Add Pattern Painters Support for BarChart (Stripes, Dots, Squares)

[Caffo17]

This PR introduces support for custom pattern painters in the BarChart component, allowing each bar to be rendered with a configurable pattern. The following features and improvements are included:

• Pattern Painters: Added three new CustomPainter classes for bar patterns:
  • StripesPatternPainter: Draws diagonal, vertical, or horizontal stripes at a configurable angle.
  • CirclePoisPatternPainter: Draws a polka dot (circle) pattern.
  • SquarePoisPatternPainter: Draws a polka dot (square) pattern.
• BarChart Integration: The BarChartRodData now accepts a patternPainter property. If set, the bar will be rendered using the provided pattern.
• Sample Update: Updated the example (bar_chart_sample1.dart) to allow users to select and preview different bar patterns via a dropdown.
• Unit Tests: Added unit tests for all new pattern painters, ensuring correct default values, equality, and rendering.
• Documentation: Updated and translated comments to English for better clarity and maintainability.

Motivation

These changes make it easy to add visual variety to bar charts, supporting use cases such as accessibility, branding, and improved data distinction. The new pattern painters are fully customizable and integrate seamlessly with the existing chart API. Related to this issue.

Showcase

showcase_compressed.mp4

[imaNNeo]

Hi, thanks for your contribution. I really like the idea.
It has performance cost (because it iterates over every single part of the pattern).
So I would like to suggest using FragmentShader as they're perfect and performant for patterns.
Can you put some time and investigation into it please?

[Caffo17]

Hi @imaNNeo, I managed to get the same result using FragmentShaders but I'm not sure how to test them. I'm working on it.

[Caffo17]

Showcase with shaders

Simulator.Screen.Recording.-.iPhone.16.Pro.-.2025-08-03.at.19.29.06.mp4

[imaNNeo]

That looks amazing! 🤩
I see that now you're initializing all the Shaders inside the main(). So suppose that someone doesn't need to have all of them, or someone doesn't want it at all. I also see that we have FlShaderManager is Singleton.
So we can improve them all using the below suggestion:

• We can create a parent class called FLSurfacePainter, and it has an abstract initialize() method that the child classes should implement it (it also has the paint() method)
• Then you can set StripesPatternPainter's parent to FLSurfacePainter. This way, it has to implement the initialize() method (as well as the paint() method)
• So it loads the shader fragment in it's initialize method, and in our bar_chart_example1 (where we're using these shaders), we can initialize those we want to use in our init method (and implement a loading state for that one until all the shaders are loaded)
• This way, the user/developer is responsible for loading the pattern/shader painters that they want. (And we're also doing that in our bar_chart_example1)
• Inside the BarChartRodData, instead of patternPainter, we can have FLSurfacePainter surfacePainter (that is a more generic name)
• This way, we have 3 built-in shader pattern painters that users can use (and initialize them when they want to use them), they're also able to implement new types of surface painters. So maybe they just want to add a black overlay on top of what has been drawn.
• So this way, we don't need to have the FlShaderManager anymore.

Please let me know what you think.
I know these are a lot of changes and takes much time to do, but I'm pretty sure they're beneficial for all the community.

[Caffo17]

Hi @imaNNeo, thanks for your feedback. I've made the changes you asked for.

Let me know what you think about them.

[Caffo17]

Hi @imaNNeo , sorry for the ping. Could you check the last changes? Thanks 🙏

[imaNNeo]

I think we can use a generic abstraction for this part, like:

abstract class FlSurfacePainter<S extends FlShader> extends CustomPainter {
  FlSurfacePainter({required this.flShader});

  /// {@macro fl_shader}
  final S flShader;

  /// Initializes the painter by setting up the shader.
  Future<void> initialize() async {
    await flShader.init();
  }

  /// Returns whether the shader is initialized.
  bool get isInitialized => flShader.isInitialized;
}

And also, for the Painters, I think we can remove the shader parameter, we can just instantiate it to make it simpler, like:

class StripesPatternPainter extends FlSurfacePainter<StripesShader> {
  StripesPatternPainter({
    this.color = Colors.black,
    this.width = 2,
    this.gap = 4,
    this.angle = 45,
  }) : super(flShader: StripesShader());

And from the user's perspective, it can be like:

final stripesPainter = StripesPatternPainter(
width: 2,
gap: 8,
angle: 45,
);

@override
void initState() {
super.initState();
initializeShaders();
}

Future<void> initializeShaders() async {
await stripesPainter.initialize();
}

So I think this change improves the developer experience and makes it more expandable

[Caffo17]

This approach makes the flShader initialization untestable.
We can inject a mock shader via an optional parameter, falling back to the default implementation when not provided:

StripesPatternPainter({
    @visibleForTesting StripesShader? mockShader,
    this.color = Colors.black,
    this.width = 2,
    this.gap = 4,
    this.angle = 45,
  }) : super(flShader: mockShader ?? StripesShader());

What do you think?

[imaNNeo]

And lastly, I think we can also add the surfacePainter property in the BarChartRodStackItem as well. So this way pepole can have different patterns for different parts of the bar.
Just like what you see here: #2009

[Caffo17]

Hi @imaNNeo, You can check the changes