Paint的设置方法
setAntiAlias：这是画笔的锯齿效
setColor
setARGB
setAlpha
setTextSize
setStyle
setStrokeWidth
getColor
getAlpha
Canvas绘制常见图形的方法
1，绘制直线
左上角是0.0点
drawLine(float startX,float startY,float stopX,float stopY,Paint paint)
2,绘制矩形
drawRect(float left,float top,float right,float bottom,Paint paint)
3,绘制圆形
drawCircle(float cx,float cy,float radius,Paint paint)
