魔棒是画板工具一个重要的功能，非常实用，只要轻轻一点，就能把触摸到的颜色区域选中，做复制、剪切、擦除等工作。

那怎么实现呢？

先来看看效果：

要实现这个效果，需要对安卓canvas和paint理解比较深才行。

原理：

1、获取画板上用户触摸点的颜色， bitmap.getPixel；

2、根据目标色对画布进行检索，符合容差范围内的像素纳入到选区内。上下左右4个方向检索，检索到连续的Point汇集成Rect，把Rect合并成Region；

3、对Region取boundaryPath，获取到选区是个Path对象

4、对Path对象描述的范围做虚线框选中显示，同时得到Rect作为选中的位置锚定。

5、把Path跟画布结合生成出剪切、复制的图像进行后续操作。

关键实现：

整个实现都在一个单独的View中操作，即在原来的画布View上添加一层半透明View。即CutView。代码太长，这里给出关键代码：

private fun startDashAnimate() {dashAnimate.setIntValues(dashMin, dashMax)dashAnimate.duration = 4000dashAnimate.addUpdateListener {val dash = it.animatedValue as IntdashPaint.pathEffect = DashPathEffect(floatArrayOf(20f, 20f), dash.toFloat())invalidate()}dashAnimate.repeatCount = ValueAnimator.INFINITEdashAnimate.start()}private fun pauseAnim() {dashAnimate.pause()}private fun resumeAnim() {dashAnimate.resume()}private fun findRegionPath(event: MotionEvent) {actionShowLoading?.invoke()GlobalScope.launch(Dispatchers.IO) {pvsEditView?.let {it.saveToPhoto(true)?.let {bitmap ->filterRegionUtils.findColorRegion(event.x.toInt(), event.y.toInt(), bitmap) {path, r ->addPath(path, r)GlobalScope.launch(Dispatchers.Main) {invalidate()actionHideLoading?.invoke()}}}}}}

这里其他的都是选区动画与绘制。主要看魔棒的入口方法：findRegionPath

findRegionPath由于耗时较长，使用了协程进行计算。

把真正的findColorRegion查找色块放到了工具类filterRegionUtils

这是核心，它返回找到的Path和Rect

整个色块查找类：

class FilterRegionUtils {data class Point(val x: Int, val y: Int)data class Segment(val point: Point, val rect: Rect)private val segmentStack = Stack<Segment>()private val tolerance = 70private var rectF = RectF()private val markedPointMap = HashMap<Int, Boolean>()private val visitedSeedMap = HashMap<Int, Boolean>()private var width: Int = 0private var height: Int = 0private var pointColor: Int = 0private lateinit var pixels: IntArrayprivate val segmentList = arrayListOf<Segment>()fun findColorRegion(x: Int, y: Int, bitmap: Bitmap, action: ((Path, RectF) -> Unit)) {markedPointMap.clear()segmentStack.clear()visitedSeedMap.clear()width = bitmap.widthheight = bitmap.heightif (x < 0 || x >= width || y < 0 || y >= height) {return}val region = Region()val path = Path()path.moveTo(x.toFloat(), y.toFloat())rectF = RectF(x.toFloat(), y.toFloat(), x.toFloat(), y.toFloat())// 拿到该bitmap的颜色数组pixels = IntArray(width * height)bitmap.getPixels(pixels, 0, width, 0, 0, width, height)pointColor = bitmap.getPixel(x, y)val point = Point(x, y)searchLineAtPoint(point)var index = 1while (segmentStack.isNotEmpty()) {val segment = segmentStack.pop()processSegment(segment)region.union(segment.rect)rectF.left = min(rectF.left, segment.rect.left.toFloat())rectF.top = min(rectF.top, segment.point.y.toFloat())rectF.right = max(rectF.right, segment.rect.right.toFloat())rectF.bottom = max(rectF.bottom, segment.point.y.toFloat())index++}val tempPath = region.boundaryPathpath.addPath(tempPath)action.invoke(path, rectF)}private fun processSegment(segment: Segment) {val left = segment.rect.leftval right = segment.rect.rightval y = segment.point.yfor (x in left .. right) {val top = y-1searchLineAtPoint(Point(x, top))val bottom = y+1searchLineAtPoint(Point(x, bottom))}}private fun searchLineAtPoint(point: Point) {if (point.x < 0 || point.x >= width || point.y < 0 || point.y >= height) returnif (visitedSeedMap[point.y * width + point.x] != null) {return}if (!markPointIfMatches(point)) return// search leftvar left = point.x;var x = point.x - 1;while (x >= 0) {val lPoint = Point(x, point.y)if (markPointIfMatches(lPoint)) {left = x} else {break}x--}// search rightvar right = point.xx = point.x + 1while (x < width) {val rPoint = Point(x, point.y)if (markPointIfMatches(rPoint)) {right = x} else {break}x++}val segment = Segment(point, Rect(left, point.y-1, right, point.y+1))segmentList.add(segment)segmentStack.push(segment)}private fun markPointIfMatches(point: Point): Boolean {val offset = point.y*width + point.xval visited = visitedSeedMap[offset]if (visited != null) return falsevar matches = falseif (matchPoint(point)) {matches = truemarkedPointMap[offset] = true}visitedSeedMap[offset] = truereturn matches}private fun matchPoint(point: Point): Boolean {val index = point.y*width + point.xval c1 = pixels[index]val t = max(max(abs(Color.red(c1)-Color.red(pointColor)), abs(Color.green(c1)-Color.green(pointColor))),abs(Color.blue(c1)-Color.blue(pointColor)))val alpha = abs(Color.alpha(c1)-Color.alpha((pointColor)))// 容差值范围内的都视作同一颜色return t < tolerance && alpha < tolerance}
}

整个算法流程还是比较简洁高效的。

再看后面，拿到了选区的Path和Rect后，怎么跟画布结合实现复制或剪切。

/*** 剪切选区*/fun cutPath(path: Path, isNormal: Boolean) {bitmap?.let {bitmap = Bitmap.createBitmap(it.width, it.height, Bitmap.Config.ARGB_8888)canvas = Canvas(bitmap!!)val paint = Paint()paint.style = Paint.Style.FILLcanvas.drawPath(path, paint)paint.xfermode = if (isNormal) {// 取原bitmap的非交集部分PorterDuffXfermode(PorterDuff.Mode.SRC_OUT)} else {// 取原bitmap的交集部分PorterDuffXfermode(PorterDuff.Mode.SRC_IN)}canvas.drawBitmap(it, 0f, 0f, paint)}}

这是剪切的方法，很简单，就是利用Paint的xfermode，用isNormal控制是正选还是反选，即取交集还是非交集。

复制选区方法也类似：

fun genAreaBitmap(src: Bitmap, action: ((Bitmap, RectF) -> Unit)){if (!canOperate()) {return}// 根据裁剪区域生成bitmapval srcCopy = Bitmap.createBitmap(src.width, src.height, Bitmap.Config.ARGB_8888)val canvas = Canvas(srcCopy)val rectF = region.bounds// 避免溢出rectF.right = min(src.width, rectF.right)rectF.bottom = min(src.height, rectF.bottom)val paint = Paint()var r = rectFpaint.style = Paint.Style.FILLval op = if (isNormal) {Region.Op.INTERSECT} else {r = Rect(0, 0, width, height)Region.Op.DIFFERENCE}canvas.clipPath(targetPath, op)canvas.drawBitmap(src, 0f, 0f, paint)val fBitmap = Bitmap.createBitmap(srcCopy, r.left, r.top,r.width(), r.height())action.invoke(fBitmap, RectF(r))finish()}

利用Cavnas的clipPath接口，在画布上裁剪出指定区域。