Custom painters (CustomPaint) in Flutter - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a CustomPaint widget with a painter.

Flutter

CustomPaint(
  size: Size(100, 100),
  painter: [1](),
)

Drag options to blanks, or click blank then click option'

AText

BContainer

CMyPainter

DScaffold

Attempts:

3 left

💡 Hint

Common Mistakes

Using widgets like Container or Text as painter.

Forgetting to create a CustomPainter subclass.

The painter property requires a class that extends CustomPainter, like MyPainter.

2fill in blank

medium

Complete the paint method signature inside a CustomPainter subclass.

Flutter

@override
void paint([1] canvas, Size size) {
  // drawing code
}

Drag options to blanks, or click blank then click option'

ACanvas

BPaint

CBuildContext

DWidget

Attempts:

3 left

💡 Hint

Common Mistakes

Using Paint instead of Canvas as parameter.

Confusing BuildContext with Canvas.

The paint method receives a Canvas object to draw on.

3fill in blank

hard

Fix the error in the shouldRepaint method to always repaint.

Flutter

@override
bool shouldRepaint([1] oldDelegate) {
  return true;
}

Drag options to blanks, or click blank then click option'

ACustomPainter

BWidget

CPaint

DBuildContext

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong parameter type like Widget or Paint.

Omitting the parameter.

The shouldRepaint method receives the old CustomPainter to compare.

4fill in blank

hard

Fill both blanks to draw a red circle at the center of the canvas.

Flutter

void paint(Canvas canvas, Size size) {
  final paint = Paint()..color = [1];
  canvas.drawCircle(size.center(Offset.zero), [2], paint);
}

Drag options to blanks, or click blank then click option'

AColors.red

B50.0

CColors.blue

D100

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong color like blue.

Using integer 100 instead of double 50.0 for radius.

Use Colors.red for the paint color and 50.0 as the circle radius.

5fill in blank

hard

Fill all three blanks to create a CustomPainter that draws a green rectangle.

Flutter

class [1] extends CustomPainter {
  @override
  void paint(Canvas canvas, Size size) {
    final paint = Paint()..color = [2];
    canvas.drawRect(Rect.fromLTWH(0, 0, size.width, size.height), [3]);
  }

  @override
  bool shouldRepaint(covariant CustomPainter oldDelegate) => false;
}

Drag options to blanks, or click blank then click option'

AGreenRectPainter

BColors.green

Cpaint

DRedRectPainter

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong class name unrelated to green rectangle.

Passing wrong color or variable to drawRect.

The class name is GreenRectPainter, color is Colors.green, and the paint object is passed to drawRect.

Practice

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

easy

A. To draw custom graphics directly on the screen

B. To manage app navigation between screens

C. To handle user input events like taps and swipes

D. To create standard UI buttons and text fields

Solution

1. Step 1: Understand CustomPainter role

   A CustomPainter is used to draw graphics directly on the canvas in Flutter apps.

2. Step 2: Compare with other options

   Options A, B, and C relate to standard UI, navigation, and input events, not drawing.

3. Final Answer:

   To draw custom graphics directly on the screen -> Option A

4. Quick Check:

   CustomPainter = Draw graphics [OK]

Hint: CustomPainter is for drawing, not UI controls or navigation [OK]

Common Mistakes:

• Confusing CustomPainter with widget layout
• Thinking it handles user input
• Assuming it manages navigation

2. Which method must you override when creating a CustomPainter subclass to draw on the canvas?

easy

A. paint

B. build

C. createState

D. initState

Solution

1. Step 1: Identify CustomPainter methods

   The paint method is where drawing commands are written.

2. Step 2: Exclude unrelated methods

   build, createState, and initState belong to widgets, not painters.

3. Final Answer:

   paint -> Option A

4. Quick Check:

   Draw in paint() method [OK]

Hint: paint() draws; build() is for widgets [OK]

Common Mistakes:

• Using build() instead of paint()
• Confusing widget lifecycle methods with painter methods
• Forgetting to override paint()

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

class MyPainter extends CustomPainter {
  @override
  void paint(Canvas canvas, Size size) {
    final paint = Paint()..color = Colors.red;
    canvas.drawCircle(Offset(size.width / 2, size.height / 2), 50, paint);
  }

  @override
  bool shouldRepaint(covariant CustomPainter oldDelegate) => false;
}

Widget build(BuildContext context) {
  return CustomPaint(
    size: Size(200, 200),
    painter: MyPainter(),
  );
}

medium

A. A blue square filling the 200x200 area

B. A red circle centered in a 200x200 area

C. Nothing is drawn because shouldRepaint returns false

D. A red circle at the top-left corner

Solution

1. Step 1: Analyze paint method

   The paint method draws a red circle at the center (width/2, height/2) with radius 50.

2. Step 2: Understand shouldRepaint effect

   shouldRepaint returning false means no redraw needed, but initial paint still happens.

3. Final Answer:

   A red circle centered in a 200x200 area -> Option B

4. Quick Check:

   Draw circle center = red circle visible [OK]

Hint: shouldRepaint false still draws initially [OK]

Common Mistakes:

• Thinking shouldRepaint false means no drawing
• Assuming circle is at top-left
• Confusing circle with square shape

4. Identify the error in this CustomPainter subclass:

class MyPainter extends CustomPainter {
  @override
  void paint(Canvas canvas, Size size) {
    final paint = Paint()..color = Colors.green;
    canvas.drawRect(Rect.fromLTWH(0, 0, size.width, size.height), paint);
  }

  @override
  bool shouldRepaint(CustomPainter oldDelegate) {
    return true
  }
}

medium

A. Canvas cannot draw rectangles

B. paint method should return a value

C. Missing semicolon after return true

D. shouldRepaint must return false

Solution

1. Step 1: Check syntax in shouldRepaint

   The return statement lacks a semicolon after true, causing a syntax error.

2. Step 2: Verify other parts

   paint method correctly draws a rectangle; canvas supports rectangles; shouldRepaint can return true.

3. Final Answer:

   Missing semicolon after return true -> Option C

4. Quick Check:

   Syntax error = missing semicolon [OK]

Hint: Check semicolons after return statements [OK]

Common Mistakes:

• Forgetting semicolon after return
• Thinking paint must return a value
• Assuming canvas can't draw rectangles

5. You want to create a CustomPainter that draws a circle only when the color changes. Which is the best way to implement shouldRepaint to optimize performance?

class CirclePainter extends CustomPainter {
  final Color color;
  CirclePainter(this.color);

  @override
  void paint(Canvas canvas, Size size) {
    final paint = Paint()..color = color;
    canvas.drawCircle(Offset(size.width/2, size.height/2), 40, paint);
  }

  @override
  bool shouldRepaint(covariant CirclePainter oldDelegate) {
    // What to write here?
  }
}

hard

A. return oldDelegate.color == color;

B. return true;

C. return false;

D. return oldDelegate.color != color;

Solution

1. Step 1: Understand shouldRepaint purpose

   shouldRepaint returns true if the painter needs to redraw due to changes.

2. Step 2: Compare colors to decide repaint

   Repaint only if the new color differs from the old one, so compare oldDelegate.color and color.

3. Step 3: Evaluate options

   return oldDelegate.color != color; returns true when colors differ, optimizing redraw. return true; always repaints (less efficient). return false; never repaints (ignores changes). return oldDelegate.color == color; repaints when colors are same (wrong logic).

4. Final Answer:

   return oldDelegate.color != color; -> Option D

5. Quick Check:

   Repaint if color changed = oldDelegate.color != color [OK]

Hint: Repaint only if properties differ for efficiency [OK]

Common Mistakes:

• Always returning true wastes performance
• Always returning false ignores changes
• Using equality check wrongly in shouldRepaint