CenterAlignedTextView

CenterAlignedTextView.kt

package me.rajin.centeralignedtextview

import android.content.Context
import android.graphics.Rect
import android.graphics.Typeface
import android.text.SpannableStringBuilder
import android.text.Spanned
import android.text.TextPaint
import android.text.style.CharacterStyle
import android.text.style.UpdateAppearance
import android.util.AttributeSet
import androidx.appcompat.widget.AppCompatTextView
import androidx.core.text.getSpans
import java.util.WeakHashMap

class CenterAlignedTextView : AppCompatTextView {
    companion object {
        /**
         * ratio is offset / fontHeight
         * fontHeight = ascent - descent
         * offset = (capHeight - ascent - descent) / 2 (in iOS)
         * Note that in android, ascent and descent is multiplied by -1 (because yAxis is downward)
         */
        val typeface2fontHeightOffsetRatio = mutableMapOf<Typeface, Float>()
        /**
         * This map is for calculated offset ratio.
         */
        val weakTypeface2fontHeightOffsetRatio = WeakHashMap<Typeface, Float>()
    }

    var realText: CharSequence? = null

    constructor(context: Context) : super(context) {
        init(context, null)
    }

    constructor(context: Context, attrs: AttributeSet?) : super(context, attrs) {
        init(context, attrs)
    }

    constructor(context: Context, attrs: AttributeSet?, defStyleAttr: Int) : super(context, attrs, defStyleAttr) {
        init(context, attrs)
    }

    private fun init(context: Context, attrs: AttributeSet?) {
        // refresh span
        this.text = text
    }

    /**
     * overriding setText(text: CharSequence?, type: BufferType?) is not complete solution.
     * There is one setText method in AppCompatTextView that does not use this method.
     * Because there is one setText method that does not call this overridable method.
     * setText(char[] text, int start, int len)
     * link : https://github.com/aosp-mirror/platform_frameworks_base/blob/e0dd9516b516bc9042ad5d6f4b7fef053ce07d0e/core/java/android/widget/TextView.java#L4063-L4070
     */
    override fun setText(text: CharSequence?, type: BufferType?) {
        realText = text
        val spannedText = text?.let {
            SpannableStringBuilder(it).apply {
                val spanList = getSpans<VerticalCenterAlignSpan>()
                spanList.forEach { span ->
                    removeSpan(span)
                }
                setSpan(
                    VerticalCenterAlignSpan(),
                    0, it.length, Spanned.SPAN_EXCLUSIVE_EXCLUSIVE
                )
            }
        } ?: text
        super.setText(spannedText, type)
    }

    private class VerticalCenterAlignSpan : CharacterStyle(), UpdateAppearance {
        override fun updateDrawState(paint: TextPaint?) {
            if (paint != null) {
                val ratio = typeface2fontHeightOffsetRatio[paint.typeface] ?: {
                    // as fallback, calculate offset using "X"'s text bounds
                    // note that this is not accurate because
                    // 1. text bounds of "X" can be different from typeface's meta data.
                    // 2. getTextBounds returns in pixel(int).
                    // to improve accuracy, we measure ratio with large text size.
                    weakTypeface2fontHeightOffsetRatio[paint.typeface] ?: {
                        val oldTextSize = paint.textSize
                        paint.textSize = 30f
                        val newFontMetric = paint.fontMetrics
                        val bounds = Rect()
                        paint.getTextBounds("X", 0, 1, bounds)
                        val capHeight = -bounds.top
                        val calculatedOffset =
                            (capHeight + newFontMetric.ascent + newFontMetric.descent) / 2f
                        val ratio =
                            calculatedOffset / (newFontMetric.descent - newFontMetric.ascent)
                        weakTypeface2fontHeightOffsetRatio[paint.typeface] = ratio
                        paint.textSize = oldTextSize
                        ratio
                    }()
                }()
                val fontMetric = paint.fontMetrics
                val offsetY = (fontMetric.descent - fontMetric.ascent) * ratio
                paint.baselineShift = offsetY.toInt()
            }
        }
    }
}

// Before using CenterAlignedTextView, set font-ratio map.
val spoqaHansRatio = -0.036148648648648626f
val spoqaFonts = listOf(
    ResourcesCompat.getFont(this, R.font.spoqa_han_sans_thin),
    ResourcesCompat.getFont(this, R.font.spoqa_han_sans_light),
    ResourcesCompat.getFont(this, R.font.spoqa_han_sans_bold),
    ResourcesCompat.getFont(this, R.font.spoqa_han_sans_regular)
)
spoqaFonts.forEach {
    if (it != null) {
        typeface2fontHeightOffsetRatio[it] = spoqaHansRatio
    }
}
// You may need to do this...
spoqaFonts.map { Typeface.create(it, Typeface.NORMAL) }.forEach {
    if (it != null) {
        typeface2fontHeightOffsetRatio[it] = spoqaHansRatio
    }
}