0.前言

最近重看了一遍封神演義，感覺QQ閱讀那個翻書的效果挺好的，準備做一個。上週五下午用了兩個小時只寫了一部分功能，以後有時間再完善

1.分析

先看效果圖

這個空間，說簡單也簡單，說難也難，簡單就在於這個效果主要就是依賴canvas的clippath才見到部分canvas，難就難在裁剪區域的點的計算，上傳一張我自己畫得草圖

其中a點就是手指的位置，b點就是控制元件右下角，cd為ab的垂直平分線，e為交點，gh為ae的垂直平分線，f為交點，i為ac與gh的交點，j為ad與gh的交點，其中icg為貝塞爾曲線，c為控制點，jdh為貝塞爾曲線，d為控制點，構建一個aigbh的path，通過canvas裁剪這個path

2.程式碼

2.1計算點

private fun calPoints(x: Float,y:Float) {
        pointA.x = x
        pointA.y = y

        calPointB()

        pointE.x = (pointA.x + pointB.x) / 2
        pointE.y = (pointA.y + pointB.y) / 2

        pointK.x = pointE.x
        pointK.y = pointB.y

        pointC.x = pointK.x - (pointK.y - pointE.y) * (pointK.y - pointE.y) / (pointB.x - pointK.x)
        pointC.y = pointB.y


        pointG.x = pointC.x - (pointB.x - pointC.x) / 2
        pointG.y = pointB.y

        pointL.x = pointB.x
        pointL.y = pointE.y

        pointD.x = pointB.x
        pointD.y = pointE.y - (pointL.x - pointE.x) * (pointL.x - pointE.x) / (pointB.y - pointL.y)

        pointF.x = (pointA.x + pointE.x) / 2
        pointF.y = (pointA.y + pointE.y) / 2

        pointH.x = pointB.x
        pointH.y = pointD.y - (pointB.y - pointD.y) / 2

        pointI = getIntercetPoint(pointA, pointC, pointH, pointG)

        pointJ = getIntercetPoint(pointA, pointD, pointH, pointG)
    }

    private fun calPointB() {
        when {
            this.touchPos == TOUCH_END -> {
                pointB.x = width.toFloat()
                pointB.y = height.toFloat()
            }
            this.touchPos == TOUCH_TOP -> {
                pointB.x = width.toFloat()
                pointB.y = 0f
            }
            else -> {
                pointB.x = pointA.x
                pointB.y = height.toFloat()
            }
        }

    }
    
     private fun getIntercetPoint(point1: Point, point2: Point, point3: Point, point4: Point): Point {

        var x1 = point1.x
        var y1 = point1.y
        var x2 = point2.x
        var y2 = point2.y
        var x3 = point3.x
        var y3 = point3.y
        var x4 = point4.x
        var y4 = point4.y

        var pointX =
            ((x1 - x2) * (x3 * y4 - x4 * y3) - (x3 - x4) * (x1 * y2 - x2 * y1)) / ((x3 - x4) * (y1 - y2) - (x1 - x2) * (y3 - y4))
        var pointY =
            ((y1 - y2) * (x3 * y4 - x4 * y3) - (x1 * y2 - x2 * y1) * (y3 - y4)) / ((y1 - y2) * (x3 - x4) - (x1 - x2) * (y3 - y4))

        return Point(pointX, pointY)
    }
複製程式碼

每個點怎麼算程式碼已經很清楚了，我也就不再贅述了。

2.2繪製

override fun draw(canvas: Canvas?) {

        canvas?.let {
            it.save()
            calPath()
            it.clipPath(path, Region.Op.XOR)
            super.draw(it)
            it.restore()
        }
    }

    private fun calPath() {
        path.reset()
        path.moveTo(pointA.x, pointA.y)
        path.lineTo(pointI.x, pointI.y)
        path.quadTo(pointC.x, pointC.y, pointG.x, pointG.y)
        path.lineTo(pointB.x, pointB.y)
        path.lineTo(pointH.x, pointH.y)
        path.quadTo(pointD.x, pointD.y, pointJ.x, pointJ.y)
        path.lineTo(pointA.x, pointA.y)
    }
複製程式碼

複寫draw方法，通過calPath構建ajgbhja這樣的一個path，裁減掉這部分後進行繪製。

2.3最大翻頁距離

當g點x的座標為0是，就停止移動了

override fun onTouchEvent(event: MotionEvent?): Boolean {

        when (event?.action) {
            MotionEvent.ACTION_DOWN -> {
                lastTouchX=event.x
                lastTouchY=event.y
                when {
                    event.y < height / 3 -> this.touchPos = TOUCH_TOP
                    event.y > height - height / 3 -> this.touchPos = TOUCH_END
                    else -> this.touchPos = TOUCH_MID
                }

            }

            MotionEvent.ACTION_MOVE -> {
                calPoints(event.x,event.y)
                if(pointG.x<=0)
                {
                    calPoints(lastTouchX,lastTouchY)
                }
                else{
                    lastTouchX=event.x
                    lastTouchY=event.y
                }
                postInvalidate()
            }

            MotionEvent.ACTION_UP -> {



            }
        }

        return true
    }
複製程式碼

3最後

最近狀態很差，寫程式碼總是寫寫停停，不能靜下心來，所以這個控制元件有很多沒有做完，以後有時間再弄吧。附github地址

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