Understanding Why Canvas is "Hidden" in VCL Controls and How to Work With It

Описание: Explore the reasons behind the protected `Canvas` in VCL controls, and discover elegant solutions for drawing on them efficiently without performance overhead.
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This video is based on the question https://stackoverflow.com/q/65050463/ asked by the user 'IceCold' ( https://stackoverflow.com/u/46207/ ) and on the answer https://stackoverflow.com/a/65052119/ provided by the user 'Andreas Rejbrand' ( https://stackoverflow.com/u/282848/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding Why Canvas is "Hidden" in VCL Controls and How to Work With It

When working with Visual Component Library (VCL) controls in Delphi or Lazarus, you may have encountered a situation where you need to draw graphics like shapes or text on different controls such as buttons or panels. However, a common frustration is that the Canvas property is protected, making it difficult to access directly for custom drawing operations. In this guide, we’ll dive into this issue, explore its roots, and present a solution that not only addresses the problem but also optimizes performance.

The Problem: Accessing the Hidden Canvas

Why Canvas is Protected

The protected status of the Canvas in VCL controls is primarily a design choice. It allows for encapsulation, ensuring that low-level drawing responsibilities are managed internally by the control itself. The intention behind this design pattern includes:

Preventing unintended modifications: By hiding the Canvas, the framework protects the integrity of the UI elements.

Encouraging best practices: This encourages developers to override the appropriate painting methods rather than directly manipulating the Canvas, leading to cleaner code and separation of concerns.

The Challenge

When you attempt to draw something like a triangle on a VCL control, you might think you can simply access the Canvas property. However, you’re met with a roadblock since it is not publicly accessible. For example, while trying to create a reusable function to draw a triangle, you find yourself constrained by the limitations of the API:

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This method can’t straightforwardly use Control.Canvas, forcing you to devise an alternative approach that often feels cumbersome and inefficient.

The Solution: An Elegant Workaround

Utilizing a “Control Cracker”

To access the hidden Canvas, you can create a subclass of TCustomControl, often called a “control cracker”. This allows you to bypass the protection layer while preserving the design philosophy.

Here is how to implement this:

Create a Control Cracker Class:

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Create a Function to Access the Canvas:

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Implement Your Drawing Function:

For example, to draw a simple “Frog” graphic:

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Full Example

Here’s a complete example to provide context:

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Alternative Approaches

While using a control cracker is a viable solution, it’s important to note that some developers prefer to pass the TCanvas (or even HDC) directly along with necessary dimensions to drawing functions. This alternative can simplify the code further and increase versatility as it allows the drawing function to work with various types of canvases, not just TCustomControl.

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Conclusion

Understanding the rationale behind the Canvas being “hidden” in VCL controls is crucial for effectively navigating this aspect of Delphi and Lazarus programming. By utilizing techniques such as the control cracker, you can elegantly draw on custom controls without unnecessary overhead, enabling resource-efficient and robust graphical applications. What’s your approach in handling such situations? Share your thoughts in the comments!