javascript - How to paragraph text drawn onto canvas

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javascript - How to paragraph text drawn onto canvas

asked Oct 24, 2021 in Technique[技术] by 深蓝 (71.8m points)

I have made a short quiz online following a tutorial, this is my first time using the canvas function. When I have typed my own questions, they appear as one line, and I am not sure how to break the lines up, in order for the question to display properly. Can anyone help?

Here is what I have so far:

<!DOCTYPE HTML>
<html>
<head>
<style>
    body{
        background-color: black;
    }

    #ccontainer{

        width: 550px;
        margin: 0 auto;
        margin-top: 110px;
    }

    #myCanvas{
        /*/ background: #FFF; /*/
    }
    </style>

    <script>
        window.onload = function(){

            var canvas = document.getElementById("myCanvas");
            var context = canvas.getContext("2d");
            var quizbg = new Image();
            var Question = new String;
            var Option1 = new String;
            var Option2 = new String;
            var Option3 = new String;
            var mx=0;                                   
            var my=0;
            var CorrectAnswer = 0;
            var qnumber = 0;
            var rightanswers=0;
            var wronganswers=0;
            var QuizFinished = false;
            var lock = false;
            var textpos1=25;
            var textpos2=145;
            var textpos3=230;
            var textpos4=325;
            var Questions = ["Which Manchester United Player won 
 the 2008 Golden Boot with 31 Goals?","At which club did Bobby Charlton start his football career?","Which year did Wayne Rooney win the BBC Young Sports Personality of the year award?"];
            var Options = [["Cristiano Ronaldo","Wayne Rooney","Ryan Giggs"],["Manchester United","Manchester City","Chelsea"],["2002","2003","2004"]];


            quizbg.onload = function(){
              context.drawImage(quizbg, 0, 0);
              SetQuestions();
            }
            quizbg.src = "quizbg.png";



            SetQuestions = function(){

                Question=Questions[qnumber];
                CorrectAnswer=1+Math.floor(Math.random()*3);

                if(CorrectAnswer==1){Option1=Options[qnumber][0];Option2=Options[qnumber][1];Option3=Options[qnumber][2];}
                if(CorrectAnswer==2){Option1=Options[qnumber][2];Option2=Options[qnumber][0];Option3=Options[qnumber][1];}
                if(CorrectAnswer==3){Option1=Options[qnumber][1];Option2=Options[qnumber][2];Option3=Options[qnumber][0];}

                context.textBaseline = "middle";
                context.font = "16pt sans-serif,Arial";
                context.fillText(Question,20,textpos1);
                context.font = "14pt sans-serif,Arial";
                context.fillText(Option1,20,textpos2);
                context.fillText(Option2,20,textpos3);
                context.fillText(Option3,20,textpos4);


            }//SetQuestions

                canvas.addEventListener('click',ProcessClick,false);

                function ProcessClick(ev) {
                    my=ev.y-canvas.offsetTop;
                    if(ev.y == undefined){
                        my = ev.pageY - canvas.offsetTop;
                }

                if(lock){
            ResetQ();
        }//if lock

        else{

        if(my>110 && my<180){GetFeedback(1);}
        if(my>200 && my<270){GetFeedback(2);}
        if(my>290 && my<360){GetFeedback(3);}

        }//!lock

            }//process click

            GetFeedback = function(a){

              if(a==CorrectAnswer){
                context.drawImage(quizbg, 0,400,75,70,480,110+(90*(a-1)),75,70);
                rightanswers++;
                //drawImage(image, sx, sy, sWidth, sHeight, dx, dy, dWidth, dHeight)
              }
              else{
                context.drawImage(quizbg, 75,400,75,70,480,110+(90*(a-1)),75,70);
                wronganswers++;
              }
              lock=true;
              context.font = "14pt sans-serif,Arial";
              context.fillText("Click again to continue",20,380);
            }//get feedback



        ResetQ= function(){
        lock=false;
        context.clearRect(0,0,550,400);
        qnumber++;
        if(qnumber==Questions.length){EndQuiz();}
        else{
        context.drawImage(quizbg, 0, 0);
        SetQuestions();}
        }


        EndQuiz=function(){
        canvas.removeEventListener('click',ProcessClick,false);
        context.drawImage(quizbg, 0,0,550,90,0,0,550,400);
        context.font = "20pt sans-serif,Arial";
        context.fillText("You have finished the quiz!",20,100);
        context.font = "16pt sans-serif,Arial";
        context.fillText("Correct answers: "+String(rightanswers),20,200);
        context.fillText("Wrong answers: "+String(wronganswers),20,240);
        }






        };
    </script>

</head>
 <body>

<div id="ccontainer">
    <canvas id="myCanvas" width="550" height="400"></canvas>
</div>

</body>
 </html>

Thanks!

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1 Answer

深蓝 · Answer 1 · 2021-10-23T18:23:39+0000

You can use context.measureText to get the width of given text.

Here's a function that wraps text using context.measureText to measure each word in a sentence and wrap to a new line when the current line exceeds a given width:

A Demo: http://jsfiddle.net/m1erickson/mQFDB/

function wrapText(context, text, x, y, maxWidth, fontSize, fontFace){
  var words = text.split(' ');
  var line = '';
  var lineHeight=fontSize;

  context.font = fontSize + "px " + fontFace;

  for(var n = 0; n < words.length; n++) {
    var testLine = line + words[n] + ' ';
    var metrics = context.measureText(testLine);
    var testWidth = metrics.width;
    if(testWidth > maxWidth) {
      context.fillText(line, x, y);
      line = words[n] + ' ';
      y += lineHeight;
    }
    else {
      line = testLine;
    }
  }
  context.fillText(line, x, y);
  return(y);
}

You can draw your text on the canvas like this:

var lastY=wrapText(context,"Hello",20,40,100,14,"verdana");

The lastY variable holds the y-coordinate of the last line of wrapped text. Therefore you can begin new text at lastY plus some padding:

lastY=wrapText(context,"World",20,lastY+20,100,14,"verdana");

This pattern lets you make text-wrapped paragraphs down the canvas (or in your case questions and multiple choice answers down the canvas).

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