CS 185C/286 - Lecture 2

Cay S. Horstmann

Lecture 2 Quiz 1

Put your answer into Piazza.

What is “detached mode” in Eclipse?

1. Executing a program without the debugger attached.
2. A view in a separate window.
3. A popup that shows content assistance.
4. A philosophical approach to contemplating the Eclipse framework.

Lecture 2 Quiz 2

What does the author least like about Eclipse?

1. The plugin architecture.
2. FindBugs.
3. Refactoring.
4. The Run As menu.

Today's Lecture

• Catch up with lab 1
• Just enough to do homework 1
• Layouts
• IDs
• Async tasks
• LogCat
• Permissions

Lab

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

Layouts

• Java Swing: Programmatic (e.g. GridBagLayout)
• HTML: Declarative
• Visual Basic: Drag and Drop
• Android: All of the above
• Easiest approach: Start with Drag and Drop
• Essential to look at the XML
• For now, just use RelativeLayout
• Pay attention to android:layout_below, android:layout_toRightOf

IDs

• Can specify TextView strings declaratively.
• Either with android:text="Fred" or with android:text="@string/main_character_name"
• Won't work when we need to update text dynamically.
• Assign an ID to each component
• Right-click in GUI builder
• In XML, set android:id="@+id/questionText"
• Get ID as R.id.questionText in Java code

  TextView t = (TextView) findViewById(R.id.questionText);
  t.setText("How old is Fred?");
  

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 Ch.3 Concurrency in Android until the end of AsyncTask and the UI Thread (not including Threads in an Android Process)