Canvas 文本点击无效。怎么修？

Question

JS代码-

var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");
var $canvas = $("#canvas");
var BB=canvas.getBoundingClientRect();
var offsetX = BB.left;
var offsetY = BB.top;
var mx;
var my;
var texts = [];
var images = [];
var dragF = -1;
var mode = "none";

function print(log) {
    console.log(log)
}

function draw() {
    ctx.clearRect(0, 0, canvas.width, canvas.height);
    for(const { text, x, y, width, height } of texts) {
        ctx.fillText(text, x, y);
        // trace
        ctx.strokeRect(x, y, width, height);
    }
}

function addNewText(string_text, arrayname) {
    var y = texts.length * 20 + 20;
    var text = {
        text: string_text,
        x: 20,
        y: y,
        name: arrayname
    };
    ctx.font = "32px verdana";
    ctx.textBaseline = "top";
    text.width = ctx.measureText(text.text).width;
    text.height = 32;

    texts.push(text);
    draw();
}

function hitDrag(x,y,textIndex) {
    var r=texts[textIndex];
    return (x>r.x && x<r.x+r.width && y>r.y && y<r.y+r.height);
}

function myDown(e) {
    e.preventDefault();
    e.stopPropagation();

    mx=parseInt(e.clientX-offsetX);
    my=parseInt(e.clientY-offsetY);

    for(var i=0;i<texts.length;i++){
        if(hitDrag(mx,my,i)){
            print("found");
            dragF = i;
        }
    }
}

$("#canvas").mousedown(function(e) { 
    myDown(e);
});

addNewText("Hello world", "text 1");

所以我听从了@Kaiido 对 BBox 的建议，并将 textBaseline 更改为“top”。非常适合。现在，当我在我的帮助论坛下运行他的代码剪接器时，它似乎可以工作并打印“found”（这表明它可以工作）。当我运行它时，它似乎不起作用。这可能是什么原因。

他的代码有效：https://gyazo.com/511bf35523fcb3ea8a26c2b088530f99

我的编码不工作：https://gyazo.com/743c38f6a33a7f1f4513bac361c23588

HTML代码-

<div id="middle_container">
            <div class="center_container">
                <canvas id="canvas"></canvas>
            </div>
        </div>

CSS代码-

  #canvas {
        height: 667px;
        width: 800px;
        touch-action: auto;
        cursor: inherit;
        visibility: visible;
        border: 1px solid grey;
    }

Answer 1

你的问题的关键在于：

 console.log("t:", texts.length); // empty array
  //empty array so does nothing
  for (var i = 0; i < texts.length; i++) {

var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");
var $canvas = $("#canvas");
var BB = canvas.getBoundingClientRect();
console.log("BB:", BB);
var offsetX = BB.left;
var offsetY = BB.top;
var mx;
var my;
var texts = [];
var images = [];
var dragF = -1;
var mode = "none";
var print = console.log;

function addNewText(string_text, arrayname) {
  var y = texts.length * 20 + 20;
  var text = {
    text: string_text,
    x: 20,
    y: y,
    name: arrayname
  };
  ctx.font = "16px verdana";
  text.width = ctx.measureText(text.text).width;
  text.height = 16;

  texts.push(text);
  draw();
}

function hitDrag(x, y, textIndex) {
  var r = texts[textIndex];
  return (x > r.x && x < r.x + r.width && y > r.y && y < r.y + r.height);
}

function myDown(e) {
  console.log("called");
  e.preventDefault();
  e.stopPropagation();
  console.log("c2");
  mx = parseInt(e.clientX - offsetX);
  my = parseInt(e.clientY - offsetY);
  console.log("t:", texts.length); // empty array
  //empty array so does nothing
  for (var i = 0; i < texts.length; i++) {
    console.log("i:", i);
    if (hitDrag(mx, my, i)) {
      print("found");
      dragF = i;
    }
  }
}

$("#canvas").mousedown(function(e) {
  console.log("triggered");
  myDown(e);
});

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<canvas id="canvas"></canvas>

Answer 2

问题是您使用的是默认 textBaseline = "alphabetic"，这使得 y 值对应于 o.

等字形的底部

您可以通过跟踪看到您的文本BBox是错误的：

var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");
var $canvas = $("#canvas");
var BB=canvas.getBoundingClientRect();
var offsetX = BB.left;
var offsetY = BB.top;
var mx;
var my;
var texts = [];
var images = [];
var dragF = -1;
var mode = "none";
var print = console.log;

function addNewText(string_text, arrayname) {
    var y = texts.length * 40 + 40;
    var text = {
        text: string_text,
        x: 20,
        y: y,
        name: arrayname
    };
    ctx.font = "32px verdana";
    text.width = ctx.measureText(text.text).width;
    text.height = 32;

    texts.push(text);
    draw();
}

function hitDrag(x,y,textIndex) {
    var r=texts[textIndex];
    return (x>r.x && x<r.x+r.width && y>r.y && y<r.y+r.height);
}

function myDown(e) {
    e.preventDefault();
    e.stopPropagation();

    mx=parseInt(e.clientX-offsetX);
    my=parseInt(e.clientY-offsetY);

    for(var i=0;i<texts.length;i++){
        if(hitDrag(mx,my,i)){
            print("found");
            dragF = i;
        }
    }
}

$("#canvas").mousedown(function(e) { 
    myDown(e);
});

function draw() {
  for(const { text, x, y, width, height } of texts) {
    ctx.fillText(text, x, y);
    // trace the text's BBox
    ctx.strokeRect(x, y, width, height);
  }
}

addNewText("Hello world", "text 1");

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<canvas id="canvas"></canvas>

将此 textBaseline 设置为 "top" 将以较低的成本将问题最小化，但这不是完美的选择，而且如果您使用特殊字形。

var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");
var $canvas = $("#canvas");
var BB=canvas.getBoundingClientRect();
var offsetX = BB.left;
var offsetY = BB.top;
var mx;
var my;
var texts = [];
var images = [];
var dragF = -1;
var mode = "none";
var print = console.log;

function addNewText(string_text, arrayname) {
    var y = texts.length * 40 + 40;
    var text = {
        text: string_text,
        x: 20,
        y: y,
        name: arrayname
    };
    ctx.font = "32px verdana";
    ctx.textBaseline = "top";
    text.width = ctx.measureText(text.text).width;
    text.height = 32;

    texts.push(text);
    draw();
}

function hitDrag(x,y,textIndex) {
    var r=texts[textIndex];
    return (x>r.x && x<r.x+r.width && y>r.y && y<r.y+r.height);
}

function myDown(e) {
    e.preventDefault();
    e.stopPropagation();

    mx=parseInt(e.clientX-offsetX);
    my=parseInt(e.clientY-offsetY);

    for(var i=0;i<texts.length;i++){
        if(hitDrag(mx,my,i)){
            print("found");
            dragF = i;
        }
    }
}

$("#canvas").mousedown(function(e) { 
    myDown(e);
});

function draw() {
  for(const { text, x, y, width, height } of texts) {
    ctx.fillText(text, x, y);
    // trace the text's BBox
    ctx.strokeRect(x, y, width, height);
  }
}

addNewText("Hello world", "text 1");

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<canvas id="canvas"></canvas>

各大浏览器终于支持TextMetrics接口的actualBoundingBoxXXX属性，所以我们现在可以得到精确的文本BBox：

function getTextBBox( ctx, text ) {
  const metrics = ctx.measureText( text );
  const left = metrics.actualBoundingBoxLeft * -1;
  const top = metrics.actualBoundingBoxAscent * -1;
  const right = metrics.actualBoundingBoxRight;
  const bottom = metrics.actualBoundingBoxDescent;
  const width = right - left;
  const height = bottom - top;
  return { left, top, right, bottom, width, height };
}

function getTextBBox(ctx, text) {
  const metrics = ctx.measureText(text);
  const left = metrics.actualBoundingBoxLeft * -1;
  const top = metrics.actualBoundingBoxAscent * -1;
  const right = metrics.actualBoundingBoxRight;
  const bottom = metrics.actualBoundingBoxDescent;
  const width = right - left;
  const height = bottom - top;
  return { left, top, right, bottom, width, height };
}
var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");
var $canvas = $("#canvas");
var BB=canvas.getBoundingClientRect();
var offsetX = BB.left;
var offsetY = BB.top;
var mx;
var my;
var texts = [];
var images = [];
var dragF = -1;
var mode = "none";
var print = console.log;

function addNewText(string_text, arrayname) {
    ctx.font = "32px verdana";
    ctx.textBaseline = "top";
    const bbox = getTextBBox(ctx, string_text);
    const prevText = texts[ texts.length - 1 ];
    const text = {
        text: string_text,
        x: 20,
        y: (prevText ? (prevText.y + prevText.bbox.bottom) : 32) + 2,
        bbox,
        name: arrayname
    };
    texts.push(text);
    draw();
}

function hitDrag(mx,my,textIndex) {
    const { x, y, bbox: { left, right, top, bottom } } = texts[textIndex];
    return (
      mx > x + left &&
      mx < x + right &&
      my > y + top &&
      my < y + bottom
    );
}

function myDown(e) {
    e.preventDefault();
    e.stopPropagation();

    mx=parseInt(e.clientX-offsetX);
    my=parseInt(e.clientY-offsetY);

    for(var i=0;i<texts.length;i++){
        if(hitDrag(mx,my,i)){
            print("found");
            dragF = i;
        }
    }
}

$("#canvas").mousedown(function(e) { 
    myDown(e);
});

function draw() {
  for(const { text, x, y, bbox } of texts) {
    ctx.fillText(text, x, y);
    // trace the text's BBox
    ctx.strokeRect(x + bbox.left, y + bbox.top, bbox.width, bbox.height);
  }
}

addNewText("Hello world", "text 1");
addNewText("Works fine?", "text 2");

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<canvas id="canvas"></canvas>

现在，如果您需要查找是否单击了字形的实际绘制像素，则需要获取 canvas 的 ImageData，我已经在 [=] 中展示了如何执行此操作23=]，这里不再赘述。

Canvas 文本点击无效。怎么修？

Canvas text click not working. How to fix?

javascript

jquery

html5-canvas