Android Canva实现渐变进度条

void drawArc (RectF oval, 
                float startAngle, 
                float sweepAngle, 
                boolean useCenter, 
                Paint paint)

/*为了便于说明，先定义几个变量*/
float mMinOvalR; //圆环外接矩形边长的一半
float mMinWidth; //圆环宽度
float mMinOffsetY; //外接矩形top属性向上的偏移（这个下文会解释）
float degree = 315; //圆弧扫过的角度
//创建画笔：
mMinPaint = new Paint();
mMinPaint.setColor(Color.BLUE); //先随便给个颜色
mMinPaint.setAntiAlias(true); //启用抗锯齿
mMinPaint.setDither(true); //启用抗颜色抖动（可以让渐变更平缓）
mMinPaint.setStyle(Paint.Style.STROKE);
mMinPaint.setStrokeWidth(mMinWidth); //设置宽度
//外接矩形
//由于手表屏幕是正圆，所以canvas正好是正方形，于是可以用下面方法算出矩形顶点坐标。
RectF rect = new RectF(canvas.getWidth() / 2 - mMinOvalR,
        canvas.getHeight() / 2 - mMinOvalR,
        canvas.getWidth() - (canvas.getWidth() / 2 - mMinOvalR),
        canvas.getHeight() - (canvas.getHeight() / 2 - mMinOvalR));
//画弧
canvas.drawArc(rect, -90, degree, false, mMinPaint);

//定义头部画笔
mMinCirclePaint = new Paint();
mMinCirclePaint.setColor(Color.BLUE);
mMinCirclePaint.setAntiAlias(true);
degree -= 90; //抵消屏幕坐标系差异
degree = (float) (Math.PI / 180f * degree); //换成弧度
canvas.drawCircle((float) (rect.left   rect.width() / 2f   mMinOvalR * Math.cos(degree)), //圆心x
        (float) (rect.top   rect.height() / 2f   mMinOvalR * Math.sin(degree)), //圆心y
        mMinWidth / 2f,  //半径
        mMinCirclePaint);

//先创建一个渲染器
SweepGradient mSweepGradient = new SweepGradient(canvas.getWidth() / 2,
        canvas.getHeight() / 2, //以圆弧中心作为扫描渲染的中心以便实现需要的效果
        mMinColors, //这是我定义好的颜色数组，包含2个颜色：#35C3D7、#2894DD
        null);
//把渐变设置到笔刷
mMinPaint.setShader(mSweepGradient);

//旋转渐变
Matrix matrix = new Matrix();
matrix.setRotate(-90f, canvas.getWidth() / 2, canvas.getHeight() / 2);
mSweepGradient.setLocalMatrix(matrix);

//创建渐变
SweepGradient mSweepGradient = new SweepGradient(canvas.getWidth() / 2,
        canvas.getHeight() / 2,
        mMinColors,
        new float[]{0f, degree / 360f - 0.017f});
// 从图肉眼不难观察出半个小圆大概占了6°的范围（刻度一格是6°）
// 6 / 360 = 0.017
//第一个元素为0表示从0°开始渐变，第二个元素表示渐变提前结束，最后的那一块是纯色。这样一来便可融为一体。

//外切矩形
//在原来基础上加上了偏移像素mMinOffsetY用来贴合背景
RectF rect = new RectF(canvas.getWidth() / 2 - mMinOvalR-mMinOffsetY,
        canvas.getHeight() / 2 - mMinOvalR-mMinOffsetY,
        canvas.getWidth() - (canvas.getWidth() / 2 - mMinOvalR),
        canvas.getHeight() - (canvas.getHeight() / 2 - mMinOvalR));

//头部圆
mMinCirclePaint.setColor(isInAmbientMode() ? mMinShimmerColors[1] : mMinColors[1]);
degree = 90f - degree; //抵消屏幕坐标系差异
degree = (float) (Math.PI / 180f * degree);
float a = rect.top,
        b = rect.left,
        c = rect.width(),
        d = rect.height();
canvas.drawCircle((float) (b   c / 2f * (Math.cos(degree)   1)),
        (float) (a   -1 * d / 2f * (Math.sin(degree) - 1)),
        mMinWidth / 2f, mMinCirclePaint);