http://www.jianshu.com/p/2d9f42dc0945

一阶 
	canvas.drawLine(start.x,start.y,end.x,end.y);

二阶 Path.quadTo
	@Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        mPaint.setColor(Color.RED); //画笔颜色
        mPaint.setStrokeWidth(10); //画笔宽度
        mPaint.setStyle(Paint.Style.STROKE);

        mPath.reset();
        //起点
        mPath.moveTo(startPoint.x, startPoint.y);
        //mPath
        mPath.quadTo(assistPoint.x, assistPoint.y, endPoint.x, endPoint.y);
        //画path
        canvas.drawPath(mPath, mPaint);
        //画控制点
        canvas.drawPoint(assistPoint.x, assistPoint.y, mPaint);
        //画线
        canvas.drawLine(startPoint.x, startPoint.y, assistPoint.x, assistPoint.y, mPaint);
        canvas.drawLine(endPoint.x, endPoint.y, assistPoint.x, assistPoint.y, mPaint);
    }

三阶 Path.cubicTo
	@Override
	protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        mPaint.setColor(Color.RED); //画笔颜色
        mPaint.setStrokeWidth(10); //画笔宽度
        mPaint.setStyle(Paint.Style.STROKE);
        mPath.reset();
        //起点
        mPath.moveTo(startPoint.x, startPoint.y);
        //mPath
        mPath.cubicTo(assistPoint1.x, assistPoint1.y, assistPoint2.x, assistPoint2.y, endPoint.x, endPoint.y);
        //画path
        canvas.drawPath(mPath, mPaint);
        //画控制点
        canvas.drawPoint(assistPoint1.x, assistPoint1.y, mPaint);
        canvas.drawPoint(assistPoint2.x, assistPoint2.y, mPaint);

        //画线
        canvas.drawLine(startPoint.x, startPoint.y, assistPoint1.x, assistPoint1.y, mPaint);
        canvas.drawLine(endPoint.x, endPoint.y, assistPoint2.x, assistPoint2.y, mPaint);
        canvas.drawLine(assistPoint1.x, assistPoint1.y, assistPoint2.x, assistPoint2.y, mPaint);
    }