Workshop › Conway's Game of Life › Step 26 of 32

Drag to paint

What we are doing

While the pointer is down, every move writes painting into the cell under it. Letting go sets painting back to null.

```js
canvas.addEventListener('pointermove', (event) => {
  if (painting === null) return
  const cell = cellAt(toCanvas(event))
  if (cell) grid[cell.r][cell.c] = painting
})

document.addEventListener('pointerup', () => {
  painting = null
})
```

What it means

• pointermove: if nothing is being painted, stop; otherwise write painting into the cell under the pointer.
• pointerup is heard on the whole document, not the canvas: if you let go outside the canvas, the canvas would never hear it and painting would go on.

Your turn

Write the two listeners under the pointerdown listener, above function update() {.

Skills: Keyboard and mouse input

← Paint or erase? Patterns from text →

All steps

1. The board
2. Measure the grid
3. One living cell
4. A function that draws
5. The world in memory
6. Draw every living cell
7. A random soup
8. Count the neighbours
9. Four rules
10. Draw again and again
11. Next generation with N
12. Count the generations
13. Show the generation
14. A world without edges
15. Count the living
16. A generation every few frames
17. Let it live
18. Running or not
19. One place for the buttons
20. Space to play, N to step
21. Clear and shuffle
22. Where is the pointer?
23. Which cell?
24. Click a cell
25. Paint or erase?
26. Drag to paint
27. Patterns from text
28. The glider gun
29. Fire the gun
30. Buttons on the screen
31. Tap the buttons
32. Build it yourself: a world at rest