Monday 16 July 2018

Building a retro game with React.js. Part 4 - Drawing the line somewhere

In the previous instalment of this series, I was able to get the player's "sprite" (actually just a div with a border!) to move around the existing lines on the edge of the screen. The next logical step is to allow the player to draw their own lines, which, upon joining at both ends to existing lines, will become part of the "navigable" world the player can manoeuvre through.

Bugs Galore
It was at this point where I started being plagued by off-by-one errors; it seemed everywhere I turned I was encountering little one-pixel gaps when drawing lines, because:
  • My on-screen lines are actually 2px wide
  • My line-drawing function was doing an incorrect length calculation (had to do (right - left) + 1)
  • I was not updating my position at the right time, so was storing my "old" position as the current line's end point; and;
  • I was naively using setState and expecting the new this.state to be visible immediately

My solution to almost all of these problems (with the exception of the UI line-drawing function) was to write a heap of unit tests; these generally flushed things out pretty quickly.

Writing the line-drawing function was a weird experience. Virtually every software development "environment" I've ever used before, from BBC Basic on my Acorn Electron on, has had a function like drawLine(startX, startY, endX, endY);. And I could do that with an HTML canvas, but I'm (possibly/probably wrongly) not going to use a canvas. I'm drawing divs dammit! Here's what my function looks like:
renderLine = (line, lineIndex) => {
  const [top, bottom] = minMax(line[2], line[4]);
  const [left, right] = minMax(line[1], line[3]);
  const height = (bottom - top) + 1;
  const width = (right - left) + 1;
  return <Line key={`line-${lineIndex}`}
                  style={{ top: `${top}px`, 
                           left: `${left}px`,
                           width: `${width}px`,
                           height: `${height}px` }} />;
Where minMax is a simple function that returns [min, max] of its inputs, and Line is a React-Emotion styled div:
const Line = styled('div')`
  position: absolute;
  border: 1px solid ${FrenzyColors.CYAN};
  box-sizing: border-box;
  padding: 0;
Notice that I resisted the temptation to pass the top, left etc into the Line wrapper. The reason for this is that doing so results in a whole new CSS class being created, and getting applied to the line, every time one of these computed values changes. This seems wasteful when most of the line's attributes remain the same! So I use an inline style to position the very-thin divs where I need it.
Time Tracking
Up to about 12 hours by my rough estimate.

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