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Android Views (part 2) - Button and ImageButton

11/5/2011 3:31:21 PM
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2. Button and ImageButton

The Button View is just a button, printed with some text to identify it, that the user can click to invoke some action. The previous section created a Button and connected it to an OnClickListener method that executes when the Button is clicked.

Android has a very visual, mobile-oriented user interface, so you might want to use a button with an image on it rather than one with text. Android provides the ImageButton View for just that purpose. You can adapt Example 2 to use an ImageButton by making one change in the XML file and another in the Java code:

  1. In main.xml, replace the Button definition for btnDone with an ImageButton:

    ...
    <ImageButton
    android:id="@+id/btnDone"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    />
    ...
  2. In TextViewDemo.java, redefine btn1 as an ImageButton and add a line to set the image to a PNG image in the drawable directory:

    ...
    private static ImageButton btn1;

    ...
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);

    //Get pointers to the Views defined in main.xml
    txt1 = (TextView) findViewById(R.id.txtDemo);
    etxt1 = (EditText) findViewById(R.id.eTxtDemo);
    btn1 = (ImageButton) findViewById(R.id.btnDone);
    ...
    //Set the image for the Done button
    btn1.setImageResource(R.drawable.log);
    ...

The button now appears as shown in Figure 2.

Figure 2. Text boxes with an ImageButton


3. Adapters and AdapterViews

The AdapterView is a generic, list-oriented view of data. Any collection of data objects that can be ordered in some relatively stable way can be displayed through an AdapterView. An AdapterView is always associated with an Adapter, which acts as the bridge between it and the underlying data collection. The Adapter has two responsibilities:

  • At the request of the AdapterView, the Adapter must be able to find the data object that corresponds to a particular index. It must, in other words, be able to find the data object that is visible in the AdapterView at a particular location.

  • Inversely, the Adapter must be able to supply a view through which the data at a particular index can be displayed.

It takes only a moment’s reflection to understand how the AdapterView works: It is a ViewGroup that contains all the machinery necessary to serve as both the View and Controller for a collection of generic widgets. It can lay them out on the display, pass in clicks and keystrokes, and so on. It need never concern itself with what the subviews actually display; it distinguishes them only by their indexes. Whenever it needs to perform either of the two operations that are not entirely generic—creating a new view or getting the data object attached to a particular view—it relies on the Adapter to convert an index into either a data object or the view of a data object.

The AdapterView requests new views from an implementation of the Adapter interface, as it needs them, for display. For instance, as a user scrolls though a list of contacts, the AdapterView requests a new view for each new contact that becomes visible. As an optimization, the AdapterView may offer a view that is no longer visible (in this case, one that has scrolled off the display) for reuse. This can dramatically reduce memory churn and speed up display.

When offered a recycled view, however, the Adapter must verify that it is the right kind of view through which to display the data object at the requested index. This is necessary because the Adapter is not limited to returning instances of a single view class in response to the request for a view. If the Adapter represents several kinds of objects, it might create several different types of views, each applicable to some subset of the data objects in the collection. A list of contacts, for instance, might have two entirely different view classes: one for displaying acquaintances that are currently online and another for those who are not. The latter might completely ignore clicks, whereas the former would open a new chat session when clicked.

Although AdapterView and Adapter are both abstract and cannot be directly instantiated, the UI toolkit includes several prebuilt Adapters and AdapterViews that can be used unmodified or further subclassed to provide your own customizations. ListAdapter and SpinnerAdapter are particularly useful Adapters, while ListView, GridView, Spinner, and Gallery are all handy subclasses of AdapterView. If you plan to create your own subclass of AdapterView, a quick look at the code for one of these classes will get you off to a running start.

 
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