TextFormField in Flutter - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a TextFormField in Flutter?

A TextFormField is a widget that allows users to enter and edit text in a form. It integrates with form validation and saving mechanisms.

beginner

How do you validate user input in a TextFormField?

You provide a validator function that returns an error message string if the input is invalid, or null if it is valid.

beginner

What property do you use to save the input value from a TextFormField?

Use the onSaved property, which takes a function to save the input value when the form is saved.

intermediate

How can you control the text inside a TextFormField programmatically?

By using a TextEditingController assigned to the controller property of the TextFormField.

intermediate

Why should you wrap TextFormField widgets inside a Form widget?

Wrapping inside a Form allows collective validation and saving of multiple fields together using a GlobalKey<FormState>.

Which property of TextFormField is used to check if the input is valid?

A validator

B onChanged

C controller

D onSaved

The validator property is a function that checks the input and returns an error message if invalid.

What does the onSaved callback do in a TextFormField?

A It validates the input

B It saves the input value when the form is saved

C It clears the input field

D It controls the keyboard type

onSaved is called when the form is saved to store the input value.

Which widget should wrap TextFormField widgets to manage form state?

A Scaffold

B Container

C Form

D Column

The Form widget manages the state of multiple TextFormField widgets.

How do you programmatically change the text inside a TextFormField?

A Use a <code>TextEditingController</code>

B Use the <code>onChanged</code> callback

C Use the <code>validator</code> function

D Use the <code>onSaved</code> callback

A TextEditingController lets you read and change the text programmatically.

What happens if the validator function returns a non-null string?

A The form input is considered valid

B The keyboard is dismissed

C The input field is cleared

D The form input is considered invalid and shows the error message

Returning a string from validator means the input is invalid and displays that string as an error.

Explain how to use TextFormField inside a form to collect and validate user input.

Think about how you group fields and check their values before saving.

Describe how TextEditingController works with TextFormField and why it might be useful.

Consider when you want to change or read the text from code, not just user typing.

Practice

1. What is the main purpose of a TextFormField widget in Flutter?

easy

A. To let users type and edit text input

B. To display images in the app

C. To create buttons for user interaction

D. To navigate between screens

Solution

1. Step 1: Understand the widget's role

   TextFormField is designed to accept text input from users.

2. Step 2: Compare with other widget functions

   Other widgets like Image or ElevatedButton serve different purposes, not text input.

3. Final Answer:

   To let users type and edit text input -> Option A

4. Quick Check:

   Text input = TextFormField [OK]

Hint: TextFormField is for typing text only [OK]

Common Mistakes:

• Confusing TextFormField with buttons or images
• Thinking it handles navigation
• Assuming it only displays static text

2. Which of the following is the correct way to add a label inside a TextFormField?

easy

A. TextFormField(labelText: 'Name')

B. TextFormField(hint: 'Name')

C. TextFormField(decoration: InputDecoration(labelText: 'Name'))

D. TextFormField(text: 'Name')

Solution

1. Step 1: Recall how decoration works

   Labels and hints are added inside the decoration property using InputDecoration.

2. Step 2: Check each option's syntax

   Only TextFormField(decoration: InputDecoration(labelText: 'Name')) correctly uses decoration: InputDecoration(labelText: 'Name').

3. Final Answer:

   TextFormField(decoration: InputDecoration(labelText: 'Name')) -> Option C

4. Quick Check:

   Label goes inside decoration [OK]

Hint: Use decoration with InputDecoration for labels [OK]

Common Mistakes:

• Putting labelText directly on TextFormField
• Using hint instead of labelText for labels
• Confusing text property with label

3. What will happen when the following Flutter code runs and the user submits an empty form?

final _formKey = GlobalKey<FormState>();

Form(
  key: _formKey,
  child: TextFormField(
    validator: (value) {
      if (value == null || value.isEmpty) {
        return 'Please enter text';
      }
      return null;
    },
  ),
)

medium

A. The form submits successfully without errors

B. The app crashes due to missing onSaved callback

C. The TextFormField clears automatically

D. The validator returns 'Please enter text' and shows an error

Solution

1. Step 1: Understand validator behavior

   The validator checks if the input is empty and returns an error message if true.

2. Step 2: Analyze empty input case

   If the user submits empty text, the validator returns 'Please enter text', showing an error below the field.

3. Final Answer:

   The validator returns 'Please enter text' and shows an error -> Option D

4. Quick Check:

   Empty input triggers validator error [OK]

Hint: Empty input triggers validator error message [OK]

Common Mistakes:

• Assuming form submits without validation
• Thinking missing onSaved causes crash
• Believing TextFormField clears on submit

4. Identify the error in this Flutter code snippet using TextFormField:

TextFormField(
  validator: (value) {
    if (value.isEmpty) {
      return 'Required';
    }
    return null;
  },
)

medium

A. Validator should return a boolean, not a string

B. Missing null check for value before calling isEmpty

C. TextFormField requires an onChanged callback

D. Validator cannot be used without a Form widget

Solution

1. Step 1: Check validator parameter type

   The validator receives a nullable String? value, so it can be null.

2. Step 2: Identify missing null check

   The code calls value.isEmpty without checking if value is null, causing a runtime error.

3. Final Answer:

   Missing null check for value before calling isEmpty -> Option B

4. Quick Check:

   Always check for null before isEmpty [OK]

Hint: Check for null before using isEmpty on value [OK]

Common Mistakes:

• Ignoring nullable value in validator
• Expecting validator to return boolean
• Thinking onChanged is mandatory

5. You want to create a form with a TextFormField that only accepts numbers and shows a hint 'Enter age'. Which code snippet correctly implements this?

hard

A. TextFormField(
keyboardType: TextInputType.number,
decoration: InputDecoration(hintText: 'Enter age'),
validator: (value) {
if (value == null || value.isEmpty) return 'Age required';
if (int.tryParse(value) == null) return 'Enter a valid number';
return null;
},
)

B. TextFormField(
keyboardType: TextInputType.text,
decoration: InputDecoration(labelText: 'Enter age'),
validator: (value) => null,
)

C. TextFormField(
decoration: InputDecoration(hintText: 'Enter age'),
validator: (value) {
if (value!.isEmpty) return 'Age required';
return null;
},
)

D. TextFormField(
keyboardType: TextInputType.number,
decoration: InputDecoration(hintText: 'Age'),
validator: (value) {
if (value == null) return 'Age required';
return null;
},
)

Solution

1. Step 1: Identify required properties for numeric input with hint

   The correct implementation requires keyboardType: TextInputType.number, decoration: InputDecoration(hintText: 'Enter age'), and a validator checking for null/empty and valid integer parse.

2. Step 2: Verify validator handles all cases

   Validator must return error for null, empty, or non-numeric input using if (value == null || value.isEmpty) and int.tryParse(value) == null.

3. Final Answer:

   TextFormField( keyboardType: TextInputType.number, decoration: InputDecoration(hintText: 'Enter age'), validator: (value) { if (value == null || value.isEmpty) return 'Age required'; if (int.tryParse(value) == null) return 'Enter a valid number'; return null; }, ) -> Option A

4. Quick Check:

   number keyboard + exact hint + null/empty/parse validator [OK]

Hint: Use keyboardType.number and validate parsing for numbers [OK]

Common Mistakes:

• Using text keyboard for numeric input
• Not validating numeric format
• Confusing hintText and labelText