Android Resource Annotation Applier

resources.xml

<resources>
   <string name="label">This is a string
    <annotation textStyle="italic"><annotation textColor="@color/grey">that can have</annotation></annotation>
    different styles. 
  </string>
  <color name="grey">#222222</color>
</resources>

TextAnnotations.kt

package com.vinaysshenoy.util

import android.content.Context
import android.graphics.Typeface
import android.support.annotation.StringRes
import android.support.v4.content.res.ResourcesCompat
import android.text.Annotation
import android.text.Spannable
import android.text.SpannableString
import android.text.SpannedString
import android.text.style.ForegroundColorSpan
import android.text.style.StyleSpan
import timber.log.Timber

/**
 * Singleton that encapsulates parsing of [Annotation] markup from string resources and provides
 * a standard way of applying them to string resources.
 *
 * See this [blog post](https://medium.com/androiddevelopers/styling-internationalized-text-in-android-f99759fb7b8f)
 * for more information.
 **/
object TextAnnotations {

  private val processors = listOf(
      LoggingProcessor(),
      TextStyleProcessor(),
      TextForegroundColorResourcesProcessor()
  )

  fun processAnnotatedString(context: Context, @StringRes resourceId: Int): CharSequence {
    val source = context.getText(resourceId) as SpannedString

    return source
        .getSpans(0, source.length, Annotation::class.java)
        .fold(SpannableString(source)) { input, annotation ->
          val spanStart = source.getSpanStart(annotation)
          val spanEnd = source.getSpanEnd(annotation)

          processors
              .asSequence()
              .filter { it.canProcess(annotation) }
              .fold(input) { spannable, processor -> processor.process(context, source, annotation, spanStart, spanEnd, spannable) }
        }
  }

  private class TextForegroundColorResourcesProcessor : Processor {

    override fun canProcess(annotation: Annotation) = annotation.key == "textColor"

    override fun process(
        context: Context,
        source: SpannedString,
        annotation: Annotation,
        spanStart: Int,
        spanEnd: Int,
        input: SpannableString
    ): SpannableString {
      val colorResourceId = context.resources.getIdentifier(annotation.value, null, context.packageName)
      val textColor = ResourcesCompat.getColor(context.resources, colorResourceId, null)

      input.setSpan(ForegroundColorSpan(textColor), spanStart, spanEnd, Spannable.SPAN_EXCLUSIVE_EXCLUSIVE)

      return input
    }
  }

  private class TextStyleProcessor : Processor {

    override fun canProcess(annotation: Annotation) = annotation.key == "textStyle"

    override fun process(
        context: Context,
        source: SpannedString,
        annotation: Annotation,
        spanStart: Int,
        spanEnd: Int,
        input: SpannableString
    ): SpannableString {
      val textStyleToApply = annotation.value

      val style = when (textStyleToApply) {
        "bold" -> Typeface.BOLD
        "italic" -> Typeface.ITALIC
        "bold_italic" -> Typeface.BOLD_ITALIC
        else -> Typeface.NORMAL
      }

      input.setSpan(StyleSpan(style), spanStart, spanEnd, Spannable.SPAN_EXCLUSIVE_EXCLUSIVE)
      return input
    }
  }

  private class LoggingProcessor : Processor {

    override fun canProcess(annotation: Annotation) = true

    override fun process(
        context: Context,
        source: SpannedString,
        annotation: Annotation,
        spanStart: Int,
        spanEnd: Int,
        input: SpannableString
    ): SpannableString {
      Timber.d("Found annotation: [${annotation.key}:${annotation.value}] on [$source] at [$spanStart, $spanEnd]")
      return input
    }
  }

  private interface Processor {

    fun canProcess(annotation: Annotation): Boolean

    fun process(
        context: Context,
        source: SpannedString,
        annotation: Annotation,
        spanStart: Int,
        spanEnd: Int,
        input: SpannableString
    ): SpannableString
  }
}

View.kt

class MyActivity: AppCompatActivity() {
  
  lateinit var tv: TextView by lazy { findViewById(R.id.textView) }
  
  override fun onCreate(savedInstanceState: Bundle?) {
    tv.text = TextAnnotations.processAnnotatedString(context, R.string.label)
  }
}