1. Networking: Part 2 (Accessing the Internet)

2. The UI Thread When an application is launched, the system creates a “main” UI thread responsible for handling interaction with running components of the Android UI toolkit. Performing long operations such as network access or database queries on the UI thread will block the whole user interface. From the user's perspective, the application appears hung. Even worse, if the UI thread is blocked for more than a few seconds, the user is presented with the infamous “application not responding” (ANR) dialog. To keep the UI thread responsive, long operations should be performed in background/worker threads. Slide 2©SoftMoore Consulting

3. The UI Single-Thread Model Two rules for using the UI single-thread model: 1. Do not block the UI thread. 2. The Android UI toolkit is not thread-safe and must always be manipulated on the UI thread. Slide 3©SoftMoore Consulting

4. Example: Downloading a Bitmap Image (Violates Rule #1) public void onClick(View v) { Bitmap b = loadImageFromNetwork(); myImageView.setImageBitmap(b); } Slide 4©SoftMoore Consulting The user interface is blocked while the image is downloading from the Internet.

5. Example: Downloading a Bitmap Image (Violates Rule #2) public void onClick(View v) { new Thread(new Runnable() { public void run() { Bitmap b = loadImageFromNetwork(); myImageView.setImageBitmap(b); } }).start(); } Slide 5©SoftMoore Consulting myImageView is manipulated on a worker thread, not the UI thread.

6. Accessing the UI Thread from Other Threads Android offers several ways to access the UI thread from other threads. Activity.runOnUiThread(Runnable) View.post(Runnable) View.postDelayed(Runnable, long) Handler AsyncTask Slide 6©SoftMoore Consulting

7. Using the View.post(Runnable) Method public void onClick(View v) { new Thread(new Runnable() { public void run() { final Bitmap bitmap = loadImageFromNetwork(); myImageView.post(new Runnable() { public void run() { myImageView.setImageBitmap(bitmap); } }); } }).start(); } Slide 7©SoftMoore Consulting worker thread − code not run on the UI thread another thread that posts results back on the UI thread

8. Class AsyncTask Class AsyncTask (package android.os ) is a generic, abstract helper class for managing background operations that will eventually publish results back to the UI thread. It provides a convenient, easy-to-use alternative to manipulating threads and message handlers for background processing. Using class AsyncTask –create a subclass specifying types for three generic parameters –override callback methods as necessary (must override method doInBackground(Params...) ) Slide 8©SoftMoore Consulting

9. Generic Parameters for AsyncClass public class AsyncTask The three generic types used by an asynchronous task are as follows: –Params : the type of the parameters sent to the task upon execution. –Progress : the type of the progress units published during the background computation. –Result : the type of the result of the background computation. Not all types are always used by an asynchronous task. To mark a type as unused, simply use the type Void: private class MyTask extends AsyncTask {... } Slide 9©SoftMoore Consulting

10. Four Callback Methods in AsyncTask onPreExecute() – invoked on the UI thread immediately before doInBackground(). –normally used to setup the task, for instance by showing a progress bar in the user interface doInBackground(Params...) – invoked on the background thread immediately after onPreExecute() finishes executing. –used to perform background computations that can take a long time –parameters of the asynchronous task are passed to this step –result of the computation must be returned by this step –can also use publishProgress(Progress...) to publish one or more units of progress to the UI thread Slide 10©SoftMoore Consulting

11. Four Callback Methods in AsyncTask (continued) onProgressUpdate(Progress...) – invoked on the UI thread after a call to publishProgress(Progress...). –used to display any form of progress in the user interface while the background computation is still executing. For instance, it can be used to animate a progress bar or show logs in a text field. onPostExecute(Result) – invoked on the UI thread after doInBackground() finishes. –result of the background computation is passed to this step as a parameter Slide 11©SoftMoore Consulting

12. Understanding How to Use AsyncTask Using generics you can specify the types of the parameters, the progress values, and the final value of the task. The method doInBackground() executes automatically on a worker thread. Methods onPreExecute(), onPostExecute(), and onProgressUpdate() are all invoked on the UI thread. The value returned by doInBackground() is sent to onPostExecute(). Slide 12©SoftMoore Consulting

13. Understanding How to Use AsyncTask (continued) You can call publishProgress() at anytime in doInBackground() to execute onProgressUpdate() on the UI thread. You can cancel the task at any time, from any thread. Slide 13©SoftMoore Consulting Important: An AsyncTask instance must be created on the UI thread and can be executed only once; e.g., create a new AsyncTask with every button click.

14. Outline for Using AsyncTask // sample call; e.g., in an onClick() method for a button DownloadStockTask stockTask = new DownloadStockTask(); stockTask.execute("GOOG"); // inner class defined within the activity class private class DownloadStockTask extends AsyncTask { protected Stock doInBackground(String... symbols) {... // download stock information from internet } protected void onPostExecute(Stock stock) {... } Slide 14©SoftMoore Consulting string passed to method doInBackbround() returned value passed to method doOnPostExecute()

15. Example: Downloading a Bitmap Image Using AsyncTask // inner class defined within the onCreate() method private class DownloadImageTask extends AsyncTask { protected Bitmap doInBackground(String... urls) { return loadImageFromNetwork(urls[0]); } protected void onPostExecute(Bitmap result) { myImageView.setImageBitmap(result); } } Slide 15©SoftMoore Consulting

16. Example: Downloading a Bitmap Image (continued) public void onClick(View v) { AsyncTask downloadTask = new DownloadImageTask(); downloadTask.execute("http://example.com/image.png"); } Slide 16©SoftMoore Consulting

17. Getting Stock Quotes from Yahoo You can use HTTP to obtain stock quotes from yahoo.com in a comma-separated values (csv) file format. The URL has the form http://finance.yahoo.com/d/quotes.csv?s= &f= Selected special tags: –s: symbol– o : open –l: last trade with time– c : change and percent change –w: 52-week range– n : name –r: P/E ratio– x : stock exchange Slide 17©SoftMoore Consulting

18. Example: Getting Stock Quotes from Yahoo Example http://finance.yahoo.com/d/quotes.csv?s=GOOG&f=lcn Format for returned line: Slide 18©SoftMoore Consulting last trade time/price change/percent name "3:21pm - 610.97 ","+10.18 - +1.69%", "Google Inc." Important: When accessing the internet, remember to add before the application element in AndroidManifest.xml.

19. ©SoftMoore ConsultingSlide 19 Networking Application: Checking Stocks public class Stock { private static final String QUOTE_FORMAT = "&f=lcwn"; private String symbol; private String lastTradePrice; private String lastTradeTime; private String change; private String range; private String name; public Stock(String symbol) { this.symbol = symbol.toUpperCase(); }... // continued on next several slides }

20. ©SoftMoore ConsultingSlide 20 Networking Application: Checking Stocks (continued) public void load() throws MalformedURLException, IOException { URL url = new URL("http://finance.yahoo.com/d/quotes.csv?s=" + symbol + QUOTE_FORMAT); URLConnection connection = url.openConnection(); BufferedReader in = new BufferedReader(new InputStreamReader(connection.getInputStream())); String line = in.readLine(); // consume any data remaining in the input stream while (in.readLine() != null) ; in.close();... // continued } Note: This method is the part of the application that accesses the network.

21. ©SoftMoore ConsultingSlide 21 Networking Application: Checking Stocks (continued) if (line != null && line.length() > 0) { // parse the line and remove quotes where necessary String[] values = line.split(","); change = values[1].substring(1, values[1].length() - 1); range = values[2].substring(1, values[2].length() - 1); name = values[3].substring(1, values[3].length() - 1); // parse last trade time and price String lastTrade = values[0]; int start = 1; // skip opening quote int end = lastTrade.indexOf(" - "); lastTradeTime = lastTrade.substring(start, end); start = lastTrade.indexOf(">") + 1; end = lastTrade.indexOf("<", start); lastTradePrice = lastTrade.substring(start, end); }

22. Checking Stocks − Sample Screen Shot Slide 22©SoftMoore Consulting

23. Relevant Links Processes and Threads http://developer.android.com/guide/components/processes-and-threads.html AsyncTask http://developer.android.com/reference/android/os/AsyncTask.html Painless Threading http://android-developers.blogspot.com/2009/05/painless-threading.html Using the Yahoo Finance API for CSV http://www.jarloo.com/yahoo_finance/ Slide 23©SoftMoore Consulting