Programming for Mobile II¶

A Spinner is the Android term for a dropdown interface, like select in HTML
The Android documentation recomends if you have more choices than can fit on a screen horizonitally, to use a Spinner rather than a CheckBox
The entries for a Spinner are defined either programatically or through a seperate XML file with the choices
```
<Spinner
android:entries="@array/ChoicesFromFile" />
```

Images¶

To display an image, the XML is almost idenctical to an ImageButton
The element is named ImageView
Once again, the location of the image it defined by the android:src attribute

<ImageView
        android:src="@drawable/ImageFile" />

As part of the building process, a static class known as R is dynamically created
This class holds constants that refer to all the resources specified in the res directory
For example, to get the numerical id associated with the id string "myId"
```
int numericalId = R.id.myId;
```

As mentioned last class, each screen in an application is known as an activity
All activities have their own class, which inherits from Activity or one of Activity's subclasses
There are several methods that should be overridden, the most important being onCreate
- This is the first method called when the Activity starts up
- This method is responsible for specifiying the layout file and setting up any event listeners usually.

Another method inherited from the Activity class is findViewById
- This method takes an integer id and returns the View object coressponding to the UI element
- Use the R class to get the interger Id
This method returns a View object, but the element we are getting is usually a subclass of View
- The return value should be casted to the appropriate subclass

Android provides two ways to attach event handlers
- The attachment method dictates how the event handler is implemented
The event handler is always passed an object of the View class, which is the base class for all UI elements
- This object represents the specific UI element that the event was triggered from
Common events for input are onClick, onLongClick, onKey, and onFocusChanged

To attach the handler for the onClick event to a button, use the following:

<Button
              android:text="New Button"
              android:id="@+id/button"
              android:onClick="aMethod"/>

The value of android:onClick is the name of a method located in the activity class
- The method must be public and return void
- The method must take one parameter, a View object

Android offers methods to attach events programatically

Button myButton = (Button) getViewById('myButtonId');
myButton.setOnClickListener(new MyListenerObject());

The MyListenerObject in the previous code is a class created by the programmer
The class must implement the approriate Listener interface.
- For an onClick event, the class will implement OnClickListener
- Each interface only has one method to override, which is named after the event, ie onCLick
```
class MyListenerObject implements OnClickListener{
      public void onClick(View v) {
          \\Respond to event
      }
}
```

Sometimes it is useful to list to events on the activity
These can serve as default events
- ie onTouchEvent will handle all clicks,etc not handled by the UI elements themselves
Some events are to enable your program to be a "good citizen" in the OS enviroment
- The onLowMemory event is triggered when the OS is low on memory and all applications should evaluate their memory and free any unneeded memory.
All events on the Activity are methods implemented directly in the activity class

The hardware on Android devices is usually equipped with various sensors that can also cause events
These event must be attached programatically
The events are registed with a SensorManager object by passing the listener object and the sensor to listen for events on
- SensorManager is provided by the OS and retrieved using
```
mySensorManager = (SensorManager) getSystemService(Context.SENSOR_SERVICE);
```

Just like for input events, the sensor events are handled by classes that implement SensorEventListener
This provides two methods that need to be overridden
- onSensorChanged(SensorEvent event) which is called when the sensor value changes
- onAccuracyChanged(SensorEvent event, int accuracy) which is called when the accuracy of the sensor changes for some reason

To attach the listener, an object representing the sensor must be retrieved from the sensor manager object
```
temp = mSensorManager.getDefaultSensor(Sensor.TYPE_TEMPERATURE);
```
The listener is then attached using registerListener and passing the listener, the sensor, and the delay between sensor events

mySensorManager.registerListener(new SensorEventListener(), temperature, SensorManager.SENSOR_DELAY_NORMAL);