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User at 8:12 pm, Jun 7, 2024

How can I write tests that check the content of the <canvas>

Codeium at 8:12 pm, Jun 7, 2024

Testing the content of a <canvas> element involves verifying that the correct drawings, images, or animations are present. Since <canvas> is a bitmap and doesn't retain a DOM structure, conventional DOM-based testing tools won't work. Instead, you can use the following approaches:

1. Visual Regression Testing: This involves taking snapshots of your canvas at various states and comparing them with baseline images to detect changes. Tools like Puppeteer can automate browser interactions and take screenshots.

2. Mocking Canvas Context: You can mock the canvas context to intercept and assert the calls to its methods. This won't test the visual output but can ensure the correct drawing functions are called with expected parameters.

3. Pixel Testing: Read individual pixels or regions of the canvas using context.getImageData() and assert their color values. This is more granular but can be complex to manage for larger areas or intricate drawings.

Here's an example of a simple test checking that a rectangle is drawn on the canvas using Jest and jest-canvas-mock:

import { render } from '@testing-library/react';
import '@testing-library/jest-dom';
import 'jest-canvas-mock'; // Mocks canvas when running tests
import MyCanvasComponent from './MyCanvasComponent'; // Import your canvas component

test('draws a blue rectangle', () => {
  const { getByTestId } = render(<MyCanvasComponent />);
  const canvas = getByTestId('my-canvas'); // Ensure your canvas has a data-testid attribute
  const context = canvas.getContext('2d');

  // Call your method that draws the rectangle here, if it's not done on component mount
  // myCanvasComponentInstance.drawBlueRectangle();

  expect(context.fillRect).toHaveBeenCalledWith(10, 10, 150, 80); // Check that fillRect was called with the right args
  expect(context.fillStyle).toBe('blue'); // Check that the fillStyle was set to blue
});

For more complex assertions, like checking if the Hero runs when typing in the terminal, you might need to simulate keypress events and then check the canvas for changes. This could involve using a combination of the methods above and potentially a game loop that processes the input and updates the canvas accordingly.

Keep in mind that for more advanced game logic, testing might involve mocking out dependencies and using game state snapshots or expected outcomes to verify the correctness of the logic.

If your tests need to interact with other parts of your app, like @xterm/xterm or other React components, make sure to mock or stub them appropriately, especially if they have side effects or asynchronous behavior.