ShadowView

public interface ShadowView {
    float getElevation();

    void setElevation(float elevation);

    float getTranslationZ();

    void setTranslationZ(float translationZ);

    ShadowShape getShadowShape();

    Shadow getShadow();

    void invalidateShadow();
}

Implementation

Layout:

    @Override
    protected boolean drawChild(Canvas canvas, View child, long drawingTime) {
        if (!child.isShown())
            return super.drawChild(canvas, child, drawingTime);

        if (!isInEditMode() && child instanceof ShadowView && Build.VERSION.SDK_INT <= Build.VERSION_CODES.KITKAT_WATCH) {
            ShadowView shadowView = (ShadowView) child;
            Shadow shadow = shadowView.getShadow();
            if (shadow != null) {
                paint.setAlpha((int) (ShadowGenerator.ALPHA * ViewHelper.getAlpha(child)));

                float childElevation = shadowView.getElevation()+shadowView.getTranslationZ();

                float[] childLocation = new float[]{(child.getLeft() + child.getRight()) / 2, (child.getTop() + child.getBottom()) / 2};
                Matrix matrix = carbon.internal.ViewHelper.getMatrix(child);
                matrix.mapPoints(childLocation);

                int[] location = new int[2];
                getLocationOnScreen(location);
                float x = childLocation[0] + location[0];
                float y = childLocation[1] + location[1];
                x -= getRootView().getWidth() / 2;
                y += getRootView().getHeight() / 2;   // looks nice
                float length = (float) Math.sqrt(x * x + y * y);

                int saveCount = canvas.save(Canvas.MATRIX_SAVE_FLAG);
                canvas.translate(
                        x / length * childElevation / 2,
                        y / length * childElevation / 2);
                canvas.translate(
                        child.getLeft(),
                        child.getTop());

                canvas.concat(matrix);
                canvas.scale(ShadowGenerator.SHADOW_SCALE, ShadowGenerator.SHADOW_SCALE);
                shadow.draw(canvas, child, paint);
                canvas.restoreToCount(saveCount);
            }
        }

        if (child instanceof RippleView) {
            RippleView rippleView = (RippleView) child;
            RippleDrawable rippleDrawable = rippleView.getRippleDrawable();
            if (rippleDrawable != null && rippleDrawable.getStyle() == RippleDrawable.Style.Borderless) {
                int saveCount = canvas.save(Canvas.MATRIX_SAVE_FLAG);
                canvas.translate(
                        child.getLeft(),
                        child.getTop());
                rippleDrawable.draw(canvas);
                canvas.restoreToCount(saveCount);
            }
        }

        return super.drawChild(canvas, child, drawingTime);
    }

View:

    private float elevation = 0;
    private float translationZ = 0;
    private Shadow shadow;

    @Override
    public float getElevation() {
        return elevation;
    }

    public synchronized void setElevation(float elevation) {
        if (elevation == this.elevation)
            return;
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP)
            super.setElevation(elevation);
        this.elevation = elevation;
        if (getParent() != null)
            ((View) getParent()).postInvalidate();
    }

    @Override
    public float getTranslationZ() {
        return translationZ;
    }

    public synchronized void setTranslationZ(float translationZ) {
        if (translationZ == this.translationZ)
            return;
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP)
            super.setTranslationZ(translationZ);
        this.translationZ = translationZ;
        if (getParent() != null)
            ((View) getParent()).postInvalidate();
    }

    @Override
    public ShadowShape getShadowShape() {
        if (cornerRadius == getWidth() / 2 && getWidth() == getHeight())
            return ShadowShape.CIRCLE;
        if (cornerRadius > 0)
            return ShadowShape.ROUND_RECT;
        return ShadowShape.RECT;
    }

    @Override
    public void setEnabled(boolean enabled) {
        super.setEnabled(enabled);
        setTranslationZ(enabled ? 0 : -elevation);
    }

    @Override
    public Shadow getShadow() {
        float elevation = getElevation() + getTranslationZ();
        if (elevation >= 0.01f && getWidth() > 0 && getHeight() > 0) {
            if (shadow == null || shadow.elevation != elevation)
                shadow = ShadowGenerator.generateShadow(this, elevation);
            return shadow;
        }
        return null;
    }

    @Override
    public void invalidateShadow() {
        shadow = null;
        if (getParent() != null)
            ((View) getParent()).postInvalidate();
    }