FabBottomNavigationView Widget custom para el manejo del BottomNavigationBar con un FAB integrado con el mismo estilo de BottomApp Bar

FabBottomNavigationView.kt

import android.animation.ValueAnimator
import android.annotation.SuppressLint
import android.content.Context
import android.graphics.Paint
import android.util.AttributeSet
import androidx.core.animation.doOnEnd
import androidx.core.view.ViewCompat
import androidx.core.view.isVisible
import com.google.android.material.bottomnavigation.BottomNavigationView
import com.google.android.material.floatingactionbutton.FloatingActionButton
import com.google.android.material.internal.ThemeEnforcement
import com.google.android.material.resources.MaterialResources
import com.google.android.material.shape.MaterialShapeDrawable

@SuppressLint("RestrictedApi","PrivateResource")
class FabBottomNavigationView @JvmOverloads constructor(
    context: Context, attrs: AttributeSet? = null, defStyleAttr: Int = 0
) : BottomNavigationView(context, attrs, defStyleAttr) {

    private var topCurvedEdgeTreatment: TopCurvedEdgeTreatment
    private var materialShapeDrawable: MaterialShapeDrawable
    private var fabSize = 0F
    var fabCradleMargin = 0F
    var fabCradleRoundedCornerRadius = 0F
    var cradleVerticalOffset = 0F

    init {
        val ta = context.theme.obtainStyledAttributes(attrs, R.styleable.FabBottomNavigationView, 0, 0)
        fabSize = ta.getDimension(R.styleable.FabBottomNavigationView_fab_size, 0F)
        fabCradleMargin = ta.getDimension(R.styleable.FabBottomNavigationView_fab_cradle_margin, 0F)
        fabCradleRoundedCornerRadius = ta.getDimension(R.styleable.FabBottomNavigationView_fab_cradle_rounded_corner_radius, 0F)
        cradleVerticalOffset = ta.getDimension(R.styleable.FabBottomNavigationView_cradle_vertical_offset, 0F)


        val tai =
        ThemeEnforcement.obtainTintedStyledAttributes(
            context,
            attrs,
            R.styleable.BottomNavigationView,
            0,
            R.style.Widget_Design_BottomNavigationView,
            R.styleable.BottomNavigationView_itemTextAppearanceInactive,
            R.styleable.BottomNavigationView_itemTextAppearanceActive)

        val backgroundTint =
        MaterialResources.getColorStateList(
            context, tai, R.styleable.BottomNavigationView_backgroundTint)

        topCurvedEdgeTreatment =
            TopCurvedEdgeTreatment(fabCradleMargin, fabCradleRoundedCornerRadius, cradleVerticalOffset).apply {
                fabDiameter = fabSize
            }

        materialShapeDrawable = MaterialShapeDrawable().apply {
            shapeAppearanceModel.topEdge = topCurvedEdgeTreatment
            tintList = backgroundTint
            //setTint(ContextCompat.getColor(context,backgroundTint))
            shadowElevation = 4
            shadowRadius = 8
            elevation = 4f
            shadowCompatibilityMode = MaterialShapeDrawable.SHADOW_COMPAT_MODE_ALWAYS
            initializeElevationOverlay(context)
            paintStyle = Paint.Style.FILL_AND_STROKE
        }

        ViewCompat.setBackground(this,materialShapeDrawable)
        //background = materialShapeDrawable
    }

    fun transform(fab: FloatingActionButton) {
        if (fab.isVisible) {
            fab.hide(object : FloatingActionButton.OnVisibilityChangedListener() {
                override fun onHidden(fab: FloatingActionButton?) {
                    super.onHidden(fab)
                    ValueAnimator.ofFloat(materialShapeDrawable.interpolation, 0F).apply {
                        addUpdateListener { materialShapeDrawable.interpolation = it.animatedValue as Float }
                        start()
                    }
                }
            })
        } else {
            ValueAnimator.ofFloat(materialShapeDrawable.interpolation, 1F).apply {
                addUpdateListener { materialShapeDrawable.interpolation = it.animatedValue as Float }
                doOnEnd { fab.show() }
                start()
            }
        }
    }
}

TopCurvedEdgeTreatment.kt

import com.google.android.material.shape.EdgeTreatment
import com.google.android.material.shape.ShapePath

class TopCurvedEdgeTreatment(
    var fabCradleMargin: Float,
    var fabCradleRoundedCornerRadius: Float,
    var cradleVerticalOffset: Float
) : EdgeTreatment() {

    var fabDiameter: Float = 0.toFloat()
    var horizontalOffset: Float = 0.toFloat()

    init {
        if (cradleVerticalOffset < 0.0f) {
            throw IllegalArgumentException("cradleVerticalOffset must be positive.")
        } else {
            this.horizontalOffset = 0.0f
        }
    }

    override fun getEdgePath(length: Float, center: Float, interpolation: Float, shapePath: ShapePath) {
        if (this.fabDiameter == 0.0f) {
            shapePath.lineTo(length, 0.0f)
        } else {
            val cradleDiameter = this.fabCradleMargin * 2.0f + this.fabDiameter
            val cradleRadius = cradleDiameter / 2.0f
            val roundedCornerOffset = interpolation * this.fabCradleRoundedCornerRadius
            val middle = length / 2.0f + this.horizontalOffset
            val verticalOffset = interpolation * this.cradleVerticalOffset + (1.0f - interpolation) * cradleRadius
            val verticalOffsetRatio = verticalOffset / cradleRadius
            if (verticalOffsetRatio >= 1.0f) {
                shapePath.lineTo(length, 0.0f)
            } else {
                val distanceBetweenCenters = cradleRadius + roundedCornerOffset
                val distanceBetweenCentersSquared = distanceBetweenCenters * distanceBetweenCenters
                val distanceY = verticalOffset + roundedCornerOffset
                val distanceX = Math.sqrt((distanceBetweenCentersSquared - distanceY * distanceY).toDouble()).toFloat()
                val leftRoundedCornerCircleX = middle - distanceX
                val rightRoundedCornerCircleX = middle + distanceX
                val cornerRadiusArcLength = Math.toDegrees(Math.atan((distanceX / distanceY).toDouble())).toFloat()
                val cutoutArcOffset = 90.0f - cornerRadiusArcLength
                shapePath.lineTo(leftRoundedCornerCircleX - roundedCornerOffset, 0.0f)
                shapePath.addArc(
                    leftRoundedCornerCircleX - roundedCornerOffset,
                    0.0f,
                    leftRoundedCornerCircleX + roundedCornerOffset,
                    roundedCornerOffset * 2.0f,
                    270.0f,
                    cornerRadiusArcLength
                )
                shapePath.addArc(
                    middle - cradleRadius,
                    -cradleRadius - verticalOffset,
                    middle + cradleRadius,
                    cradleRadius - verticalOffset,
                    180.0f - cutoutArcOffset,
                    cutoutArcOffset * 2.0f - 180.0f
                )
                shapePath.addArc(
                    rightRoundedCornerCircleX - roundedCornerOffset,
                    0.0f,
                    rightRoundedCornerCircleX + roundedCornerOffset,
                    roundedCornerOffset * 2.0f,
                    270.0f - cornerRadiusArcLength,
                    cornerRadiusArcLength
                )
                shapePath.lineTo(length, 0.0f)
            }
        }
    }

}