Activity results in Android Kotlin

Introduction

Activity results let one screen send data back to the screen that started it. This helps apps share information between screens easily.

When you open a screen to pick a photo and want the chosen photo returned.

When you ask the user to fill a form on a new screen and need the filled data back.

When you start a screen to get user confirmation and want to know their choice.

When you want to get a result from a settings screen before continuing.

Syntax

Android Kotlin

val getResult = registerForActivityResult(ActivityResultContracts.StartActivityForResult()) { result ->
    if (result.resultCode == Activity.RESULT_OK) {
        val data = result.data
        // Use the returned data here
    }
}

// To start the activity
val intent = Intent(this, OtherActivity::class.java)
getResult.launch(intent)

This uses registerForActivityResult to handle results in a modern way.

Always check resultCode to know if the result is OK or cancelled.

Examples

Starts InputActivity and expects a string extra named "text" back.

Android Kotlin

val getResult = registerForActivityResult(ActivityResultContracts.StartActivityForResult()) { result ->
    if (result.resultCode == Activity.RESULT_OK) {
        val returnedText = result.data?.getStringExtra("text")
        // Use returnedText
    }
}

val intent = Intent(this, InputActivity::class.java)
getResult.launch(intent)

Launches a system picker to select an image and returns its Uri.

Android Kotlin

val getImage = registerForActivityResult(ActivityResultContracts.GetContent()) { uri ->
    if (uri != null) {
        // Use the image Uri
    }
}

getImage.launch("image/*")

Sample App

This app has two screens. The first screen has a button that opens the second screen. The second screen has a button that sends back a message to the first screen. The first screen shows the message in a TextView.

Android Kotlin

class MainActivity : AppCompatActivity() {
    private lateinit var resultLauncher: ActivityResultLauncher<Intent>

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        resultLauncher = registerForActivityResult(ActivityResultContracts.StartActivityForResult()) { result ->
            if (result.resultCode == Activity.RESULT_OK) {
                val returnedData = result.data?.getStringExtra("message")
                findViewById<TextView>(R.id.textView).text = returnedData ?: "No message"
            }
        }

        findViewById<Button>(R.id.button).setOnClickListener {
            val intent = Intent(this, SecondActivity::class.java)
            resultLauncher.launch(intent)
        }
    }
}

class SecondActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_second)

        findViewById<Button>(R.id.returnButton).setOnClickListener {
            val resultIntent = Intent().apply {
                putExtra("message", "Hello from SecondActivity")
            }
            setResult(Activity.RESULT_OK, resultIntent)
            finish()
        }
    }
}

Important Notes

Use registerForActivityResult instead of the old startActivityForResult method.

Always check if the returned data is null before using it to avoid crashes.

Remember to call setResult and finish() in the second activity to send data back.

Summary

Activity results let one screen send data back to the screen that started it.

Use registerForActivityResult to handle results safely and simply.

Always check the result code and returned data before using it.

Practice

1. What is the main purpose of registerForActivityResult in Android Kotlin?

easy

A. To create a new fragment instance

B. To start a new activity without expecting a result

C. To register a broadcast receiver

D. To safely receive data back from another activity

Solution

1. Step 1: Understand the role of registerForActivityResult

   This function is designed to handle results from activities started for a result safely and simply.

2. Step 2: Compare with other options

   Starting an activity without expecting a result or registering broadcast receivers are unrelated to this function.

3. Final Answer:

   To safely receive data back from another activity -> Option D

4. Quick Check:

   Activity result handling = A [OK]

Hint: Remember: registerForActivityResult handles returned data safely [OK]

Common Mistakes:

• Confusing it with starting activities without results
• Thinking it registers broadcast receivers
• Assuming it creates fragments

2. Which of the following is the correct syntax to register an activity result launcher in Kotlin?

easy

A. val launcher = registerForActivityResult(ActivityResultContracts.StartActivityForResult()) { result -> /* handle result */ }

B. val launcher = registerActivityResult(ActivityResultContracts.StartActivityForResult()) { result -> /* handle result */ }

C. val launcher = registerForResult(ActivityResultContracts.StartActivityForResult()) { result -> /* handle result */ }

D. val launcher = registerActivity(ActivityResultContracts.StartActivityForResult()) { result -> /* handle result */ }

Solution

1. Step 1: Identify the correct function name

   The correct function is registerForActivityResult, not any shortened or altered form.

2. Step 2: Check the contract and lambda syntax

   The contract ActivityResultContracts.StartActivityForResult() and lambda parameter { result -> } are correctly used in val launcher = registerForActivityResult(ActivityResultContracts.StartActivityForResult()) { result -> /* handle result */ }.

3. Final Answer:

   val launcher = registerForActivityResult(ActivityResultContracts.StartActivityForResult()) { result -> /* handle result */ } -> Option A

4. Quick Check:

   Correct function name and syntax = B [OK]

Hint: Look for exact function name: registerForActivityResult [OK]

Common Mistakes:

• Using incorrect function names like registerActivityResult
• Omitting parentheses in contract instantiation
• Wrong lambda syntax

3. Given the following code snippet, what will be printed if the launched activity returns RESULT_OK with intent data containing key "username" = "Alice"?

val launcher = registerForActivityResult(ActivityResultContracts.StartActivityForResult()) { result ->
    if (result.resultCode == Activity.RESULT_OK) {
        val name = result.data?.getStringExtra("username")
        println(name)
    } else {
        println("No data")
    }
}

medium

A. Crash due to null pointer

B. Alice

C. null

D. No data

Solution

1. Step 1: Check the result code condition

   The code checks if result.resultCode == Activity.RESULT_OK, which is true in this case.

2. Step 2: Extract the "username" from intent data

   The intent data contains key "username" with value "Alice", so name will be "Alice" and printed.

3. Final Answer:

   Alice -> Option B

4. Quick Check:

   Result OK with data returns value = D [OK]

Hint: Check resultCode and safely unwrap intent data [OK]

Common Mistakes:

• Ignoring resultCode and printing 'No data'
• Assuming data is null without checking
• Not using safe call operator on intent data

4. Identify the error in this code snippet for handling activity results:

val launcher = registerForActivityResult(ActivityResultContracts.StartActivityForResult()) { result ->
    val name = result.data!!.getStringExtra("username")
    if (result.resultCode == Activity.RESULT_OK) {
        println(name)
    }
}

medium

A. Unsafe call on result.data before checking resultCode

B. Missing lambda parameter in registerForActivityResult

C. Incorrect contract used for activity result

D. No error, code is correct

Solution

1. Step 1: Analyze the order of operations

   The code force unwraps result.data!! before checking if result.resultCode == Activity.RESULT_OK.

2. Step 2: Understand potential crash

   If the result is not OK or data is null, force unwrapping causes a crash. The check should come before accessing data.

3. Final Answer:

   Unsafe call on result.data before checking resultCode -> Option A

4. Quick Check:

   Check resultCode before accessing data = A [OK]

Hint: Always check resultCode before using intent data [OK]

Common Mistakes:

• Force unwrapping nullable data without checks
• Accessing data before verifying resultCode
• Ignoring null safety in Kotlin

5. You want to start an activity to pick a contact and get the contact's URI back. Which approach correctly uses registerForActivityResult to achieve this?

hard

A. val pickContact = registerForActivityResult(ActivityResultContracts.RequestPermission()) { granted -> if (granted) println("Permission granted") }

B. val pickContact = registerForActivityResult(ActivityResultContracts.StartActivityForResult()) { result -> println(result.data?.getData().toString()) }

C. val pickContact = registerForActivityResult(ActivityResultContracts.PickContact()) { uri -> if (uri != null) println(uri.toString()) }

D. val pickContact = registerForActivityResult(ActivityResultContracts.TakePicture()) { success -> if (success) println("Picture taken") }

Solution

1. Step 1: Identify the correct contract for picking a contact

   ActivityResultContracts.PickContact() is designed to pick a contact and return its URI.

2. Step 2: Check the lambda parameter and usage

   The lambda receives a URI nullable parameter, which is safely checked before printing.

3. Step 3: Eliminate other options

   val pickContact = registerForActivityResult(ActivityResultContracts.StartActivityForResult()) { result -> println(result.data?.getData().toString()) } uses a generic contract but does not check resultCode; A and D are unrelated contracts for permissions and taking pictures.

4. Final Answer:

   val pickContact = registerForActivityResult(ActivityResultContracts.PickContact()) { uri -> if (uri != null) println(uri.toString()) } -> Option C

5. Quick Check:

   Use PickContact contract for contact URI = C [OK]

Hint: Use specific contracts like PickContact for contact picking [OK]

Common Mistakes:

• Using StartActivityForResult without checking resultCode
• Confusing permission or picture contracts with contact picking
• Not handling nullable URI safely