Retrieving Results from Activity

Sometimes, we want some pages bring some data after closing it, e.g., returning the phone number of someone.

The mainstream way is to use Activity Result API. However, in some old tutorials, startActivityForResult, onActivityResult() might be used.

Activity Result API

Activity result API is introduced to simply the procedure of returning results and decouple components. The core includes:

ActivityResultContract

which defines how to create Intent and how to parse the returned result Intent.
ActivityResultLauncher

We should first call registerForActivityResult() to register a contract and a callback, to get this launcher. Then invoking the launch() method of this launcher to start target activity.
ActivityResultCallback.

When the target activity returns, this callback will be triggered, which can be exploited to handle returned data.

Concrete Steps

Register at source activity.

This registration must be done at onCreate() or member declaration, not runtime logics.

// MainActivity.java
public class MainActivity extends AppCompatActivity {
    private TextView resultTextView;

    // 1. Register ActivityResultLauncher
    private final ActivityResultLauncher<Intent>
		secondActivityLauncher = registerForActivityResult(
			// use a preset Contract
			new ActivityResultContracts.StartActivityForResult(),
			
			// set the callback handler
            new ActivityResultCallback<ActivityResult>() {
                @Override
                public void onActivityResult(ActivityResult result) {
                    // 4. process returned result in callback.
                    if (result.getResultCode() == Activity.RESULT_OK) {
                        Intent data = result.getData();
                        if (data != null) {
                            String returnedData = data.getStringExtra("RETURN_DATA");
                            resultTextView.setText("Returned from SecondActivity: " + returnedData);
                        }
                    }
                }
            });

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        resultTextView = findViewById(R.id.resultTextView);
        Button launchButton = findViewById(R.id.launchButtonForReuslt);

        launchButton.setOnClickListener(v -> {
            // 2. create Intent
            Intent intent = new Intent(MainActivity.this, SecondActivity.class);
            intent.putExtra("GREETING", "Hello from MainActivity!");

            // 3. use launcher to start Activity
            secondActivityLauncher.launch(intent);
        });
    }
}

Process data and return at target activity.

// SecondActivity.java
public class SecondActivity extends AppCompatActivity {
    private EditText editText;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_second);

        editText = findViewById(R.id.editTextForReturn);
        Button returnButton = findViewById(R.id.returnButton);

        // retrieve data passed from source activity
        String greeting = getIntent().getStringExtra("GREETING");
        
        returnButton.setOnClickListener(v -> {
            String input = editText.getText().toString();

            // a. create an empty Intent to store data for return
            Intent returnIntent = new Intent();
            
            // b. put data into intent
            returnIntent.putExtra("RETURN_DATA", input);

            // c. set result_code and returned intent
            setResult(Activity.RESULT_OK, returnIntent);

            // d. close current Activity
            finish();
        });
    }
}