Custom painters (CustomPaint) in Flutter - Build, Publish & Deploy

Build & Publish - Custom painters (CustomPaint)

Performance Impact

Using CustomPaint lets you draw graphics directly on the screen. This can be very fast if done right because it avoids many widget rebuilds. However, complex or frequent repaints can lower frame rates below 60fps, causing janky animations. Also, heavy painting uses more CPU and battery.

Optimization Tips

To keep your app smooth at 60fps, only repaint when needed by using shouldRepaint wisely. Cache complex drawings in Picture objects or use RepaintBoundary widgets to isolate repaints. Avoid expensive operations inside the paint method. Keep your paint code simple and use efficient drawing commands.

App Size and Startup Time

Custom painters add minimal size to your app bundle because they are code-based drawings, not image assets. They do not significantly affect startup time. However, if you use many custom painters or complex shaders, it can increase code size slightly.

iOS vs Android Differences

Flutter's CustomPaint works the same on iOS and Android because it uses Skia for rendering. However, performance may vary slightly due to device GPU differences. iOS devices often have ProMotion displays supporting 120fps, so optimizing for higher frame rates can improve user experience there.

Store Review Guidelines

• Ensure your custom painting does not cause excessive battery drain or overheating, as Apple and Google review app performance.
• Follow accessibility guidelines: provide alternatives if your custom painting conveys important information.
• Do not use custom painting to mimic system UI elements in a misleading way, which can cause rejection.
• Test on multiple devices to avoid crashes or slowdowns that can lead to store rejection.

Self-Check Question

Your app takes 5 seconds to load a screen with custom painting. What is likely wrong?

Answer: You might be doing heavy painting work synchronously on the main thread or repainting too often. Consider caching drawings, reducing paint complexity, or deferring expensive calculations.

Key Result

CustomPaint enables efficient, flexible drawing with minimal app size impact, but requires careful repaint management and optimization to maintain smooth 60fps performance and pass store reviews.

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