Creating Your First Activity

Now you can create your first activity! Well, technically speaking, you already created your first activity — the MainActivity.java file — when you created a project through the New Android Project wizard in Chapter 5. Therefore, you will not be creating a new activity; you will only be working with the MainActivity.java file you already created. Open this file now.

As you read previously, the entry point into your application is the onCreate() method. The code for your MainActivity.java file already contains an implementation of the onCreate() method. Right now, your code should look like this:

public class MainActivity extends Activity {
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {                →4
        super.onCreate(savedInstanceState);                          →5
        setContentView(R.layout.main);                               →6
    }
}

You will be writing initialization code directly below the setContentView() method shortly.

Some points to note about this code:

Handling user input

For user interaction, your Screen Brightness Toggle application won't require much: only a single button. The user taps the button to dim the tablet's screen brightness, and then the user taps the button again to restore it to full brightness.

To respond to this tap event, you need to register what is known as an event listener. An event listener detects and responds to an event in the Android system. Android uses various types of events, but two of the most common are

• Keyboard events: These occur when a particular keyboard key has been pressed. Why would you want to know about this? Maybe you want your app to use hot keys, such as Alt+E, to change your app on the fly. By pressing Alt+E, then, the user could, say, toggle an app's view into Edit mode. Responding to keyboard events allows you to do this. I do not use keyboard events in this book, but if you need to in future applications, override the onKeyDown method, as shown here:

  @Override
  public boolean onKeyDown(int keyCode, KeyEvent event) {
             // TODO Auto-generated method stub
             return super.onKeyDown(keyCode, event);
  }

• Touch events (also known as touches, clicks, or taps): These occur when the user taps a widget on the screen. Examples of widgets that can respond to touch events include (but are not limited to):
  • Button
  • ImageButton
  • EditText
  • Spinner
  • List Item Row
  • Menu Item

    All views in the Android system can react to a tap; however, some of the widgets have their clickable property set to false by default. You can override this setting in your layout file or in code to allow a view to be clickable by setting the clickable attribute on the view or the setClickable() method in code.

Writing your first event handler

When the user touches the onscreen toggle button, a click event occurs. In order for your application to respond to the user's action, then, you must design the code to respond to the button's click event.

Type the code shown in Listing 7-1 into your editor. This code shows how to implement a click handler for the toggleButton. The listing shows the entire code — the original onCreate() method with the new code added. Feel free to either fill in the button code or overwrite your entire onCreate code.

Listing 7-1: The Initial Class File with a Default Button OnClickListener

public class MainActivity extends Activity {

    ToggleButton tb;                                            →3
    TextView tv;
                                                                →4

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        tb = (ToggleButton)findViewById(R.id.toggleButton);     →11
        tv = (TextView)findViewById(R.id.currentBrightness);    →12

        tb.setOnClickListener(new View.OnClickListener()
           {                                                    →14

            public void onClick(View v) {                       →16

                        }
                    });
    }
}

Whoa! A lot of stuff is going on there! I'm going to break this code down, line by line, so you can understand what's happening.

Lines 10 and 11 find views through the findViewById() method, which is available to all activities in Android. This method lets you find any view inside the activity's layout. This method always returns a View class that you must cast to the appropriate type before you can work with it. As an example, the following code (which is a snippet from the previous code) casts the returned view from findViewById() to a ToggleButton (which is a subclass of View):

tb = (ToggleButton)findViewById(R.id.toggleButton);

Immediately following the code on line 11, you retrieve the TextView that you will use to display the current brightness of the device back to the user. Though not used in this example, you need this object, so add it now to save time later in this chapter.

Importing packages

When you typed this code into your editor, you may have seen some red squiggly lines, as shown in Figure 7-2. These lines are Eclipse's way of telling you, “Hey! I don't know what this ‘Button’ thing is.” If you place your cursor over the squiggly line and leave it there for a moment, you receive a small context window that gives you several options (also shown in Figure 7-2).

Figure 7-2: When Eclipse gives you a red squiggly line, hovering your cursor over the line provides a context menu of options.

To help Eclipse understand, choose the first option from this menu, Import ‘ToggleButton’. Doing this adds the following import statement to the top of the file:

import android.widget.ToggleButton;

This import statement informs Eclipse where the ToggleButton is located in the Android packages. You may also need to import the android.view.package.

import android.widget.*;

The asterisk informs Eclipse to include all widgets in the android.widget package.