1、效果图

2、确定绘制的图形

两个圆和中间虚线框住的部分

3、计算画图的几个要素

（1）大圆O'：原点O',半径R'

O':为手指触摸点,R'初始半径

（2）小圆O：原点O,半径R

（3）虚线部分：点A，点B，点C，点D,两条曲线还需要OO'中点

可直接得到的条件有O(x,y)，O'(x,y)，R'

O(x,y):为初始图形中点(startX,startY)

O'(x,y):为手指触摸点(endX,endY)

R'初始半径DEFAULT_RADIUS

计算：

|OO'|：

    |OO'|=√((O'x-Ox)^2+(O'y-Oy)^2)

小圆半径R:

    R=R'初始半径-|OO'|*SIZE_CHANGE_SEED(SIZE_CHANGE_SEED大小变化速度)

A点:

    Ax=Ox-R* sin(tan-1((O'x - Ox) /(O'y - Oy)))

    Ay=Oy+R* cos(tan-1((O'x - Ox) / (O'y - Oy)))

B点:

    Bx=Ox +R* sin(tan-1((O'x - Ox) /(O'y - Oy)))

    By=Oy-R* cos(tan-1((O'x - Ox) / (O'y - Oy)))

C点:

    Ax=O'x + R'* sin(tan-1((O'x - Ox) /(O'y - Oy)))

    Ay=O'y-R'* cos(tan-1((O'x - Ox) / (O'y - Oy)))

D点:

    Bx=O'x-R'* sin(tan-1((O'x - Ox) /(O'y - Oy)))

    By=O'y +R'* cos(tan-1((O'x - Ox) / (O'y - Oy)))

OO'中点：   
    CPx = (Ox + O'x) / 2;
    CPx = (Oy + O'y) / 2;

转成java代码：

private static final float DEFAULT_RADIUS = 20;
private static final float SIZE_CHANGE_SEED =0.1f;

float startX, startY, endX, endY;

float dY = endY - startY;
float dX = endX - startX;

// 斜率 换算 角度
double angle = Math.atan(dY / dX);

//|OO'|
float distance = (float) Math.sqrt(Math.pow(dY, 2) + Math.pow(dX, 2));

//小圆半径R
radius = DEFAULT_RADIUS - distance * SIZE_Change_SEED;

// A、B
float offsetX1 = (float) (radius * Math.sin(angle));
float offsetY1 = (float) (radius * Math.cos(angle));

float Ax = startX - offsetX1;
float Ay = startY + offsetY1;

float Bx = startX + offsetX1;
float By = startY - offsetY1;

//C、D
float offsetX2 = (float) (DEFAULT_RADIUS * Math.sin(angle));
float offsetY2 = (float) (DEFAULT_RADIUS * Math.cos(angle));

float Cx = endX + offsetX2;
float Cy = endY - offsetY2;

float Dx = endX - offsetX2;
float Dy = endY + offsetY2;

//OO'中点
float CPx= (endX + startX) / 2;
float CPy= (endY + startY) / 2;

未完待续.....