Android仿IOS10圆盘时间选择器

// 坐标系的直线表达式
// 直线l1的表达式子:过钟表中心点和开始控件中心点
float a1 = mCenterY - mStartBtnCurY;
float b1 = mStartBtnCurX - mCenterX;
float c1 = mStartBtnCurY * mCenterX - mCenterY * mStartBtnCurX;
double d1 = (a1 * eventX + b1 * eventY + c1) / (Math.sqrt(a1 * a1 + b1 * b1));

// 直线l2的表达式:过钟表中心点且垂直直线l1
     float a2 = b1;
     float b2 = -a1;
     float c2 = -a2 * mCenterX - b2 * mCenterY;
     double d2 = (a2 * eventX + b2 * eventY + c2) / (Math.sqrt(a2 * a2 + b2 * b2));

// 以l1为基准线,顺势针半圆为0-180度,逆时针半圆为0-负180度
     double moveDegree = Math.toDegrees(Math.atan2(d1, d2));

     mStartDegree = (float) (mStartDegree + Math.floor(moveDegree));
     mStartDegree = (mStartDegree < 0) ? mStartDegree + mDegreeCycle : mStartDegree % mDegreeCycle;
     refreshStartBtnPositon();
     invalidate();