Opacity slider for a fabric object

问题

I have a canvas and it includes fabric objects.I want to do opacity slider for text area's backround "textbackroundColor" in canvas.How can I do slide and change functions?

$("#backgroundOpacity").slider( {
     min: 10,
     max: 50,
     value: 30,
     slide: text_bgColor_slider_moved,
     change: text_bgColor_slider_changed

  });

回答1:

first get active object of canvas

var activeObject = canvas.getActiveObject();

and then

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

回答2:

Another way, via a range input coupled with $(document).on

$(document).on("change", "#alpha", function () {
transval = $(this).val();
var obj = canvas.getActiveObject();
obj.set({
opacity: transval
});
canvas.renderAll();
});

来源：https://stackoverflow.com/questions/28813552/opacity-slider-for-a-fabric-object