Incorporating Opacity Slider for Selected Object

Question

I'm trying to incorporate an opacity slider so that any selected objects are set to change based on the slider's position (100 being completely visible). I'm using fabric.js/master/dist/fabric.js and jQuery 3.3.1. What am I doing wrong here? I am basically trying to incorporate the Opacity slider on fabricjs.com.

I'm getting this error: "Uncaught TypeError: Cannot read property 'opacity' of undefined"

What I have: https://codepen.io/s-harper/pen/QxeMXL

Answers I've tried incorporating: Opacity slider for a fabric object

var canvas = new fabric.Canvas("c");
canvas.isDrawingMode = true;

// select, draw
$("#select").click(function() {
  canvas.isDrawingMode = false;
});
$("#draw").click(function() {
  canvas.isDrawingMode = true;
});

var activeObject = canvas.getActiveObject();

$("#alpha").slider( {
       max : 100,
       value : activeObject.opacity * 100,
       slide: function (event, ui) {
           activeObject.setOpacity(ui.value / 100);
           canvas.renderAll();
       },
       stop : function (event, ui) {
           canvas.renderAll();
       }
});

canvas {
  border: solid 1px #000;
}

fieldset {
  max-width: 350px;
}

<script src="https://code.jquery.com/jquery-3.3.1.js"></script>
<script src="https://rawgithub.com/kangax/fabric.js/master/dist/fabric.js"></script>
<canvas id="c" width="400" height="400"></canvas>
<br>
<button id="draw">Draw</button>
<button id="select">Select</button>
<br>
<br>
<fieldset>
  <legend>Controls</legend>
  <label for="alpha">Opactity</label>
  <input type="range" id="alpha" name="alpha" min="0" max="100" value="100" step="1" />
</fieldset>

Thanks in advance!

Answer 1

You need to get activeobject using events like selection:created, selection:updated and use jquery ui for jquery slider.

DEMO

var canvas = new fabric.Canvas("c");
canvas.isDrawingMode = true;

// select, draw
$("#select").click(function() {
  canvas.isDrawingMode = false;
});
$("#draw").click(function() {
  canvas.isDrawingMode = true;
});
var activeObject = null;
canvas.on('selection:created', function(options) {
  activeObject = options.target;
  $("#alpha").slider("option", "value", activeObject.opacity);
});
canvas.on('selection:updated', function(options) {
  activeObject = options.target;
  $("#alpha").slider("option", "value", activeObject.opacity);
});
canvas.on('selection:cleared', function(options) {
  activeObject = null;
});
$("#alpha").slider({
  max: 1,
  min: 0,
  step: 0.1,
  value: 1,
  slide: function(event, ui) {
    activeObject && (activeObject.opacity = ui.value)
    canvas.requestRenderAll();
  },
  stop: function(event, ui) {
    canvas.requestRenderAll();
  }
});

canvas {
  border: solid 1px #000;
}

fieldset {
  max-width: 350px;
}

<link href="https://cdnjs.cloudflare.com/ajax/libs/jqueryui/1.12.1/jquery-ui.css" rel="stylesheet"/>
<script src="https://code.jquery.com/jquery-3.3.1.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/jqueryui/1.12.1/jquery-ui.js"></script>
<script src="https://rawgithub.com/kangax/fabric.js/master/dist/fabric.js"></script>
<canvas id="c" width="400" height="400"></canvas>
<br>
<button id="draw">Draw</button>
<button id="select">Select</button>
<br>
<br>
<fieldset>
  <legend>Controls</legend>
  <label for="alpha">Opactity</label>
  <div id="alpha" name="alpha"></div>
</fieldset>