Using Intents to Create Flows - TechGeekD/android_guides GitHub Wiki

Overview

Intent is a powerful concept within the Android universe. An intent is a message that can be thought of as a request that is given to either an activity within your own app, an external application, or a built-in Android service.

Think of an intent as a way for an Activity to communicate with the outside Android world. A few key tasks that an intent might be used for within your apps:

  • Take the user to another screen (activity) within your application
  • Take the user to a particular URL within the Android web browser
  • Take the user to the camera to have them take a picture
  • Initiate a call for the user to a given number

As you can see, the Intent is a core part of user flows in Android development. The Intent object itself is a class that represents a particular "request" including the topic of the request and any request "parameters" which are called the Bundle.

Explicit Intents

An "explicit" intent is used to launch other activities within your application. For example, if you the user presses the "compose" button and you want to bring up an activity for them to compose a message, you would launch that second activity using an explicit intent.

Using an intent is as simple as constructing the Intent with the correct parameters and then invoking that intent using the startActivity method:

// ActivityOne.java
public void launchComposeView() {
  // first parameter is the context, second is the class of the activity to launch
  Intent i = new Intent(ActivityOne.this, ActivityTwo.class);
  startActivity(i); // brings up the second activity
}

Now, in the launched second activity, the user can go back to the first screen by hitting "back" or if the developer wants to trigger the second activity to close, we need only call the finish method:

// ActivityTwo.java
public void onSubmit(View v) {
  // closes the activity and returns to first screen
  this.finish(); 
}

Note: The first argument of the Intent constructor used above is a Context which at the moment is just the current Activity in scope.

Passing Data to Launched Activities

In addition to specifying the activity that we want to display, an intent can also pass key-value data between activities. Think of this as specifying the "request parameters" for an HTTP Request. You can specify the parameters by putting key-value pairs into the intent bundle:

// ActivityOne.java
public void launchComposeView() {
  // first parameter is the context, second is the class of the activity to launch
  Intent i = new Intent(ActivityOne.this, ActivityTwo.class);
  // put "extras" into the bundle for access in the second activity
  i.putExtra("username", "foobar"); 
  i.putExtra("in_reply_to", "george"); 
  i.putExtra("code", 400);
  // brings up the second activity
  startActivity(i); 
}

Once you have added data into the bundle, you can easily access that data within the launched activity:

// ActivityTwo.java (subactivity) can access any extras passed in
protected void onCreate(Bundle savedInstanceState) {
   String username = getIntent().getStringExtra("username");
   String inReplyTo = getIntent().getStringExtra("in_reply_to");
   int code = getIntent().getIntExtra("code", 0);
}

And using this system the intent can pass useful data across activities.

Returning Data Result to Parent Activity

In the typical case of using startActivity, the activity is launched and added to the navigation stack and no result is expected. If the user wants to close the activity, the user can simply hit "back" and the parent activity is displayed.

However, in other cases the parent activity may want the launched activity to return a result back when it is finished. In this case, we use a different method to launch called startActivityForResult which allows the parent to retrieve the result based on a code that is returned (akin to an HTTP code).

// ActivityOne.java
// REQUEST_CODE can be any value we like, used to determine the result type later
private final int REQUEST_CODE = 20;
// FirstActivity, launching an activity for a result
public void onClick(View view) {
  Intent i = new Intent(ActivityOne.this, ActivityNamePrompt.class);
  i.putExtra("mode", 2); // pass arbitrary data to launched activity
  startActivityForResult(i, REQUEST_CODE);
}

This will launch the subactivity, and when the subactivity is complete then it can return the result to the parent:

// ActivityNamePrompt.java -- launched for a result
public void onSubmit(View v) {
  EditText etName = (EditText) findViewById(R.id.name);
  // Prepare data intent 
  Intent data = new Intent();
  // Pass relevant data back as a result
  data.putExtra("name", etName.getText().toString());
  data.putExtra("code", 200); // ints work too
  // Activity finished ok, return the data
  setResult(RESULT_OK, data); // set result code and bundle data for response
  finish(); // closes the activity, pass data to parent
}

Once the sub-activity finishes, the onActivityResult() method in the calling activity is be invoked:

// ActivityOne.java, time to handle the result of the sub-activity
@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
  // REQUEST_CODE is defined above
  if (resultCode == RESULT_OK && requestCode == REQUEST_CODE) {
     // Extract name value from result extras
     String name = data.getExtras().getString("name");
     int code = data.getExtras().getInt("code", 0);
     // Toast the name to display temporarily on screen
     Toast.makeText(this, name, Toast.LENGTH_SHORT).show();
  }
}

And using that process you can communicate data freely between different activities in your application.

Passing Complex Data in a Bundle

Bundles can pass complex Java objects into Intents through the use of serialization. Simple types such as integers and strings can be automatically passed as an extra but to pass Java objects, they need to be Serializable or Parcelable.

public class User implements Serializable {
	private static final long serialVersionUID = 5177222050535318633L;
	private String firstName;
	private String lastName;
	private int age;
}

Then you can pass arbitrary user objects into an intent as an extra:

User u = new User("John", "Smith", 45);
Intent  i =  new Intent(ActivityOne.this, SecondActivity.class);
i.putExtra("user", u);
startActivity(i);

and access the user data in the launched intent with getSerializableExtra:

// SecondActivity.java
User u = (User) getIntent().getSerializableExtra("user");
TextView tvUser = (TextView) findViewById(R.id.tvUser);
tvUser.setText(u.getFirstName() + " " + u.getLastName());

Implicit Intents

Implicit Intents are requests to perform an action based on a desired action and target data. This is in contrast to an explicit intent that targets a specific activity. For example, if I want to make a phone call for the user, that can be done with this intent:

Intent callIntent = new Intent(Intent.ACTION_CALL);
callIntent.setData(Uri.parse("tel:3777789888"));
startActivity(callIntent);

If I want to launch a website in the phone's browser, I might do this:

Intent browserIntent = new Intent(Intent.ACTION_VIEW, 
  Uri.parse("http://www.google.com"));
startActivity(browserIntent);

You can see a list of other common implicit intents.

References