Why Activity results in Android Kotlin? - Purpose & Use Cases

The Big Idea

What if you could get data back from another screen without messy code or lost info?

The Scenario

Imagine you have two screens in your app: one to pick a photo and another to show it. You want to send the photo back from the picker screen to the main screen manually.

You try to pass data by saving it somewhere temporary or using complicated global variables.

The Problem

This manual way is slow and confusing. You might lose data if the app closes or crashes. It's hard to keep track of where the data is and when it arrives. Bugs happen easily, and the code becomes messy.

The Solution

Activity results let you start a screen and wait for it to send back data cleanly. Android handles the communication for you. You just write simple code to get the result when the other screen finishes.

Before vs After

✗ Before

startActivity(intent)
// no easy way to get result back

✓ After

val launcher = registerForActivityResult(ActivityResultContracts.StartActivityForResult()) { result ->
  if (result.resultCode == RESULT_OK) {
    val data = result.data
    // use returned data
  }
}
launcher.launch(intent)

What It Enables

You can easily get data back from another screen, making your app interactive and user-friendly without messy code.

Real Life Example

When a user picks a contact from their address book, your app can get the chosen contact's info right away and show it on the main screen.

Key Takeaways

Manual data passing between screens is error-prone and complicated.

Activity results provide a clean, reliable way to get data back from another screen.

This makes your app easier to build, understand, and maintain.

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