Fix #456: Improvised bullet code

utility/src/main/java/org/oppia/util/parser/HtmlParser.kt

@@ -1,9 +1,13 @@
 package org.oppia.util.parser
 
+import android.os.Build
 import android.text.Html
 import android.text.Spannable
 import android.text.SpannableStringBuilder
+import android.text.Spanned
+import android.text.style.BulletSpan
 import android.widget.TextView
+import androidx.appcompat.app.AppCompatActivity
 import javax.inject.Inject
 
 private const val CUSTOM_IMG_TAG = "oppia-noninteractive-image"
@@ -14,6 +18,7 @@ private const val REPLACE_IMG_FILE_PATH_ATTRIBUTE = "src"
 /** Html Parser to parse custom Oppia tags with Android-compatible versions. */
 class HtmlParser private constructor(
   private val urlImageParserFactory: UrlImageParser.Factory,
+  private val activity: AppCompatActivity,
   private val entityType: String,
   private val entityId: String,
   private val imageCenterAlign: Boolean
@@ -44,18 +49,28 @@ class HtmlParser private constructor(
     }
 
     val imageGetter = urlImageParserFactory.create(htmlContentTextView, entityType, entityId, imageCenterAlign)
-    return if (android.os.Build.VERSION.SDK_INT >= android.os.Build.VERSION_CODES.N) {
-      trimSpannable(
-        Html.fromHtml(
-          htmlContent,
-          Html.FROM_HTML_MODE_LEGACY,
-          imageGetter, /* tagHandler= */
-          null
-        ) as SpannableStringBuilder
-      )
+
+    @Suppress("DEPRECATION")
+    val htmlSpannable = if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.N) {
+      Html.fromHtml(htmlContent, Html.FROM_HTML_MODE_LEGACY, imageGetter, LiTagHandler())
     } else {
-      trimSpannable(Html.fromHtml(htmlContent, imageGetter, /* tagHandler= */ null) as SpannableStringBuilder)
+      Html.fromHtml(htmlContent, imageGetter, LiTagHandler())
+    }
+
+    val spannableBuilder = SpannableStringBuilder(htmlSpannable)
+    val bulletSpans = spannableBuilder.getSpans(0, spannableBuilder.length, BulletSpan::class.java)
+    bulletSpans.forEach {
+      val start = spannableBuilder.getSpanStart(it)
+      val end = spannableBuilder.getSpanEnd(it)
+      spannableBuilder.removeSpan(it)
+      spannableBuilder.setSpan(
+        ImprovedBulletSpan(activity),
+        start,
+        end,
+        Spanned.SPAN_INCLUSIVE_EXCLUSIVE
+      )
     }
+    return trimSpannable(spannableBuilder)
   }
 
   private fun trimSpannable(spannable: SpannableStringBuilder): SpannableStringBuilder {
@@ -77,9 +92,9 @@ class HtmlParser private constructor(
     return spannable.delete(0, trimStart).delete(spannable.length - trimEnd, spannable.length)
   }
 
-  class Factory @Inject constructor(private val urlImageParserFactory: UrlImageParser.Factory) {
+  class Factory @Inject constructor(private val urlImageParserFactory: UrlImageParser.Factory, private val activity: AppCompatActivity) {
     fun create(entityType: String, entityId: String, imageCenterAlign: Boolean): HtmlParser {
-      return HtmlParser(urlImageParserFactory, entityType, entityId, imageCenterAlign)
+      return HtmlParser(urlImageParserFactory, activity, entityType, entityId, imageCenterAlign)
     }
   }
 }

utility/src/main/java/org/oppia/util/parser/ImprovedBulletSpan.kt

@@ -0,0 +1,75 @@
+package org.oppia.util.parser
+
+import android.content.Context
+import android.graphics.Canvas
+import android.graphics.Paint
+import android.graphics.Path
+import android.graphics.Path.Direction
+import android.text.Layout
+import android.text.Spanned
+import android.text.style.LeadingMarginSpan
+import androidx.appcompat.app.AppCompatActivity
+import org.oppia.util.R
+
+/**
+ * Copy of [android.text.style.BulletSpan] from android SDK 28 with removed internal code.
+ *
+ * Reference: https://github.com/davidbilik/bullet-span-sample
+ */
+class ImprovedBulletSpan(activity: AppCompatActivity) : LeadingMarginSpan {
+
+  private var bulletRadius: Int = 0
+  private var gapWidth: Int = 0
+  private var yOffset: Int = 0
+
+  init {
+    bulletRadius = (activity as Context).resources.getDimensionPixelSize(R.dimen.bullet_radius)
+    gapWidth = (activity as Context).resources.getDimensionPixelSize(R.dimen.bullet_gap_width)
+    yOffset = (activity as Context).resources.getDimensionPixelSize(R.dimen.bullet_y_offset)
+  }
+
+  private var mBulletPath: Path? = null
+
+  override fun getLeadingMargin(first: Boolean): Int {
+    return 2 * bulletRadius + gapWidth
+  }
+
+  override fun drawLeadingMargin(
+    canvas: Canvas, paint: Paint, x: Int, dir: Int,
+    top: Int, baseline: Int, bottom: Int,
+    text: CharSequence, start: Int, end: Int,
+    first: Boolean,
+    layout: Layout?
+  ) {
+    if ((text as Spanned).getSpanStart(this) == start) {
+      val style = paint.style
+      paint.style = Paint.Style.FILL
+
+      var yPosition = if (layout != null) {
+        val line = layout.getLineForOffset(start)
+        layout.getLineBaseline(line).toFloat() - bulletRadius * 2f
+      } else {
+        (top + bottom) / 2f
+      }
+      yPosition += yOffset
+
+      val xPosition = (x + dir * bulletRadius).toFloat()
+
+      if (canvas.isHardwareAccelerated) {
+        if (mBulletPath == null) {
+          mBulletPath = Path()
+          mBulletPath!!.addCircle(0.0f, 0.0f, bulletRadius.toFloat(), Direction.CW)
+        }
+
+        canvas.save()
+        canvas.translate(xPosition, yPosition)
+        canvas.drawPath(mBulletPath!!, paint)
+        canvas.restore()
+      } else {
+        canvas.drawCircle(xPosition, yPosition, bulletRadius.toFloat(), paint)
+      }
+
+      paint.style = style
+    }
+  }
+}

utility/src/main/java/org/oppia/util/parser/LiTagHandler.kt

@@ -0,0 +1,37 @@
+package org.oppia.util.parser
+
+import android.text.Editable
+import android.text.Html
+import android.text.Spannable
+import android.text.Spanned
+import android.text.style.BulletSpan
+import org.xml.sax.XMLReader
+
+/**
+ * [Html.TagHandler] implementation that processes <li> tags and creates bullets.
+ *
+ * Reference: https://github.com/davidbilik/bullet-span-sample
+ */
+class LiTagHandler : Html.TagHandler {
+  /**
+   * Helper marker class. Idea stolen from [Html.fromHtml] implementation
+   */
+  class Bullet
+
+  override fun handleTag(opening: Boolean, tag: String, output: Editable, xmlReader: XMLReader) {
+    if (tag == "li" && opening) {
+      output.setSpan(Bullet(), output.length, output.length, Spannable.SPAN_INCLUSIVE_EXCLUSIVE)
+    }
+    if (tag == "li" && !opening) {
+      output.append("\n\n")
+      val lastMark = output.getSpans(0, output.length, Bullet::class.java).lastOrNull()
+      lastMark?.let {
+        val start = output.getSpanStart(it)
+        output.removeSpan(it)
+        if (start != output.length) {
+          output.setSpan(BulletSpan(), start, output.length, Spanned.SPAN_INCLUSIVE_EXCLUSIVE)
+        }
+      }
+    }
+  }
+}

utility/src/main/res/values/dimens.xml

@@ -0,0 +1,6 @@
+<?xml version="1.0" encoding="utf-8"?>
+<resources>
+  <dimen name="bullet_radius">4dp</dimen>
+  <dimen name="bullet_gap_width">16dp</dimen>
+  <dimen name="bullet_y_offset">2dp</dimen>
+</resources>