Stack and Positioned in Flutter - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ ui_behavior

intermediate

What is the visual output of this Flutter code?

Consider this Flutter widget tree using Stack and Positioned. What will you see on the screen?

Flutter

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

A A red square with a smaller blue square inside it, offset 20 pixels from top-left

B A blue square behind a red square, both fully overlapping

C Only a blue square visible at top-left corner

D A red square with a blue square positioned outside its bounds

💡 Hint

Remember, Positioned places its child relative to the Stack's edges.

The red container forms the base 100x100 square. The blue container is positioned 20 pixels from the left and top inside the Stack, so it appears inside the red square offset by 20 pixels.

❓ lifecycle

intermediate

How does Flutter handle hit testing in a Stack with Positioned widgets?

Given multiple Positioned widgets inside a Stack, which statement about touch event hit testing is true?

A Only widgets without <code>Positioned</code> can receive touch events

B The bottom widget always receives the touch event regardless of position

C The topmost widget visually receives the touch event if it is hit

D Touch events are ignored inside a Stack

💡 Hint

Think about which widget is drawn last and visible on top.

Flutter hit tests widgets from top to bottom visually. The topmost widget under the touch point receives the event.

📝 Syntax

advanced

Which code snippet correctly uses Positioned.fill inside a Stack?

Choose the Flutter code that fills the entire Stack area with a green container using Positioned.fill.

A Stack(children: [Positioned.fill(child: Container(color: Colors.green))])

B Stack(children: [Positioned.fill(left: 0, top: 0, child: Container(color: Colors.green))])

C Stack(children: [Positioned.fill(child: Container(color: Colors.green), left: 0)])

D Stack(children: [Positioned(child: Container(color: Colors.green), fill: true)])

💡 Hint

Positioned.fill does not take positional parameters like left.

Positioned.fill is a named constructor that fills the Stack. It only takes a child parameter.

🔧 Debug

advanced

Why does this Positioned widget cause a layout error?

Examine this code snippet inside a Stack. Why does it cause a layout error?

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

A Because Stack requires all Positioned widgets to have all four sides set

B Because <code>width</code> and <code>height</code> cannot be set inside Positioned children

C Because <code>left</code> must be zero or null

D Because <code>top</code> or <code>bottom</code> is missing, so vertical position is undefined

💡 Hint

Think about how Flutter calculates vertical position when only left is set.

Positioned requires at least two constraints for each axis. Here, vertical constraints are missing, so Flutter cannot determine vertical position.

🧠 Conceptual

expert

How does Stack's fit property affect its size and children layout?

Given a Stack with fit: StackFit.expand, what is the effect on the Stack and its children?

A Stack shrinks to fit only positioned children, ignoring non-positioned ones

B Stack expands to fill its parent, and non-positioned children fill the Stack

C Stack ignores the fit property and sizes to zero

D Stack sizes itself to the largest child, ignoring parent constraints

💡 Hint

Consider how StackFit.expand changes Stack's size behavior.

With StackFit.expand, the Stack fills its parent constraints. Non-positioned children are forced to fill the Stack's size.

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)