Formatting text on HTML canvas

Question

I'm making a game using HTML Canvas and vanilla javascript. I'm new to javascript so this may be easier than I think. I have two array's of objects that represent cards, each object has a 'text' property of between 40-100 characters that's drawn onto the screen dynamically onto a card that is 130 x 70px.

I need to format the text to fit the width restriction of the card (130px) and create a new line whenever necessary.

Any help is appreciated

Edited to make clearer

Answer 1

You can use the measureText() method from the canvas API.
As noted by Ken Fyrstenberg in this awesome answer,

canvas' measureText doesn't currently support measuring height (ascent + descent).

Below attempt uses an hardcoded lineHeight value that you'd have to find before rendering text. Ken's answer does provide a way to programmatically find it.

[ Edit: thanks to markE's comment below, it now uses an approximation of 1.286*the font-size. ]

So here it is, dirty and there must be better ways to do so but anyway...

var input = document.querySelector('input');
input.addEventListener('keyup', write, false);
var c = document.createElement('canvas'),ctx = c.getContext('2d');
c.width = 400, c.height = 150; document.body.appendChild(c);

// simple box object for the card
var card = {x: 25, y: 25, w: 130, h: 70};
ctx.fillStyle = "#CCCCCC";
ctx.fillRect(card.x, card.y, card.w, card.h);

var fontSize = 12;
ctx.font=fontSize+'px arial';

// Margins multipliers are chosen arbitrarly here, just because they fit well to my eyes
var margins = {t: 1.25*fontSize, l: .7*fontSize, b: 2*fontSize, r: .7*fontSize},
marginsX = margins.l+margins.r,
marginsY = margins.t+margins.b;

// As suggested by markE, lineHeight is set to 1.286* the fontSize, 
// for a calculated way, see Ken's answer here : https://stackoverflow.com/a/17631567/3702797
var lineHeight = 1.286*fontSize;

// just a shortcut
var lineWidth = function(text) {
  return ctx.measureText(text).width;
  };

function write() {
  var txt = this.value;
  // Redraw our card
  ctx.fillStyle = "#CCCCCC";
  ctx.fillRect(card.x, card.y, card.w, card.h);
  // Split the input at any white space, you might want to change it
  var txtLines = txt.split(/\s/);
  var i = 0, maxWidth = card.w - marginsX;

  if(lineWidth(txt[0])>card.w || lineHeight>card.h-(margins.t/4) ){
      console.warn('TOO BIG FONT!!');
      return;
      }

  while(txtLines[i]) {
    // If our current line merged with the next line width isn't greater than the card one
    if (txtLines[i+1] && lineWidth(txtLines[i] + ' ' + txtLines[i+1]) < maxWidth) {
      // Merge them
      txtLines[i] += ' ' + txtLines.splice(i + 1, 1);
    } 
    else {
      // Is the one word too big? --> Dirtyphenation !
      if (lineWidth(txtLines[i]) > maxWidth) {
        // Add a new index with the two last chars since we'll add a dash
        txtLines.splice(i+1, 0, "");
        // If it's still too big
        while (lineWidth(txtLines[i]) > maxWidth) {
          var lastChars = txtLines[i].length - 2;
          // Append those two last chars to our new array index
          txtLines[i+1] = txtLines[i].substring(lastChars) + txtLines[i+1];
          // Remove them from our current index
          txtLines[i] = txtLines[i].substring(0, lastChars);
        }
        // Add the final dash
        txtLines[i] += "-";
      }
 
      // Is our text taller than the card height?
   if (lineHeight*i > card.h-marginsY){
        // If there is more text coming after...
        if (txtLines[i+1]){
            // ...and it fits in the line
            if(lineWidth(txtLines[i]+' '+txtLines[i+1])<maxWidth){
               continue;
            }
            // ...and it doesn't fit in the line
            else{
             // Does a single char fit with the ellipsis ?
                if(lineWidth(txtLines[i][0]+'...')<maxWidth){
                    // remove a char until we can put our ellipsis
     while (lineWidth(txtLines[i]+'...') > maxWidth){
      txtLines[i] = txtLines[i].substring(0,txtLines[i].length-1)
     }
    }else{
     return;
     }
               txtLines[i] += '...';
                // remove the surplus from the array
               txtLines = txtLines.slice(0,i+1);
            }
        }
      // stop looping here since we don't have space anymore
      break;
      }
      // Go to next line
   i++;
  }
  }
  ctx.fillStyle = "#000";
  // Where to draw
  var x = card.x + (margins.l);
  var y = card.y + (margins.t);
  // Iterate through our lines
  for (var i = 0; i < txtLines.length; i++) {
    ctx.fillText(txtLines[i], x, y + (i * lineHeight));
  }
}

canvas {border: 1px solid}

<input type="text" />