Building a Detail View

LEARNING OBJECTIVES

After this lesson, you will be able to:

Use a Singleton to open a database connection from master and detail views
Determine the data to pass from master to detail view
Create a detail View in a new Activity
Override the correct methods to open a detail view from a single item in a list
Pass a database connection between master and detail views

STUDENT PRE-WORK

Before this lesson, you should already be able to:

Create new Activities with Intents
Pass data between activities
Retrieve data from a SQLite database

INSTRUCTOR PREP

Before this lesson, instructors will need to:

Read through the lesson
Add additional instructor notes as needed
Edit language or examples to fit your ideas and teaching style
Open, read, run, and edit (optional) the starter and solution code to ensure it's working and that you agree with how the code was written

Opening (5 mins)

All of our apps with databases so far have been in a single Activity. As you can imagine, most apps have more than one screen and often need to access the database across each screen. Even something as simple as a to-do-list would need access to the same information in different activities, such as the list item on one screen, but all of the details on the next. All of this information needs to be pulled from the same database. Today we will learn about the correct way to access the same database to gather information across multiple activities.

Check: Ask the students what parameters we pass into the Database Helper to open the connection. Emphasize the context parameter.

Introduction: Opening Multiple Database Connections (15 mins)

In our SQLiteOpenHelper, one of the parameters we pass is the context. So far we have used MainActivity.this and left it at that.

If you remember, context works similar to scope, and if we move to another activity, we don't want to be using the context of the previous activity. Opening a new database connection on each activity can become very expensive in terms of system resources, and it's possible to run into conflicts with multiple things trying to change the database at once. How do we get around this?

The first way is to use the Singleton pattern. Instead of creating a new instance of the Helper class in each activity, we can initialize it once, and use it everywhere after that.

The second way is by using something called a ContentProvider, which we will cover later in the course. Using a Singleton is a very common way to go about doing this though.

Check: Ask the students why we can't just pass the Helper object between activities.

Codealong: Using a Singleton with SQLiteOpenHelper (20 mins)

The first step to making this a Singleton is to make a private static instance of the class as a member variable:

private static ExampleSQLiteOpenHelper mInstance;

Next, we have to make the constructor private, and create a getInstance method. In getInstance, we need to use the context getApplicationContext:

public static ExampleSQLiteOpenHelper getInstance(Context context){
  if(mInstance == null){
    mInstance = new ExampleSQLiteOpenHelper(context.getApplicationContext());
  }
  return mInstance;
}

Finally, in our MainActivity, we need to use getInstance instead of calling new:

ExampleSQLiteOpenHelper helper = ExampleSQLiteOpenHelper.getInstance(MainActivity.this);

That's it! Now our SQLiteOpenHelper is using a Singleton!

Check: Ask why we called getApplicationContext?

Codealong: Adding a detail view (20 mins)

Let's use this to create a detail view which will also access the database.

Since we want to get the data for the individual item we click on, we need to pass the id column value into the intent in our MainActivity in the item click listener.

Since we have the cursor, we can move it to the clicked position, and get the id.

listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
  @Override
  public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
    Intent intent =  new Intent(MainActivity.this,DetailActivity.class);
    cursor.moveToPosition(position);
    intent.putExtra("id",cursor.getInt(cursor.getColumnIndex(ExampleSQLiteOpenHelper.COL_ID)));
    startActivity(intent);
  }
});

Let's add a helper method to get the description for the item by id.

public String getDescriptionById(int id){
  SQLiteDatabase db = this.getReadableDatabase();

  Cursor cursor = db.query(EXAMPLE_LIST_TABLE_NAME,
                new String[]{COL_ITEM_DESCRIPTION},
                COL_ID+" = ?",
                new String[]{String.valueOf(id)},
                null,
                null,
                null,
                null);

  if(cursor.moveToFirst()){
    return cursor.getString(cursor.getColumnIndex(COL_ITEM_DESCRIPTION));
  } else {
    return "No Description Found";
  }
}

Then in our DetailActivity, we have to get the description and put it in our TextView.

ExampleSQLiteOpenHelper helper = ExampleSQLiteOpenHelper.getInstance(DetailActivity.this);

int id = getIntent().getIntExtra("id",-1);

if(id >= 0){
    String description = helper.getDescriptionById(id);
    TextView textView = (TextView)findViewById(R.id.description_text);
    textView.setText(description);
}

Independent Practice: Icon List v2 (25 mins)

Instructor Note: This can be a pair programming activity or done independently.

We are going to take the Icon List example from the Cursor Adapters lesson and make it into two activities. The first activity will just show the icon, and when you click on it, the second activity will show the text for that icon.

Check: Were students able to create the desired deliverable(s)? Did it meet all necessary requirements / constraints?

Conclusion (5 mins)

Since many apps are so data-intensive today, databases play a large role in Android app development. Now you know how to create, manipulate, and access your database across multiple activities in your apps.

Detail Views