Error display patterns in Flutter - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ ui_behavior

intermediate

Displaying error messages in Flutter widgets

Which option correctly displays a red error message below a TextField when the input is invalid?

Flutter

TextField(
  decoration: InputDecoration(
    labelText: 'Email',
    errorText: isValid ? null : 'Invalid email address',
  ),
)

A TextField(decoration: InputDecoration(labelText: 'Email', errorText: isValid ? null : 'Invalid email address'))

B TextField(decoration: InputDecoration(labelText: 'Email', errorText: isValid))

C TextField(decoration: InputDecoration(labelText: 'Email', errorText: 'Invalid email address'))

D TextField(decoration: InputDecoration(labelText: 'Email', errorText: isValid ? 'Error' : null))

💡 Hint

The errorText should be null when there is no error to hide the message.

Option A correctly sets errorText to null when input is valid, so no error message shows. When invalid, it shows the red error message below the TextField. Other options either show error always or use wrong types.

❓ lifecycle

intermediate

Handling error state updates in Flutter StatefulWidget

In a StatefulWidget, which method should you call to update the UI after setting an error message string in the state?

Flutter

setState(() {
  errorMessage = 'Network error';
});

A refresh(() { errorMessage = 'Network error'; });

B setState(() { errorMessage = 'Network error'; });

C updateUI(() { errorMessage = 'Network error'; });

D rebuild(() { errorMessage = 'Network error'; });

💡 Hint

Flutter uses a special method to notify the framework about state changes.

Only setState is the correct Flutter method to update the UI after changing state variables. Other methods do not exist in Flutter.

🔧 Debug

advanced

Identifying error in Flutter error display code

What error will this Flutter code cause? TextField( decoration: InputDecoration( labelText: 'Username', errorText: errorMessage, ), ) where errorMessage is a boolean variable.

Flutter

bool errorMessage = true;

TextField(
  decoration: InputDecoration(
    labelText: 'Username',
    errorText: errorMessage,
  ),
)

A Runtime error: NullPointerException

B No error, displays error message 'true'

C SyntaxError: missing quotes around errorMessage

D TypeError: errorText expects a String or null, but got bool

💡 Hint

Check the expected type of errorText property.

errorText must be a String or null. Passing a boolean causes a type error at runtime.

❓ navigation

advanced

Error handling during navigation in Flutter

Which option correctly shows an error dialog if navigation to a new screen fails?

Flutter

try {
  await Navigator.pushNamed(context, '/details');
} catch (e) {
  showDialog(context: context, builder: (_) => AlertDialog(title: Text('Error'), content: Text('Failed to open screen')));
}

A Navigator.pushNamed(context, '/details').then((_) {}, onError: (e) => showDialog(context: context, builder: (_) => AlertDialog(title: Text('Error'), content: Text('Failed to open screen'))));

B Navigator.pushNamed(context, '/details').catchError((e) => showDialog(context: context, builder: (_) => AlertDialog(title: Text('Error'), content: Text('Failed to open screen'))));

C try { await Navigator.pushNamed(context, '/details'); } catch (e) { showDialog(context: context, builder: (_) => AlertDialog(title: Text('Error'), content: Text('Failed to open screen'))); }

D Navigator.pushNamed(context, '/details').onError((e) => showDialog(context: context, builder: (_) => AlertDialog(title: Text('Error'), content: Text('Failed to open screen'))));

💡 Hint

Use try-catch with async-await for error handling in Flutter navigation.

Option C uses try-catch with await correctly. Option C and C use catchError or then with onError but Navigator.pushNamed returns a Future, so catchError works but only if awaited or returned. Option C uses a non-existent onError method.

🧠 Conceptual

expert

Best practice for error display patterns in Flutter apps

Which approach is best to show error messages consistently across multiple screens in a Flutter app?

A Create a reusable ErrorDisplay widget and use it wherever errors need to be shown

B Write separate error message code in each screen widget manually

C Use print statements to show errors in the console only

D Ignore errors and let the app crash to detect issues

💡 Hint

Think about code reuse and consistent UI design.

Creating a reusable ErrorDisplay widget ensures consistent look and behavior for errors across the app. Writing separate code duplicates effort and risks inconsistency. Printing errors to console is not user-friendly. Ignoring errors harms user experience.

Practice

1. What is the best way to show an error message for a text input field in Flutter?

easy

A. Show a SnackBar without any text input changes

B. Use the errorText property in InputDecoration of TextField

C. Use a Container with red background below the field

D. Change the TextField font size to smaller

Solution

1. Step 1: Understand Flutter form error display

   Flutter provides errorText in InputDecoration to show inline error messages below text fields.

2. Step 2: Compare options for error display

   Using errorText is the standard and clear way. SnackBar is for brief messages, Container with red background is not standard, and font size change does not convey error.

3. Final Answer:

   Use the errorText property in InputDecoration of TextField -> Option B

4. Quick Check:

   Inline error message = errorText [OK]

Hint: Use errorText for inline field errors [OK]

Common Mistakes:

• Using SnackBar for field errors
• Changing font size to indicate error
• Adding unrelated colored containers

2. Which Flutter widget is used to show a brief message at the bottom of the screen for errors?

easy

A. SnackBar

B. AlertDialog

C. Tooltip

D. Dialog

Solution

1. Step 1: Identify widget for brief messages

   SnackBar is designed to show short messages at the bottom of the screen temporarily.

2. Step 2: Differentiate from other widgets

   AlertDialog and Dialog are modal dialogs requiring user action. Tooltip shows hints on hover or long press, not error messages.

3. Final Answer:

   SnackBar -> Option A

4. Quick Check:

   Brief bottom message = SnackBar [OK]

Hint: SnackBar shows quick messages at screen bottom [OK]

Common Mistakes:

• Confusing AlertDialog with SnackBar
• Using Tooltip for error messages
• Using Dialog without user action

3. What will happen if you run this Flutter code snippet?

ScaffoldMessenger.of(context).showSnackBar(
  SnackBar(content: Text('Error occurred!'))
);

medium

A. A red error dialog appears in the center

B. App crashes due to missing AlertDialog

C. A brief message 'Error occurred!' appears at the bottom

D. Nothing happens because SnackBar needs a Scaffold

Solution

1. Step 1: Understand SnackBar usage with ScaffoldMessenger

   Using ScaffoldMessenger.of(context).showSnackBar() correctly shows a SnackBar message at the bottom of the screen.

2. Step 2: Analyze the code behavior

   The SnackBar with text 'Error occurred!' will appear briefly at the bottom. It does not show a dialog or crash.

3. Final Answer:

   A brief message 'Error occurred!' appears at the bottom -> Option C

4. Quick Check:

   ScaffoldMessenger.showSnackBar = brief message at bottom [OK]

Hint: SnackBar shows text at bottom, not dialog [OK]

Common Mistakes:

• Thinking SnackBar is a dialog
• Assuming SnackBar needs AlertDialog
• Believing SnackBar causes crash

4. You want to show an error alert dialog in Flutter but your code throws an exception. Which is the likely cause?

showDialog(
  context: context,
  builder: (context) => AlertDialog(
    title: Text('Error'),
    content: Text('Something went wrong'),
  ),
);

medium

A. Using showDialog without a valid context

B. Not wrapping AlertDialog in a MaterialApp

C. Missing Navigator.pop(context) to close dialog

D. AlertDialog requires a SnackBar inside

Solution

1. Step 1: Check context usage in showDialog

   showDialog requires a valid BuildContext from a widget tree that includes a MaterialApp or WidgetsApp.

2. Step 2: Identify common error cause

   Passing an invalid or null context causes exceptions. Missing Navigator.pop only affects closing, not showing. AlertDialog does not require SnackBar.

3. Final Answer:

   Using showDialog without a valid context -> Option A

4. Quick Check:

   Valid context needed for showDialog [OK]

Hint: Always pass valid context to showDialog [OK]

Common Mistakes:

• Forgetting to provide valid context
• Confusing SnackBar with AlertDialog requirements
• Not handling dialog dismissal

5. You want to display multiple form field errors and a general error alert in Flutter. Which approach correctly combines inline errors and an alert dialog?

hard

A. Change field colors to red and show no dialogs

B. Show all errors only in a SnackBar at the bottom

C. Use AlertDialog for each field error and ignore inline errors

D. Use errorText for each field and show an AlertDialog for general errors

Solution

1. Step 1: Understand error display roles

   Inline errors for fields use errorText to show specific issues. General errors affecting the whole form use AlertDialog to grab attention.

2. Step 2: Evaluate options for combined error display

   Use errorText for each field and show an AlertDialog for general errors correctly uses inline errors for fields and an alert dialog for general errors. SnackBar is brief and not suitable for multiple detailed errors. Using AlertDialog for each field is disruptive. Changing colors alone is unclear.

3. Final Answer:

   Use errorText for each field and show an AlertDialog for general errors -> Option D

4. Quick Check:

   Inline errors + alert dialog [OK]

Hint: Combine errorText for fields and AlertDialog for general errors [OK]

Common Mistakes:

• Showing all errors only in SnackBar
• Using AlertDialog for each field error
• Ignoring inline error messages