"Dynamically preventing scrolling on selected ViewPager pages" for blog.jakelee.co.uk

fragment_viewpager.xml

<com.example.LockableViewPager
        android:id="@+id/pager"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />

LockableViewPager.kt

import android.content.Context
import android.support.v4.view.ViewPager
import android.util.AttributeSet
import android.view.MotionEvent

enum class SwipeDirection { BOTH, LEFT, RIGHT, NONE }

// https://stackoverflow.com/a/34076649/608312
class LockableViewPager(context: Context, attrs: AttributeSet) : ViewPager(context, attrs) {

    private var initialXValue: Float = 0f
    private var direction: SwipeDirection? = null

    init {
        this.direction = SwipeDirection.BOTH
    }

    override fun onTouchEvent(event: MotionEvent): Boolean {
        return if (this.isSwipeAllowed(event)) {
            super.onTouchEvent(event)
        } else false
    }

    override fun onInterceptTouchEvent(event: MotionEvent): Boolean {
        return if (this.isSwipeAllowed(event)) {
            super.onInterceptTouchEvent(event)
        } else false
    }

    private fun isSwipeAllowed(event: MotionEvent): Boolean {
        if (this.direction === SwipeDirection.BOTH) {
            return true
        } else if (direction === SwipeDirection.NONE) {
            return false
        }

        if (event.action == MotionEvent.ACTION_DOWN) {
            initialXValue = event.x
            return true
        }

        if (event.action == MotionEvent.ACTION_MOVE) {
            try {
                val diffX = event.x - initialXValue
                if (diffX > 0 && direction === SwipeDirection.RIGHT) {
                    // swipe from left to right detected
                    return false
                } else if (diffX < 0 && direction === SwipeDirection.LEFT) {
                    // swipe from right to left detected
                    return false
                }
            } catch (e: Exception) {
                e.printStackTrace()
            }
        }
        return true
    }

    fun setAllowedSwipeDirection(direction: SwipeDirection) {
        this.direction = direction
    }
}

ViewPagerFragment.kt

    private fun pageChangeListener(): ViewPager.SimpleOnPageChangeListener =
        object : ViewPager.SimpleOnPageChangeListener() {
            override fun onPageSelected(position: Int) {
                setSwipeability()
            }
        }    

    private fun setSwipeability() {
        pager.setAllowedSwipeDirection(getSwipeDirection(pager))
    }

    override fun getSwipeDirection(pager: LockableViewPager): SwipeDirection {
        if (isUserRequired(pager.currentItem)) {
            return SwipeDirection.NONE
        }

        val isFirstUser = pager.currentItem == 0
        val isLastUser = pager.currentItem == pager.adapter!!.count - 1
        val isUserToLeft = !isFirstUser && pager.currentItem > 0
        val isUserToRight = !isLastUser && pager.currentItem < pager.adapter!!.count - 1
        val isOptionalUserOnLeft = isUserToLeft && !isUserRequired(pager.currentItem - 1)
        val isOptionalUserOnRight = isUserToRight && !isUserRequired(pager.currentItem + 1)

        if (isOptionalUserOnLeft && isOptionalUserOnRight) {
            return SwipeDirection.BOTH
        } else if (isOptionalUserOnLeft) {
            return SwipeDirection.LEFT
        } else if (isOptionalUserOnRight) {
            return SwipeDirection.RIGHT
        } else {
            return SwipeDirection.NONE
        }
    }