Intents

An Intent is an object used to specify the transition from one android activity to another.

You can create a second activity in your project by right-clicking on the package name (such as com.example.app) under src/main/java, and selecting New » Activity. Suppose our new activity is called SecondActivity.

To initiate a transition from the MainActivity to the SecondActivity (perhaps in response to a button click), you can enter this code:

    Intent intent = new Intent(getActivity(), SecondActivity.class);
    startActivity(intent);

The getActivity() works if this code is somewhere within the inner Fragment class. If instead you are doing this directly in the Activity class, you can replace it with this:

    Intent intent = new Intent(this, SecondActivity.class);
    startActivity(intent);

Implicit resolution

In the above example, we transitioned to an explicit activity, indicated by SecondActivity.class. In other cases, we may avoid specifying a particular activity and instead let the Android system (and perhaps the user) decide which activity to open.

For example, suppose I want to link to a map view of some location on the Earth, specified by latitude and longitude. In that case, we specify that we want to VIEW a geo: URL:

    Intent intent = new Intent(Intent.ACTION_VIEW,
            Uri.parse("geo:0,0?q=20.629595,-87.069032(Playa)"));
    startActivity(intent);

The format of this URL is geo:0,0?q=LATITUDE,LONGITUDE(LABEL). When I click the button to activate this intent, my phone has two applications that handle geo URLs, as shown in the screen-shot.

On my tablet, I don’t have VZ Navigator, but instead I have Google Earth, so it still offers me a choice. The user can select an app and then “Always” so that this association is used as the default in the future.

A similar thing happens with YouTube URLs. This Intent transitions to a YouTube video:

    Intent intent = new Intent(Intent.ACTION_VIEW,
            Uri.parse("http://www.youtube.com/watch?v=Yexc19j3TjE"));
    startActivity(intent);

My phone has three ways to visit a YouTube URL, as shown in the screen-shots.

For a ton of extra information about how Android (and perhaps the user) is choosing the target activity, you can set a flag before starting the activity, like this:

    Intent intent = new Intent(Intent.ACTION_VIEW,
            Uri.parse("http://www.youtube.com/watch?v=Yexc19j3TjE"));
    intent.addFlags(Intent.FLAG_DEBUG_LOG_RESOLUTION);
    startActivity(intent);

and then it will dump the details into the system log. Search for the tag IntentResolver to find them.