圆环-颜色渐变

private int [] colors=new int [2]; // 渐变色数组

SweepGradient  sweepGradient = new SweepGradient(centreX, centreY,colors, null);  //角度颜色渐变

// 设置渐变起始位置
matrix.reset();每次都要重置，不然颜色渐变起始位置不起作用
matrix.preRotate(startAngle-5 , centreX, centreY);  //颜色起始角度
sweepGradient.setLocalMatrix(matrix);
mPaint.setShader(sweepGradient);

canvas.drawArc(rectF, startAngle, sweepAngle * roundTodayProportion,
false, mPaint);