Question

I’m fairly new to Javascript so I was hoping for a bit of help. I’ve been playing with animating a graph using canvas and javascript. I have it so that when the page loads, it will fill up the graph to a pre-determined percentage.

I also have some buttons that have an onclick on them, and the aim is for clicking these buttons to execute the animating function again but with a different percentage. However, clicking on them doesn’t so a thing.

Any help you could give me on this would be great.

Code:

    window.onload = function(){
    //canvas initialization
    var canvas = document.getElementById("canvas");
    var ctx = canvas.getContext("2d");
    //dimensions
    var W = canvas.width;
    var H = canvas.height;
    //Variables
    var degrees = 0;
    var new_degrees = 0;
    var difference = 0;
    var color = "#c70505"; //green looks better to me
    var bgcolor = "#222";
    var text;
    var animation_loop, redraw_loop;

    function init()
    {
        //Clear the canvas everytime a chart is drawn
        ctx.clearRect(0, 0, W, H);

        //Background 360 degree arc
        ctx.beginPath();
        ctx.strokeStyle = bgcolor;
        ctx.lineWidth = 30;
        ctx.arc(W/2, H/2, 100, 0, Math.PI*2, false); //you can see the arc now
        ctx.stroke();

        //gauge will be a simple arc
        //Angle in radians = angle in degrees * PI / 180
        var radians = degrees * Math.PI / 180;
        ctx.beginPath();
        ctx.strokeStyle = color;
        ctx.lineWidth = 30;
        //The arc starts from the rightmost end. If we deduct 90 degrees from the angles
        //the arc will start from the topmost end
        ctx.arc(W/2, H/2, 100, 0 - 90*Math.PI/180, radians - 90*Math.PI/180, false); 
        //you can see the arc now
        ctx.stroke();

        //Lets add the text
        ctx.fillStyle = color;
        ctx.font = "50px bebas";
        text = Math.floor(degrees/360*100) + "%";
        //Lets center the text
        //deducting half of text width from position x
        text_width = ctx.measureText(text).width;
        //adding manual value to position y since the height of the text cannot
        //be measured easily. There are hacks but we will keep it manual for now.
        ctx.fillText(text, W/2 - text_width/2, H/2 + 15);
    }

    function draw(percent)
    {
        //Cancel any movement animation if a new chart is requested
        if(typeof animation_loop != undefined) clearInterval(animation_loop);

        //random degree from 0 to 360
        new_degrees = percent
        difference = new_degrees - degrees;
        //This will animate the gauge to new positions
        //The animation will take 1 second
        //time for each frame is 1sec / difference in degrees
        animation_loop = setInterval(animate_to, 1000/difference);
    }

    //function to make the chart move to new degrees
    function animate_to()
    {
        //clear animation loop if degrees reaches to new_degrees
        if(degrees == new_degrees) 
            clearInterval(animation_loop);

        if(degrees < new_degrees)
            degrees++;
        else
            degrees--;

        init();
    }

        draw(100);

}

<canvas id="canvas" width="300" height="300"></canvas>


<button onclick="draw(360)">100%</button>
<button onclick="draw(270)">75%</button>

Answer 1

The draw function is out of scope as soon as the anonymous function you have assigned to window.onload finishes executing. If you declare the functions in the global scope instead, you’ll be able to call them anywhere. Here’s a working fiddle.