April 20, 2014

Tabs With Intents

If you look at the Android widgets and classes related to tabs, you’ll see that a tab can have either a View or an Activity as its contents. Most of the time, people use Views, as they are simpler to implement and lighter-weight to run. However, there may be circumstances where you feel putting your activities in tabs would be useful, so let’s take a look at how that is done, in a modified excerpt from Version 1.9 of The Busy Coder’s Guide to Android Development.

If you want to use an Activity as the content of a tab, you provide an Intent that will launch the desired Activity; Android’s tab-management framework will then pour the Activity’s user interface into the tab. So, let’s create a tabbed Web browser.

Your natural instinct might be to use an http: Uri as the target of the Intents, one Intent per tab. That way, you could use the built-in Browser application and get all of the features that it offers. Alas, this does not work. It appears you cannot host other applications’ activities in your tabs, only your own activities, presumably for security reasons.

But, we can always use the WebView widget to provide a simplified browsing experience.

Here is the source to the main activity, the one hosting the TabView:

[sourcecode language="java"]
public class IntentTabDemo extends TabActivity {
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);

TabHost host=getTabHost();

host.addTab(host.newTabSpec(“one”)
.setIndicator(“CW”)
.setContent(new Intent(this, CWBrowser.class)));
host.addTab(host.newTabSpec(“two”)
.setIndicator(“Android”)
.setContent(new Intent(this, AndroidBrowser.class)));
}
}
[/sourcecode]

As you can see, we are using TabActivity as the base class, and so we do not need our own layout XML — TabActivity supplies it for us. All we do is get access to the TabHost and add two tabs, each specifying an Intent that directly refers to another class. In this case, our two tabs will host a CWBrowser and an AndroidBrowser, respectively.

Those latter two activities are simple wrappers around WebView:

[sourcecode language="java"]
public class CWBrowser extends Activity {
WebView browser;

@Override
public void onCreate(Bundle icicle) {
super.onCreate(icicle);

browser=new WebView(this);
setContentView(browser);
browser.loadUrl(“http://commonsware.com”);
}
}
[/sourcecode]

[sourcecode language="java"]
public class AndroidBrowser extends Activity {
WebView browser;

@Override
public void onCreate(Bundle icicle) {
super.onCreate(icicle);

browser=new WebView(this);
setContentView(browser);
browser.loadUrl(“http://code.google.com/android”);
}
}
[/sourcecode]

They simply load a different URL into the browser: the CommonsWare home page in one, the Android home page in the other.

The resulting UI shows what tabbed browsing could look like on Android:

Using distinct subclasses for each targeted page is rather wasteful. Instead, we could have packaged the URL to open as an “extra” in an Intent and used that Intent to spawn a general-purpose BrowserTab activity, which would read the URL out of the Intent “extra” and use this. The proof of this is left as an exercise for the reader.

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