Align text around ImageSpan center vertical

Question

I have an ImageSpan inside of a piece of text. What I\'ve noticed is that the surrounding text is always drawn at the bottom of the text line -- to be more precise, the size

Answer 1

This solution provides a vertical centering based on actual letter size. It supports centering using capital letters and lower-case letters. For example, look at the marker character near a letter: X•. This solution achieves a similar effect.

This is a modified version of @WindRider's answer. Also, it's in Kotlin. And it supports drawable size customization.

The reason why this solution is created is to provide a better visual result. A lot of other solutions use font ascent. But it appears to cause visual problems in some cases. Android's default Roboto font, for example, has ascent higher than a typical capital letter top border. And because of it, some manual adjustments were needed to properly center an image.

class CenteredImageSpan(context: Context,
                        drawableRes: Int,
                        private val centerType: CenterType = CenterType.CAPITAL_LETTER,
                        private val customHeight: Int? = null,
                        private val customWidth: Int? = null) : ImageSpan(context, drawableRes) {

    private var mDrawableRef: WeakReference? = null

    override fun getSize(paint: Paint, text: CharSequence,
                         start: Int, end: Int,
                         fontMetrics: FontMetricsInt?): Int {

        if (fontMetrics != null) {
            val currentFontMetrics = paint.fontMetricsInt
            // keep it the same as paint's Font Metrics
            fontMetrics.ascent = currentFontMetrics.ascent
            fontMetrics.descent = currentFontMetrics.descent
            fontMetrics.top = currentFontMetrics.top
            fontMetrics.bottom = currentFontMetrics.bottom
        }

        val drawable = getCachedDrawable()
        val rect = drawable.bounds
        return rect.right
    }

    override fun draw(canvas: Canvas,
                      text: CharSequence,
                      start: Int,
                      end: Int,
                      x: Float,
                      lineTop: Int,
                      baselineY: Int,
                      lineBottom: Int,
                      paint: Paint) {
        val cachedDrawable = getCachedDrawable()
        val drawableHeight = cachedDrawable.bounds.height()

        val relativeVerticalCenter = getLetterVerticalCenter(paint)

        val drawableCenter = baselineY + relativeVerticalCenter
        val drawableBottom = drawableCenter - drawableHeight / 2

        canvas.save()
        canvas.translate(x, drawableBottom.toFloat())
        cachedDrawable.draw(canvas)
        canvas.restore()
    }

    private fun getLetterVerticalCenter(paint: Paint): Int =
         when (centerType) {
            CenterType.CAPITAL_LETTER -> getCapitalVerticalCenter(paint)
            CenterType.LOWER_CASE_LETTER -> getLowerCaseVerticalCenter(paint)
        }

    private fun getCapitalVerticalCenter(paint: Paint): Int {
        val bounds = Rect()
        paint.getTextBounds("X", 0, 1, bounds)
        return (bounds.bottom + bounds.top) / 2
    }

    private fun getLowerCaseVerticalCenter(paint: Paint): Int {
        val bounds = Rect()
        paint.getTextBounds("x", 0, 1, bounds)
        return (bounds.bottom + bounds.top) / 2
    }


    // Redefined here because it's private in DynamicDrawableSpan
    private fun getCachedDrawable(): Drawable {

        val drawableWeakReference = mDrawableRef
        var drawable: Drawable? = null
        if (drawableWeakReference != null) drawable = drawableWeakReference.get()
        if (drawable == null) {
            drawable = getDrawable()!!

            val width = customWidth ?: drawable.intrinsicWidth
            val height = customHeight ?: drawable.intrinsicHeight

            drawable.setBounds(0, 0,
                               width, height)
            mDrawableRef = WeakReference(drawable)
        }
        return drawable

    }

    enum class CenterType {
        CAPITAL_LETTER, LOWER_CASE_LETTER
    }

}