for the socket to receive data and when it does it calls the UI thread using the runOnUiThread method. This is done since only the thread creating the UI is allowed to interact with controls created there. The thread runs in a loop checking that the thread hasn't been interrupted (more on this soon), the check to see if the result from readLine is null is to handle when the server closes the connection. In onPause I first interrupt the thread, this will cause the while loop in the receive thread to end, then i close the input and output streams followed by the socket itself. By closing the input stream the receive thread gets a result from readline and since the thread has been interrupted the thread reaches it's end. The last little method is the one used from say a button to send a string to the server.

public class MainActivity extends Activity {
	private Socket sock;
	private BufferedReader r;
	private BufferedWriter out;
	private Thread thrd;

	@Override
	public void onResume() {
		super.onResume();
		try {
			sock = new Socket("10.0.2.2", 12345);
			r = new BufferedReader(new InputStreamReader(sock.getInputStream()));
			out = new BufferedWriter(new OutputStreamWriter(sock
					.getOutputStream()));

			thrd = new Thread(new Runnable() {
				public void run() {
					while (!Thread.interrupted()) {
						try {
							final String data = r.readLine();
							if (data != null)
								runOnUiThread(new Runnable() {
									@Override
									public void run() {
										// do something in ui thread with the data var
									}
								});

						} catch (IOException e) { }
					}
				}
			});
			thrd.start();
		} catch (IOException ioe) { }
	}

	@Override
	public void onPause() {
		super.onPause();
		if (thrd != null)
			thrd.interrupt();
		try {
			if (sock != null) {
				sock.getOutputStream().close();
				sock.getInputStream().close();
				sock.close();
			}
		} catch (IOException e) {}
		thrd = null;
	}

	private void sendText() {
		String text = inputText.getText().toString();
		try {
			out.write(text + "\n");
			out.flush();
		} catch (IOException e) {}
	}
}

Things to improve, since I connect in onResume that should be the place to put logic to load host name/port from some preferences and also checks to see if the phone has any network connection active (3g or wifi). And to have something to talk to during testing here's a simple ruby server that sends the current time to the client when it connects and then just echos whatever the client sends.

require 'socket'
serv = TCPServer.new(12345)
while true do
  begin
    s = serv.accept
    while true do
      s.puts Time.now
      foo = s.gets
      break if foo == nil
      puts foo
      s.puts foo
    end
    s.close
  rescue
  end
end

To continue the theme of pdf creation in websites. Today I bring you a way of using wkhtmltopdf to render html pages as pdfs, since it uses webkit to render html the result looks the same as the page does in Chrome or Safari. To avoid creating a bunch of pdfs on disk and then reading them back before sending them to the client I choose to use the option of sending the pdf to the standard outputstream for the wkhtmltopdf process, that way I simply read the result for that stream and write it on the asp.net Response. To get wkhtmltopdf to send the result on stdout I set the second parameter to a - instead of a output filename. The first is set to the url of the html page I want turned into a pdf. One thing that might cause problems for you here is that since the request to get the html is send for an external process it won't share the session data so if your page relies on some information that is present in the clients session (authentication etc) you need to pass that information in the request. Another possible solution is presented after the code example.

string args = string.Format("\"{0}\" - ",Request.Url.AbsoluteUri);
var startInfo = new ProcessStartInfo(Server.MapPath("tools\\wkhtmltopdf.exe"), args)
{
  UseShellExecute = false,
  RedirectStandardOutput = true
};
var proc = new Process {StartInfo = startInfo};
proc.Start();

string output = proc.StandardOutput.ReadToEnd();
byte[] buffer = proc.StandardOutput.CurrentEncoding.GetBytes(output);
proc.WaitForExit();
proc.Close();
Response.ContentType = "application/pdf";
Response.BinaryWrite(buffer);
Response.End();

Possible improvements, since wkthmltopdf also supports sending the html to convert to the process on standard input (by setting the first argument to -) it's possible to start the process and send the rendered page to process and then the result to the client.

I've been working on a Robot for a while and it's finally in a state that I can call working. Roamerbot seemed like a good name for it since it roams my house trying not to run into things. A previous post covers the parts used, if I find a good tool I'll try to create a nice schematic. Until then the setup is as following: 4 dc motors connected to the motor shield, 1 server connected to servo 1 on the shield and a Ping))) sensor connected to pin 2. To power the bot I use 2 sources, a 9v battery to power the Arduino board and Ping sensor and 5x1.5 AAA batteries to power the motors and servo. To hold everything in place i decided to use zip ties, not the most aesthetic solution but it works. The sketch used to run the bot is hosted on github. Most parts are quite simple, the only hack I had to add was that each time I measure range I do it 3 times and return the average. The reason for this is that in some cases (once every 20 or so times) I got a reading that was of, usually way to low, which caused the bot to start to get surrounding readings and then turn. Left to solve is to handle when the Bot runs into items that sits below the sensor so that they're not detected and the wheels just spins, might be a slightly harder problem to solve. Other fun things to add would be some kind of mapping to make it possible to build a 2d mapping of the room the bot is in. Roamerbot in action, had to chase it since it was running way for me.