PercentVisibleCardView

PercentVisibleCardView.kt

import android.content.Context
import android.util.AttributeSet
import android.view.ViewTreeObserver
import androidx.cardview.widget.CardView
import ru.dpd.core.extension.getViewVisibilityPercent

class PercentVisibleCardView @JvmOverloads constructor(
    context: Context, attrs: AttributeSet? = null
) : CardView(context, attrs) {

    var visibilityCallback: () -> Unit = {}

    private var shouldNotify = true

    private val scrollListener = ViewTreeObserver.OnScrollChangedListener {
        val visibilityPercent = getViewVisibilityPercent()
        if (shouldNotify && visibilityPercent > VISIBILITY_THRESHOLD) {
            visibilityCallback.invoke()
            shouldNotify = false
        } else if (visibilityPercent < VISIBILITY_THRESHOLD) {
            shouldNotify = true
        }
    }

    override fun onAttachedToWindow() {
        super.onAttachedToWindow()
        viewTreeObserver.addOnScrollChangedListener(scrollListener)
    }

    override fun onDetachedFromWindow() {
        super.onDetachedFromWindow()
        viewTreeObserver.removeOnScrollChangedListener(scrollListener)
    }

    private companion object {
        const val VISIBILITY_THRESHOLD = 0.7
    }
}

/**
 * 1f == 100%
 */
fun View.getViewVisibilityPercent(): Float {
    val viewArea = width * height.toFloat()
    val currentVisibleArea = Rect()
        .takeIf { this.isShown }
        ?.takeIf(this::getGlobalVisibleRect)
        ?.let { it.height() * it.width() }
        ?.toFloat()
        ?: 0f
    return when (viewArea) {
        0f -> 0f
        else -> currentVisibleArea / viewArea
    }
}