MAP524/DPS924 Lecture 5

More complex views

Adapters

We'll finish the Spinner example from last week's lecture. Unlike the other Views we looked at last week it requires the use of an Adapter. See the official documentation here. Note that in the setup you have:

• A store of data (an array of values)
• A UI element to display those values (the spinner)
• An adapter to connect the first two (specifically here an ArrayAdapter)

Here's a nice tutorial on Adapters generally with a ListView as a specific example.

ListView

This should be as simple as a spinner, it appears to do essentially the same thing, but it's more complicated to build. Follow these steps:

• Create a new Android app and name it My List.
• Add a ListView below the TextView in your activity_main.xml file.
• Create a second layout file named "activity_listview.xml" with root element set to TextView (no layout element). It should look like this

<!?xml version="1.0" encoding="utf-8"?>

<textview xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent" 
    android:layout_height="match_parent"/>

• Create a new arrays.xml file inside your values directory.
• Add a String array named colourNames as a class variable in MainActivity like this:

String[] colourNames;

• Tie your array to your xml array like this :

colourNames = getResources().getStringArray(R.array.listArray);

• Create a ListView object pointing to your list view like this:

ListView lv = (ListView) findViewById(R.id.listView);

• Create an array adapter and attach it to your list view like this:

ArrayAdapter aa = new ArrayAdapter(this, R.layout.activity_listview, colourNames);
lv.setAdapter(aa);

• You can now run your app to see your scrollable list of colours.
• Now add an onClickListener to your list view like this:

lv.setOnItemClickListener(new AdapterView.OnItemClickListener() {
    public void onItemClick(AdapterView parent, View view, int position, long id) {
        Toast.makeText(getApplicationContext(), ((TextView) view).getText(), Toast.LENGTH_SHORT).show();
    }
});

• Run your app again and this time select a colour by clicking it.
• You should see a Toast with the selected colour name pop up.

Context Menu

Add a Context Menu to the Scrollable List in the ListView example above. Official documentation here.

• First let's register the list view for a Context Menu by adding this line to our activity :

registerForContextMenu(lv);

• Now add this method to create our context menu:

@Override
public void onCreateContextMenu(ContextMenu menu, View v, ContextMenu.ContextMenuInfo menuInfo) {
    super.onCreateContextMenu(menu, v, menuInfo);
    menu.setHeaderTitle("Select The Action");
    menu.add(0, v.getId(), 0, "Make a Phone Call");
    menu.add(0, v.getId(), 0, "Send an SMS Message");
}

• Now add the listener method like this:

@Override
public boolean onContextItemSelected(MenuItem item) {
    if (item.getTitle() == "Make a Phone Call") {
        Toast.makeText(getApplicationContext(), "making a call", Toast.LENGTH_LONG).show();
    } else if (item.getTitle() == "Send an SMS Message") {
        Toast.makeText(getApplicationContext(), "sending a message", Toast.LENGTH_LONG).show();
    } else {
        return false;
    }
    return true;
}

• You can now run your app and test the context menu.

Threads

You shouldn't (and Android often enforces it) run long operations on the UI thread. That would result in a blocked user interface which is annoying on the desktop but extremely annoying on a slow mobile device. See the official documentation for an overview.

We'll look at one means of doing hard processing work in the background without affecting the user interface: AsyncTask.

AsyncTask

Basically just a thread but with a built-in function to update the UI after it's finished. To use it you'll need to:

• Extend AsyncTask to make your own class, and decide on the types of parameters and return value.
• Implement doInBackground() to do all the work, and parse the parameters first.
• Implement onPostExecute() to update the UI after doInBackground() completes.