To Lecture Notes

IT 372 -- May 8, 2019

Review Questions

1. Locate the classes.dex file in the Android Studio file system for a project.
   Ans: app/build/intermediates/dex/debug/mergeProjectDexDebug/out/
2. Using the Device File Explorer, locate the students.db file for the StudentsDb1 or StudentsDb2 Example.
   Ans: /data/data/<package name>/databases
3. Which classes/objects are used when working with Sqlite databases in an Android app?

Array Adapters

• Practice Problem: How can you add items programatically to a Spinner widget?
  Hint: use an ArrayAdapter object.
  Ans: Add a Spinner widget to the layout, with id set to spinner1. Then
  
  1. place the array of Integer objects to display in the spinner in an array list,
  2. set up an ArrayAdapter object that connects the spinner to the array list.
  Here is the Java code:
  

  // Set up array list.
  ArrayList<Integer> arrList = new ArrayList<Integer>( );
  for(int n = 1001; n <= 1005; n++) {
      arrList.add(n);
  }
  
  // Set up array adapter.
  ArrayAdapter<Integer> adapter = new ArrayAdapter<Integer>(
      this, android.R.layout.simple_list_item_1, arrList);
  spinner1.setAdapter(adapter);
  

• Practice Project: ChineseHoroscope
  
  1. Input a year of birth using a Spinner widget. Use an array adapter to load the years into the spinner.
  2. Determine the Chinese horoscope animal corresponding to the year. The animal number can be obtained from year % 12.
  3. Display the animal in a TextView widget.

StudentsDb2 Example

• Look at the StudentsDb2 Example.
• Possible modifications to project:
  
  • Display all of the rows of the SQL table in a TextView widget.
  • Add a button that deletes all of the rows in the database.

Debugging Android Apps

• What techniques do you use for debugging your Android apps?
  Ans: (a) Write lines to the Logcat window with Log.i or to the Run window with System.out.println.
  (b) Comment out lines and add them back in one by one to locate the line that causes the app to crash.
  (c)Use the Android Debugger: put breakpoints in your code, which show up as maroon disks in the gray area to the left of the editor lines. Use the Debug 'app' button in the toolbar to the right to start the debugger. Step Over, Step Into, and Step Out buttons above the Debug Window at the bottom of Android Studio. Check the values of variables in the Variable Window. Here are, respectively, the Android Studio Debug 'app', Step Over, Step Into, and Step Out buttons:
  

Drawing in a View

• To draw in a view, create a subclass class derived from View, which overrides the onDraw method.
• This is similar to Swing, where you create a subclass of Panel and override the onDraw method.
• Here is a simple onDraw method:
  

  @Override
  public void onDraw(Canvas c) {
      Paint p = new Paint( );
      p.setColor(Color.RED);
      c.drawCircle(100, 120, 30, p);
  }
  

• The MyView class should also have this constructor:
  

  public MyView(Context context) {
      super(context);
  }
  

• Finally, a MyView object must be instantiated and added to the layout in the Java activity code:
  

   MyView mv = new MyView(this);
  LinearLayout ll = (LinearLayout) findViewById(R.id.layout);
  ll.addView(mv);
  

• Discuss the Drawing1 and Drawing2 Examples.

Responding to Touch Events

• The touch events that we will discuss come in three forms:
  
  1. A finger not in contact with a view comes in contact with the view (ACTION_DOWN).
  2. A finger in contact with a view moves to a different location on the view without losing contact (ACTION_MOVE).
  3. A finger in with a view raises and loses contact with the view (ACTION_UP).
• Use the setOnTouchListener to add a touch event handler to a view.
• Look at the TouchEvent1 Example.