Stack and Positioned in Flutter

Introduction

Stack lets you place widgets on top of each other like layers. Positioned helps you put these widgets exactly where you want inside the Stack.

You want to show a badge on top of an icon.

You want to overlap text on an image.

You want to create a custom button with an icon floating at a corner.

You want to place widgets at specific spots inside a container.

Syntax

Flutter

Stack(
  children: [
    Widget1(),
    Positioned(
      top: 10.0,
      left: 20.0,
      child: Widget2(),
    ),
  ],
)

Stack's children are drawn in order, so later children appear on top.

Positioned requires at least one of top, bottom, left, or right to place the child.

Examples

A blue square with a smaller red square positioned 10 pixels from top and left.

Flutter

Stack(
  children: [
    Container(color: Colors.blue, width: 100, height: 100),
    Positioned(
      top: 10,
      left: 10,
      child: Container(color: Colors.red, width: 50, height: 50),
    ),
  ],
)

Text with a star icon positioned at the bottom-right corner.

Flutter

Stack(
  children: [
    Text('Hello'),
    Positioned(
      bottom: 0,
      right: 0,
      child: Icon(Icons.star),
    ),
  ],
)

Stack with two containers layered, second container is semi-transparent and on top.

Flutter

Stack(
  children: [
    Container(color: Colors.green, width: 100, height: 100),
    // No Positioned, child fills Stack by default
    Container(color: Colors.yellow.withOpacity(0.5), width: 50, height: 50),
  ],
)

Sample App

This app shows a blue square with a smaller red square positioned inside it near the top-left corner. The text 'Top Layer' is positioned near the bottom-right corner on top of both squares.

Flutter

import 'package:flutter/material.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(title: const Text('Stack and Positioned Example')),
        body: Center(
          child: Stack(
            children: [
              Container(
                width: 150,
                height: 150,
                color: Colors.blue,
              ),
              Positioned(
                top: 20,
                left: 20,
                child: Container(
                  width: 100,
                  height: 100,
                  color: Colors.red,
                ),
              ),
              Positioned(
                bottom: 10,
                right: 10,
                child: const Text(
                  'Top Layer',
                  style: TextStyle(color: Colors.white, fontWeight: FontWeight.bold),
                ),
              ),
            ],
          ),
        ),
      ),
    );
  }
}

Important Notes

Stack paints children in order, so later children appear on top.

Positioned widgets must have at least one position property: top, bottom, left, or right.

Using Stack and Positioned is great for overlapping widgets or custom layouts.

Summary

Stack lets you layer widgets on top of each other.

Positioned places a child widget at a specific position inside the Stack.

Use Stack and Positioned to create complex, overlapping UI elements.

Practice

1. What does the Stack widget do in Flutter?

easy

A. It layers widgets on top of each other.

B. It arranges widgets in a vertical list.

C. It creates a scrollable grid of widgets.

D. It displays widgets side by side horizontally.

Solution

1. Step 1: Understand Stack widget purpose

   The Stack widget allows placing widgets on top of each other in layers.

2. Step 2: Compare with other layout widgets

   Other widgets like Column or Row arrange widgets vertically or horizontally, not layered.

3. Final Answer:

   It layers widgets on top of each other. -> Option A

4. Quick Check:

   Stack = layering widgets [OK]

Hint: Stack means layering widgets over each other [OK]

Common Mistakes:

• Confusing Stack with Column or Row
• Thinking Stack scrolls content
• Assuming Stack arranges widgets side by side

2. Which of the following is the correct syntax to position a widget 10 pixels from the top and 20 pixels from the left inside a Stack?

easy

A. Positioned(left: 10, top: 20, child: Text('Hello'))

B. Positioned(top: 10, left: 20, child: Text('Hello'))

C. Positioned(bottom: 10, right: 20, child: Text('Hello'))

D. Positioned(10, 20, child: Text('Hello'))

Solution

1. Step 1: Check parameter order and values

   Positioned requires named parameters; order doesn't matter but values must match position.

2. Step 2: Verify correct top and left values

   Positioned(top: 10, left: 20, child: Text('Hello')) correctly sets top: 10 and left: 20 with child widget.

3. Final Answer:

   Positioned(top: 10, left: 20, child: Text('Hello')) -> Option B

4. Quick Check:

   top=10 and left=20 in Positioned [OK]

Hint: Use named parameters top and left with correct values [OK]

Common Mistakes:

• Swapping top and left values
• Placing child before position parameters
• Using bottom and right instead of top and left

3. What will be the visual result of this Flutter code?

Stack(
  children: [
    Container(width: 100, height: 100, color: Colors.red),
    Positioned(top: 20, left: 20, child: Container(width: 50, height: 50, color: Colors.blue))
  ]
)

medium

A. Only a blue square visible at top-left corner.

B. Two squares side by side, red on left and blue on right.

C. A red square with a smaller blue square overlapping near the top-left corner.

D. A blue square behind a red square, both fully visible.

Solution

1. Step 1: Understand Stack layering

   The red container is the base layer sized 100x100 pixels.

2. Step 2: Analyze Positioned child

   The blue container is 50x50 pixels placed 20 pixels from top and left, overlapping the red.

3. Final Answer:

   A red square with a smaller blue square overlapping near the top-left corner. -> Option C

4. Quick Check:

   Stack layers with Positioned overlap [OK]

Hint: Positioned child overlaps base widget at given offset [OK]

Common Mistakes:

• Thinking widgets appear side by side
• Assuming blue is behind red
• Ignoring Positioned offsets

4. Identify the error in this Flutter code snippet:

Stack(
  children: [
    Positioned(top: 10, child: Text('Hello')),
    Positioned(left: 20, child: Text('World'))
  ]
)

medium

A. Both Positioned widgets must have both top and left properties.

B. Positioned widgets cannot be direct children of Stack.

C. Positioned widgets must have at least two positioning properties (top, left, bottom, right).

D. Positioned widgets can have only one property; this code is correct.

Solution

1. Step 1: Review Positioned requirements

   Positioned widgets can use zero, one, or more positioning properties (top, left, bottom, right). Partial constraints are allowed (unspecified directions default to 0).

2. Step 2: Check given Positioned widgets

   The first uses top: 10 (Hello positioned 10px from top, left=0). The second uses left: 20 (World positioned 20px from left, top=0). No errors; valid code.

3. Final Answer:

   Positioned widgets can have only one property; this code is correct. -> Option D

4. Quick Check:

   Positioned allows partial properties [OK]

Hint: Positioned supports partial positioning properties [OK]

Common Mistakes:

• Thinking Positioned requires at least two properties
• Believing partial positioning is invalid
• Confusing Positioned requirements

5. You want to create a Stack where a small green circle is always centered over a larger yellow square, regardless of screen size. Which code snippet achieves this correctly?

hard

A. Stack(
children: [
Container(width: 200, height: 200, color: Colors.yellow),
Positioned.fill(child: Align(alignment: Alignment.center, child: Container(width: 50, height: 50, decoration: BoxDecoration(color: Colors.green, shape: BoxShape.circle))))
]
)

B. Stack(
alignment: Alignment.topLeft,
children: [
Container(width: 200, height: 200, color: Colors.yellow),
Container(width: 50, height: 50, decoration: BoxDecoration(color: Colors.green, shape: BoxShape.circle))
]
)

C. Stack(
children: [
Container(width: 200, height: 200, color: Colors.yellow),
Positioned(top: 75, left: 75, child: Container(width: 50, height: 50, decoration: BoxDecoration(color: Colors.green, shape: BoxShape.circle)))
]
)

D. Stack(
children: [
Container(width: 200, height: 200, color: Colors.yellow),
Positioned(bottom: 75, right: 75, child: Container(width: 50, height: 50, decoration: BoxDecoration(color: Colors.green, shape: BoxShape.circle)))
]
)

Solution

1. Step 1: Understand centering in Stack

   Using Positioned.fill with Align centers the child relative to the Stack size.

2. Step 2: Analyze each option

   Stack(
     children: [
       Container(width: 200, height: 200, color: Colors.yellow),
       Positioned.fill(child: Align(alignment: Alignment.center, child: Container(width: 50, height: 50, decoration: BoxDecoration(color: Colors.green, shape: BoxShape.circle))))
     ]
   )

   uses Positioned.fill and Align with Alignment.center, ensuring the green circle stays centered regardless of screen size.

3. Final Answer:

   Stack with Positioned.fill and Align to center the circle. -> Option A

4. Quick Check:

   Use Positioned.fill + Align for responsive centering [OK]

Hint: Use Positioned.fill with Align to center inside Stack [OK]

Common Mistakes:

• Hardcoding top and left offsets (Option A)
• Missing Positioned for precise control (Option B)
• Using bottom and right offsets incorrectly (Option D)