Image / drawable span that is vertically centered relative to its surrounding text

CenterVerticalDrawableSpan.kt

private abstract class CenterVerticalDrawableSpan : ReplacementSpan() {

    abstract fun getDrawable(): Drawable?

    private var drawableRef: WeakReference<Drawable>? = null

    private val cachedDrawable: Drawable?
        get() = drawableRef?.get() ?: getDrawable()?.also { drawableRef = WeakReference(it) }

    override fun getSize(paint: Paint, text: CharSequence,
                         start: Int, end: Int, fm: Paint.FontMetricsInt?): Int {
        val drawable = cachedDrawable ?: return 0
        val bounds = drawable.bounds

        if (fm != null) {
            val height = bounds.height()
            val bottom = fm.descent + (height - (fm.descent - fm.ascent)) / 2
            val top = bottom - height

            fm.descent = bottom
            fm.bottom = bottom
            fm.ascent = top
            fm.top = top
        }

        return bounds.width()
    }

    override fun draw(canvas: Canvas, text: CharSequence,
                      start: Int, end: Int, x: Float, top: Int, y: Int, bottom: Int, paint: Paint) {
        val drawable = cachedDrawable ?: return
        canvas.apply {
            save()
            translate(x, 0f)
            drawable.draw(this)
            restore()
        }
    }

}