// 坐標系的直線表達式// 直線l1的表達式子:過鐘表中心點和開始控件中心點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));// 直線l2的表達式:過鐘表中心點且垂直直線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為基準線,順勢針半圓為0-180度,逆時針半圓為0-負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();