TextFormField in Flutter - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ ui_behavior

intermediate

What is the output of this Flutter code snippet?

Consider this Flutter widget code using TextFormField. What will the UI show when the app runs?

Flutter

TextFormField(
  decoration: InputDecoration(
    labelText: 'Enter your name',
    border: OutlineInputBorder(),
  ),
  initialValue: 'John',
  enabled: false,
)

A An enabled text field showing 'John' with no border.

B An enabled empty text field with label 'Enter your name'.

C A disabled empty text field with no label.

D A disabled text field showing 'John' with a label 'Enter your name'.

💡 Hint

Check the enabled and initialValue properties.

The enabled: false disables input, so the field is read-only. The initialValue sets the text shown. The label and border appear as specified.

❓ lifecycle

intermediate

What happens when you call _formKey.currentState!.validate() in a Flutter form?

Given a Flutter form with a TextFormField that has a validator, what does calling _formKey.currentState!.validate() do?

A It runs all validators and returns true if all fields are valid, false otherwise.

B It clears all the text fields in the form.

C It submits the form data to the server automatically.

D It disables all text fields in the form.

💡 Hint

Think about what validation means in forms.

Calling validate() runs each field's validator function and returns true only if all validators return null (no error).

📝 Syntax

advanced

What error does this Flutter code cause?

Analyze this Flutter code snippet using TextFormField. What error will it produce?

Flutter

TextFormField(
  validator: (value) {
    if (value!.isEmpty) {
      return 'Please enter text';
    }
    return null;
  },
)

A Missing return statement in validator causes a warning but runs fine.

B The code causes a syntax error due to missing else branch.

C The validator returns null implicitly if input is not empty, so no error occurs.

D The validator must always return a String, so this causes a runtime error.

💡 Hint

What does a validator return when input is valid?

If the validator returns null, it means input is valid. The code returns a string only when empty, otherwise returns null implicitly.

❓ navigation

advanced

How to focus the next TextFormField after pressing 'Next' on the keyboard?

In Flutter, you want to move focus from one TextFormField to the next when the user presses the 'Next' button on the keyboard. Which approach works best?

A Use <code>FocusScope.of(context).nextFocus()</code> inside <code>onFieldSubmitted</code> callback.

B Call <code>setState()</code> to rebuild the widget tree on submit.

C Use <code>TextEditingController.clear()</code> to clear the current field on submit.

D Wrap the fields in a <code>GestureDetector</code> and detect taps to move focus.

💡 Hint

Think about how Flutter manages keyboard focus.

Calling FocusScope.of(context).nextFocus() moves the focus to the next focusable widget, which is the next text field.

🔧 Debug

expert

Why does this TextFormField not update its text when typing?

This Flutter code uses a TextFormField with a TextEditingController. The user types, but the text does not update on screen. Why?

Flutter

final controller = TextEditingController(text: 'Hello');

TextFormField(
  controller: controller,
  onChanged: (value) {
    controller.text = value;
  },
)

A The controller must be disposed inside onChanged to update text.

B Setting controller.text inside onChanged causes the cursor to jump and text not to update properly.

C The initial text must be set inside onChanged, not in controller constructor.

D The TextFormField needs a validator to update text.

💡 Hint

Think about what happens when you set controller.text inside onChanged.

Setting controller.text inside onChanged resets the text and cursor position, causing input to not update as expected.