This, of course, could be handled more simply by having a single TextView and changing the text and color on each click. However, you can imagine that the ViewFlipper contents could be much more complicated, like the contents you might put into a TabView.
As with the TabWidget, sometimes, your ViewFlipper contents may not be known at compile time. As with TabWidget, though, you can add new contents on the fly with ease.
For example, let’s look at another sample activity (Fancy/Flipper2), using this layout:
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
>
<ViewFlipper android:id="@+id/details"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
>
</ViewFlipper>
</LinearLayout>
Notice that the ViewFlipper has no contents at compile time. Also note that there is no Button for flipping between the contents — more on this in a moment.
For the ViewFlipper contents, we will create large Button widgets, each containing one of a set of random words. And, we will set up the ViewFlipper to automatically rotate between the Button widgets, using an animation for transition:
public class FlipperDemo2 extends Activity {
static String[] items={"lorem", "ipsum", "dolor", "sit", "amet",
"consectetuer", "adipiscing", "elit",
"morbi", "vel", "ligula", "vitae",
"arcu", "aliquet", "mollis", "etiam",
"vel", "erat", "placerat", "ante",
"porttitor", "sodales", "pellentesque",
"augue", "purus"};
ViewFlipper flipper;
@Override
public void onCreate(Bundle icicle) {
super.onCreate(icicle);
setContentView(R.layout.main);
flipper = (ViewFlipper)findViewById(R.id.details);
flipper.setInAnimation(AnimationUtils.loadAnimation(this,
R.anim.push_left_in));
flipper.setOutAnimation(AnimationUtils.loadAnimation(this,
R.anim.push_left_out));
for (String item : items) {
Button btn = new Button(this);
btn.setText(item);
flipper.addView(btn,
new ViewGroup.LayoutParams(
ViewGroup.LayoutParams.FILL_PARENT,
ViewGroup.LayoutParams.FILL_PARENT));
}
flipper.setFlipInterval(2000);
flipper.startFlipping();
}
}
After getting our ViewFlipper widget from the layout, we first set up the “in” and “out” animations. In Android terms, an animation is a description of how a widget leaves (”out”) or enters (”in”) the viewable area. Animations are a complex beast, eventually worthy of their own chapter but not covered in this text. For now, realize that animations are resources, stored in res/anim/ in your project. For this example, we are using a pair of animations supplied by the SDK samples, available under the Apache 2.0 License. As their names suggest, widgets are “pushed” to the left, either to enter or leave the viewable area.
After iterating over the funky words, turning each into a Button, and adding the Button as a child of the ViewFlipper, we set up the flipper to automatically flip between children (flipper.setFlipInterval(2000);) and to start flipping (flipper.startFlipping();).
The result is an endless series of buttons, each appearing, then sliding out to the left after 2 seconds, being replaced by the next button in sequence, wrapping around to the first after the last has been shown (see Figure 10-11).
Figure 10-11. The Flipper2 application, showing an animated transition
The auto-flipping ViewFlipper is useful for status panels or other situations where you have a lot of information to display, but not much room. The key is that, since it automatically flips between views, expecting users to interact with individual views is dicey — the view might switch away part-way through their interaction.
Other Containers of Note
Android offers AbsoluteLayout, where the contents are laid out based on specific coordinate positions. You tell AbsoluteLayout where to place a child in precise X,Y coordinates, and Android puts it there, no questions asked. On the plus side, this gives you precise positioning. On the minus side, it means your views will only look “right” on screens of a certain dimension, or it requires you to write a bunch of code to adjust the coordinates based on screen size. Since Android screens might run the gamut of sizes, plus have new sizes crop up periodically, using AbsoluteLayout could get quite annoying.
Android also has a new flavor of list, the ExpandableListView. This provides a simplified tree representation, supporting two levels of depth: groups and children. Groups contain children; children are “leaves” of the tree. This requires a new set of adapters, since the ListAdapter family does not provide any sort of group information for the items in the list.
CHAPTER 11
Applying Menus
Like applications for the desktop and some mobile operating systems, such as Palm OS and Windows Mobile, Android supports activities with application menus. Some Android phones will have a dedicated menu key for popping up the menu; others will offer alternate means for triggering the menu to appear.
Also, as with many GUI toolkits, you can create context menus. On a traditional GUI, this might be triggered by the right mouse button. On mobile devices, context menus typically appear when the user taps-and-holds over a particular widget. For example, if a TextView had a context menu and the device was designed for finger-based touch input, you could push the TextView with your finger, hold it for a second or two, and a pop-up menu would appear for you to choose from.
Android differs from most other GUI toolkits in terms of menu construction. While you can add items to the menu, you do not have full control over the menu’s contents, nor of the timing of when the menu is built. Part of the menu is system-defined, and that portion is managed by the Android framework itself.