Canvas: pointerevents to move drawing around

  1. draw a red and a blue square and connect corners of blue square to corners of canvas
  2. use pointerevents to drag blue square around (hold down mouse key )
  3. should work for mouse or touchscreen

Source /home/ch45859/web/wlkl.ch/public_html/inf/js/c02canvasPoi.php 

<html>
<head>
  <title> <?php echo basename(__file__, '.php'); ?> </title>
  <meta name="viewport" content="width=device-width, initial-scale=1.0, maximum-scale=3.0, user-scalable=yes"/>
  <meta name="HandheldFriendly" content="true" />
  <meta name="apple-mobile-web-app-capable" content="YES" /> 
  <style>
      canvas {
        touch-action:none;
        border: 1px solid black;
      }
  </style>
</head>
<body onload="draw();">

<h1>Canvas: pointerevents to move drawing around</h1>
<ol><li>draw a red and a blue square and connect corners of blue square to corners of canvas 
</li><li>use pointerevents to drag blue square around (hold down mouse key ) 
</li><li>should work for mouse or touchscreen
</li></ol>
   <div style="float: left; margin-right: 30px;"> <canvas  id="tutorial" width="150" height="150"></canvas> </div>
<ul><li>event = <span id="ty"></span>
</li><li>x, y = <span id="xy"></span>
</li><li>pressure = <span id="press">empty</span>
</li></ul>
<h1 style="clear: both;">Source <?php echo __file__; ?> </h1>
<?php highlight_file(__file__) ?>
</body>

<script>
    const can = document.getElementById("tutorial");
    const ctx = can.getContext("2d");
    var o1 = {x: 30, y:30, w:50, h:50, sel: false}  
        , cnt = 0;

    function draw() {
        //a = Object.assign({}, aEv);
        ctx.clearRect(0, 0, 150, 150);
        // ctx.setTransform(0, 1, -1, 0, 150, 0);
        ctx.fillStyle = "rgb(200, 0, 0)";
        ctx.fillRect(10, 10, 50, 50);

        ctx.fillStyle = "rgba(" + (o1.sel ? "100, 100" : "0, 0") + ", 200, 0.5)";
        ctx.fillRect(o1.x, o1.y, o1.w, o1.h);  

        ctx.beginPath();
        ctx.lineWidth = 3;
        ctx.strokeStyle = "rgba(0, 0, 200, 0.5)";
        ctx.moveTo(o1.x, o1.y);
        ctx.lineTo(0, 0);
        ctx.moveTo(o1.x + o1.w, o1.y);
        ctx.lineTo(150, 0);
        ctx.stroke();    
        
        ctx.beginPath();
        const gradient = ctx.createLinearGradient(0, 0, 150, 0);
        // Add color stops
        gradient.addColorStop(0, "green");
        gradient.addColorStop(0.5, "cyan");
        gradient.addColorStop(1, "green");
        ctx.strokeStyle = gradient;
        ctx.moveTo(o1.x, o1.y+o1.h);
        ctx.lineTo(0, 150);
        ctx.moveTo(o1.x+o1.w, o1.y+o1.h);
        ctx.lineTo(150, 150);
        ctx.stroke();    
    }

    function pointerAna(e) {
        can.aX = Math.round(e.offsetX);
        can.aY = Math.round(e.offsetY);
        document.getElementById("ty").innerHTML = e.type + ' ' + (cnt+=1);
        document.getElementById("xy").innerHTML = can.aX + ", " + can.aY;
        document.getElementById("press").innerHTML = e.pressure;
    }

    can.addEventListener("pointerdown", (e) => { // select blue square, if cursor on it
        pointerAna(e);
        if (e.button === 0 && can.aX >= o1.x && can.aX <= o1.x + o1.w && can.aY >= o1.y && can.aY <= o1.y + o1.h ) {
            o1.sel = true;
            can.mvX = o1.x - can.aX;
            can.mvY = o1.y - can.aY;
            draw();
            // window.requestAnimationFrame(draw);
        }
    });

    can.addEventListener("pointerup", (e) => { // unselect
        pointerAna(e);
        o1.sel = false;
        draw();
    });

    can.addEventListener("pointermove", (e) => { // move blue square, if it is selected        
        pointerAna(e);
        if (o1.sel) {
            o1.x = can.aX + can.mvX;
            o1.y = can.aY + can.mvY;
            // window.requestAnimationFrame(draw);
            draw();
        } 
    });

</script>
</html>