CS 175/286 - Lecture 3

Cay S. Horstmann

Lecture 3 Quiz 1

Put your answer into Piazza.

What are the most feared words in the English language?

1. I'm from the government and I'm here to help you
2. Honey, we need to talk
3. We're doing this for your own good
4. But it builds on my machine

Lecture 3 Quiz 2

Put your answer into Piazza.

In the DO NOT USE!!! version of the initGame example, which call(s) take so long that they should be called outside the UI thread?

1. start
2. initialize
3. stop
4. All of them

Today's Lecture

• Async tasks
• LogCat
• Permissions

AsyncTask

• UI activities occur in UI thread
• Your app is called whenever something interesting happens (startup, button press, etc.)
• When it's your turn, do your thing quickly and return from callback
• If you do something time-consuming (e.g. read data from the web), your app (and maybe the phone) could freeze.
• Android suspends apps with unresponsive UI
• Android won't let you do I/O on the UI thread
• Do time-consuming work on a background thread
• AsyncTask convenience class makes this easy.

AsyncTask

• class DownloadTextTask extends AsyncTask<URL, Void, String> {
       protected String doInBackground(URLs... urls) {
           Scanner in = new Scanner(urls[0].openStream());
           StringBuilder result = new StringBuilder();
           while (in.hasNext()) { result.append(in.next()); result.append("\n"); }
           return result;
       }
  
       protected void onPostExecute(String result) {
           view.setText(result);
       }
   }

• Start with

  new DownloadTextTask().execute(new URL("http://example.com/fred.txt"));
  

• Makes a new thread that runs doInBackground
• When that thread is completed, calls onPostExecute on the UI thread
• First type parameter is for launch arguments (using varargs)
• Second type parameter is for progress reports (not covered now)
• Third type parameter is result type

LogCat

• Remember—no System.out
• Log manager collects, presents logging messages
• Can see LogCat output in Eclipse
• Can attach device to USB and get logs
• Lots of apps to inspect the logs on a device, e.g. CatLog
• API makes it as easy as possible:

  Log.e(getClass().toString(), exception.getMessage());
  

• Log.d for debug, Log.w for warnings etc.
• First parameter is tag (for filtering), second is message text
• This view is very cramped; makes sense to detach.

Permissions

• Any app can put up buttons, images, etc. on the screen
• Everything that might cost the user money or compromise their privacy requires permission
• Permissions displayed to user at install time
• Permissions must be entered in app manifest
• To read data from internet, need android.permission.INTERNET
• Can set in Eclipse—choose Use Permission button and select permission from menu
• Or edit XML file and add

  <uses-permission android:name="android.permission.INTERNET"/>

Reading Before Next Class

• Programming Android Chapter 6 until Wiring up the Controller. Stop just before Listening to the Model.

Lab

• Bring your laptop to each class
• You will work with a buddy
• One of you writes code, the other types up answers
• Switch roles each lab
• Submit lab work to github

Reading from the Web

• In the preceding lab and homework, you built an app that displayed a question with four choices. Now let's read the question and four answer options from the web, specifically from here. Here is the code for reading the contents of a web page into a list, a line at a time:

                  ArrayList<String> result = new ArrayList<String>();                
                  try {
                      URL url = new URL(urlString);
                      Scanner in = new Scanner(url.openStream());
                      while (in.hasNextLine()) 
                          result.add(in.nextLine());
                  in.close();
                  } catch (IOException e) {
                      Log.e(getClass().toString(), e.getMessage());
                  }

  Place this code in your onCreate method, and then set each TextView to results.get(0), results.get(1), and so on.
• Be sure to set the internet permission in the manifest.
• Also in the manifest, set the minimum and target API levels to 17 (4.2). What happens when you run your program?
• Look in the LogCat output. What error message do you see that is related to the problem?
• Do you see any output that could have come from the Log.e call? How do you know for sure?
• Put breakpoints in the three red lines above. Run the debugger. The first breakpoint will hit. Then click on Continue in the debugger. Which of the breakpoints get hit next?
• What do you conclude from these observations?
• Use an AsyncTask to solve this problem. How did you do this?