How to pass a parameter from an Activity b to Activity A

Question

Good evening, I need help to solve the following problem, I’m doing a Million Dollar Show style game. I have a main activity that is where the questions occur and has the buttons that trigger the aids. When the user clicks on the card button, it opens a second Activity (detail, the main Activity remains open), when generating the letter I need to pass the value of that letter to Main Activity.

The problem is that if I use the code below:

Note: This code is used in Activity 2, which would be a temporary screen that runs a code and goes back to Main Screen

Intent intent = new Intent(this, TelaPrincipal.class);

intent.putExtras("valor", valor); // value is the attribute I need to pass to main screen.

startActivity(intent);

I’m going to restart the main Activity, and I can’t do that, because I would be missing the progress of the game. So I need to pass this parameter another way.

Detail, I don’t have much experience I’m starting to program on this platform, so who can send a detailed reply thank you.

Answer 1

You need to use the Activity Result API methods for this. Really, if you call a startActivityForResult in the second Activity, the first Activity will be started from scratch, and is not what you want (inclusive, the animation transition between these screens will also give the impression that you are "going" to this screen instead of "returning").

To do what you want, you need to use another set of methods. Considering that you want to send the ID of the clicked letter (as int) to the second Activity, and that you need to return this "value" as String, you need to define a contract class for this exchange of information between the Activities:

class MyContract : ActivityResultContract<Int, String>() {
    override fun createIntent(context: Context, cardId: Int) =
        Intent(context, SecondActivity::class.java).apply {
            putExtra(CARD_ID_EXTRA, cardId)
        }

    override fun parseResult(resultCode: Int, intent: Intent?): String? {
        if (resultCode != Activity.RESULT_OK) {
            return null
        }

        return intent?.getStringExtra(VALUE_EXTRA)
    }

    companion object {
        const val CARD_ID_EXTRA = "card_id"
        const val VALUE_EXTRA = "valor"
    }
}

This class defines that you will pass an Int and receive a String back. See where Intent is created and how the "value" will be extracted from the return Intent. This will be done implicitly in your main code, because it is already defined here.

Now, in your Mainactivity, create an instance of that contract between the two Activities:

val contractLauncher = registerForActivityResult(MyContract()) { str ->
    // faça o que quiser com str, que é "valor" retornado pela segunda Activity
}

That one contractLauncher will be used to start the second Activity. Then, at the click of the letter button, you will call the following code:

contractLauncher.launch(cardId)

That will create an Intent with that cardId and open the second Activity with it. Everything as already defined in the class MyContract.

Now, in your second Activity, you need to pick up the value of the card:

val cardId = intent.extras?.getInt(MyContract.CARD_ID_EXTRA) ?: -1

The value of cardId will be what was passed there in Mainactivity (or -1, if someone starts this second Activity without passing a pro value cardID).

And yet in this second Activity, the moment the person chooses the "value", you need to do the following to indicate that it is to return him:

setResult(Activity.RESULT_OK, Intent().apply {
    putExtra(MyContract.VALUE_EXTRA, valor)
})
finish()

That one finish() will finish the second Activity and return to Mainactivity (and not start Mainactivity again) and will pass this value back to her. That callback with str set up there, at the time you created the contractLauncher, will be called and you will be able to use this value back in Mainactivity. The transition animation will now be as if you have "gone back" to previous Activity, which is what you want.

See more details on this in the official documentation on: https://developer.android.com/training/basics/intents/result