如何自己实现Android View Touch事件分发流程

class MotionEvent {
 companion object {
  const val ACTION_DOWN = 0
  const val ACTION_MOVE = 1
  const val ACTION_UP = 2
  const val ACTION_CANCEL = 3
 }
 var x = 0
 var y = 0
 var action = 0
 override fun toString(): String {
  return "MotionEvent(x=$x, y=$y, action=$action)"
 }
}

open class View {
 var left = 0
 var right = 0
 var top = 0
 var bottom = 0//1

 var enable = true
 var clickable = false
 var onTouch: ((View, MotionEvent) -> Boolean)? = null
 var onClick: ((View) -> Unit)? = null//3
  set(value) {
   field = value
   clickable = true
  }

 private var downed = false

 open fun layout(l: Int, t: Int, r: Int, b: Int) {
  left = l
  top = t
  right = r
  bottom = b
 }//2

 open fun onTouchEvent(ev: MotionEvent): Boolean {
  var handled: Boolean
  if (enable && clickable) {
   when (ev.action) {
    MotionEvent.ACTION_DOWN -> {
     downed = true
    }
    MotionEvent.ACTION_UP -> {
     if (downed && ev.inView(this)) {//7
      downed = false
      onClick?.invoke(this)
     }
    }
    MotionEvent.ACTION_MOVE -> {
     if (!ev.inView(this)) {//7
      downed = false
     }
    }
    MotionEvent.ACTION_CANCEL -> {
     downed = false
    }
   }
   handled = true
  } else {
   handled = false
  }
  return handled
 }//5

 open fun dispatchTouchEvent(ev: MotionEvent): Boolean {
  var result = false
  if (onTouch != null && enable) {
   result = onTouch!!.invoke(this, ev)
  }
  if (!result && onTouchEvent(ev)) {
   result = true
  }
  return result
 }//4
}
fun MotionEvent.inView(v: View) = v.left <= x && x <= v.right && v.top <= y && y <= v.bottom//6

open class ViewGroup(private vararg val children: View) : View() {//1
 private var mFirstTouchTarget: View? = null

 open fun onInterceptTouchEvent(ev: MotionEvent): Boolean {
  return false
 }//2

 override fun dispatchTouchEvent(ev: MotionEvent): Boolean {//3
  val intercepted: Boolean
  var handled = false

  if (ev.action == MotionEvent.ACTION_DOWN) {
   mFirstTouchTarget = null
  }//4
  if (ev.action == MotionEvent.ACTION_DOWN || mFirstTouchTarget != null) {
   intercepted = onInterceptTouchEvent(ev)//5
  } else {
   intercepted = true//6
  }

  val canceled = ev.action == MotionEvent.ACTION_CANCEL
  var alreadyDispatchedToNewTouchTarget = false
  if (!intercepted) {
   if (ev.action == MotionEvent.ACTION_DOWN) {//7
    for (child in children.reversed()) {//8
     if (ev.inView(child)) {//9
      if (dispatchTransformedTouchEvent(ev, false, child)) {//10
       mFirstTouchTarget = child
       alreadyDispatchedToNewTouchTarget = true//12
      }
      break
     }
    }
   }
  }

  if (mFirstTouchTarget == null) {
   handled = dispatchTransformedTouchEvent(ev, canceled, null)//17
  } else {
   if (alreadyDispatchedToNewTouchTarget) {//13
    handled = true
   } else {
    val cancelChild = canceled || intercepted//14
    if (dispatchTransformedTouchEvent(ev, cancelChild, mFirstTouchTarget)) {
     handled = true
    }
    if (cancelChild) {
     mFirstTouchTarget = null//16
    }
   }
  }

  if (canceled || ev.action == MotionEvent.ACTION_UP) {
   mFirstTouchTarget = null
  }//4
  return handled
 }

 private fun dispatchTransformedTouchEvent(ev: MotionEvent, cancel: Boolean, child: View?): Boolean {
  if (cancel) {
   ev.action = MotionEvent.ACTION_CANCEL//15
  }
  val oldAction = ev.action
  val handled = if (child == null) {
   super.dispatchTouchEvent(ev)//18
  } else {
   child.dispatchTouchEvent(ev)//11
  }
  ev.action = oldAction
  return handled
 }
}

class VG1(vararg children: View) : ViewGroup(*children)
class VG2(vararg children: View) : ViewGroup(*children)
class V : View() {
 override fun onTouchEvent(ev: MotionEvent): Boolean {
  println("V onTouchEvent $ev")
  return super.onTouchEvent(ev)
 }

 override fun dispatchTouchEvent(ev: MotionEvent): Boolean {
  println("V dispatchTouchEvent $ev")
  return super.dispatchTouchEvent(ev)
 }
}

fun produceEvents(startX: Int, startY: Int, endX: Int, endY: Int, stepNum: Int): List<MotionEvent> {
 val list = arrayListOf<MotionEvent>()
 val stepX = (endX - startX) / stepNum
 val stepY = (endY - startY) / stepNum
 for (i in 0..stepNum) {
  when (i) {
   0 -> {
    list.add(MotionEvent().apply {
     action = MotionEvent.ACTION_DOWN
     x = startX
     y = startY
    })
   }
   stepNum -> {
    list.add(MotionEvent().apply {
     action = MotionEvent.ACTION_UP
     x = endX
     y = endY
    })
   }
   else -> {
    list.add(MotionEvent().apply {
     action = MotionEvent.ACTION_MOVE
     x = stepX * i + startX
     y = stepY * i + startY
    })
   }
  }
 }
 return list
}

fun main() {
 val page = VG1(
  VG2(
   V().apply { layout(0, 0, 100, 100); onClick = { println("Click in V") } }//2
  ).apply { layout(0, 0, 200, 200) }
 ).apply { layout(0, 0, 300, 300) }//3

 val events = produceEvents(50, 50, 90, 90, 5)
 events.forEach {
  page.dispatchTouchEvent(it)//1
 }
}

V dispatchTouchEvent MotionEvent(x=50, y=50, action=0)
V onTouchEvent MotionEvent(x=50, y=50, action=0)
V dispatchTouchEvent MotionEvent(x=58, y=58, action=1)
V onTouchEvent MotionEvent(x=58, y=58, action=1)
V dispatchTouchEvent MotionEvent(x=66, y=66, action=1)
V onTouchEvent MotionEvent(x=66, y=66, action=1)
V dispatchTouchEvent MotionEvent(x=74, y=74, action=1)
V onTouchEvent MotionEvent(x=74, y=74, action=1)
V dispatchTouchEvent MotionEvent(x=82, y=82, action=1)
V onTouchEvent MotionEvent(x=82, y=82, action=1)
V dispatchTouchEvent MotionEvent(x=90, y=90, action=2)
V onTouchEvent MotionEvent(x=90, y=90, action=2)
Click in V