3.canvas实现刻度仪表盘的绘制

css和html代码没什么好说的，就直接开始js实现：

 var canvas = document.getElementById('canvas'),
        context = canvas.getContext('2d'),
        centroid_radius = 10,
        centroid_stroke_style = 'rgba(0,0,0,0.5)',
        centroid_fill_style = 'rgba(80,190,240,0.6)',
        ring_inner_radius = 35,
        ring_outer_radius = 55,
        annotations_fill_style = 'rgba(0,0,230,0.9)',
        annotations_text_size = 12,
        tick_width = 10,
        tick_long_stroke_style = 'rgba(100,140,230,0.9)',
        tick_short_stroke_style = 'rgba(100,140,240,0.7)',
        tracking_dial_stroking_style = 'rgba(100,140,230,0.5)',
        guidewire_stroke_style = 'goldenrod',
        guidewire_fill_style = 'rgba(250,250,0,0.6)',
        circle = {
            x : canvas.width / 2,
            y: canvas.height / 2,
            radius: 150
        };
    function drawDial(){
        var loc = {x: circle.x, y: circle.y};
        drawCentroid();
        drawCentroidGuidewire(loc);
        drawRing();
        drawTickInnerCircle();
        drawTicks();
        drawAnnotations();
    }
    function drawCentroid(){
        context.beginPath();
        context.save();
        context.strokeStyle = centroid_stroke_style;
        context.fllStyle = centroid_fill_style;
        context.arc(circle.x, circle.y, centroid_radius, 0
            , Math.PI * 2, false);
        context.stroke();
        context.fill();
        context.restore();
    }
    function drawCentroidGuidewire(loc){
        var angle = - Math.PI / 4,
            radius, endpt;0
        radius = circle.radius + ring_outer_radius;
        if(loc.x >= circle.x){
            endpt = {
                x: circle.x + radius * Math.cos(angle),
                y: circle.y + radius * Math.sin(angle)
            };
        }
        else{
            endpt = {
                x: circle.x - radius * Math.cos(angle),
                y: circle.y - radius * Math.sin(angle)
            };
        }
        context.save();
        context.strokeStyle = guidewire_stroke_style;
        context.fillStyle = guidewire_fill_style;
        context.beginPath();
        context.moveTo(circle.x, circle.y);
        context.lineTo(endpt.x, endpt.y);
        context.stroke();
        context.beginPath();
        context.strokeStyle = tick_long_stroke_style;
        context.arc(endpt.x, endpt.y, 5, 0, Math.PI * 2, false);
        context.fill();
        context.stroke();
        context.restore();
    }
    function drawRing(){
        drawRingOuterCircle();
        context.strokeStyle = 'rgba(0, 0, 0, 0.1)';
        context.arc(circle.x,  circle.y, circle.radius + ring_inner_radius,
            0, Math.PI * 2, false);
        context.fillStyle = 'rgba(100, 140, 230, 0.1)';
        context.fill();
        context.stroke();
    }
    function drawRingOuterCircle(){
        context.beginPath();
        context.strokeStyle=tracking_dial_stroking_style;
        context.arc(circle.x, circle.y, circle.radius +ring_outer_radius,
            0, Math.PI * 2, true);
        context.stroke();
    }
    function drawTickInnerCircle(){
        context.save();
        context.beginPath();
        context.strokeStyle = 'rgba(0, 0, 0, 0.1)';
        context.arc(circle.x, circle.y, circle.radius + ring_inner_radius
            -tick_width, 0, Math.PI * 2, false);
        context.stroke();
        context.restore();
    }
    function drawTick(angle, radius, cnt){
        var tickWidth = cnt % 4 ===0 ? tick_width : tick_width / 2;
        context.beginPath();
        context.moveTo(circle.x + Math.cos(angle) *
            (radius - tickWidth), circle.y + Math.sin(angle) *
            (radius - tickWidth));
        context.lineTo(circle.x + Math.cos(angle) * radius,
            circle.y + Math.sin(angle) * radius);
        context.strokeStyle = tick_short_stroke_style;
        context.stroke();
    }
    function drawTicks(){
        var radius = circle.radius + ring_inner_radius,
            angle_max = 2 * Math.PI,
            angle_delta = Math.PI / 64;
        context.save();
        for(var angle = 0, cnt = 0; angle < angle_max;
            angle += angle_delta, cnt++){
            drawTick(angle, radius, cnt++);
        }
        context.restore();
    }
    function drawAnnotations(){
        var radius = circle.radius + ring_inner_radius;
        context.save();
        context.fillStyle = annotations_fill_style;
        context.font = annotations_text_size + 'px Helvetica';
        for(var angle = 0; angle < 2 * Math.PI;angle += Math.PI / 6){
            context.beginPath();
            context.fillText((angle * 180 / Math.PI).toFixed(0),
                circle.x + Math.cos(angle) * (radius - tick_width * 2),
                circle.y - Math.sin(angle) * (radius - tick_width * 2));
        }
        context.restore();
    }
    context.shadowColor = 'rgba(0, 0, 0, 0.5)';
    context.shadowOffsetX = 2;
    context.shadowOffsetY = 2;
    context.shadowBlur = 4;
    context.textAlign = 'center';
    context.textBaseline = 'middle';
    drawDial();