mastodon-web-like trailing hashtag bar (#4761)

Rationale: Since the mastodon web UI has started stripping "trailing"
hashtags from post content and shoving it into an ellipsized section at
the bottom of posts, the general hashtag : content ratio is rising.

This is an attempt at adopting a similar functionality for Tusky.

Before:

<img width="420" alt="Screenshot of a hashtag-heavy post on Tusky
nightly"
src="https://github.com/user-attachments/assets/09c286e8-6822-482a-904c-5cb3323ea0e1">


After:
![Screenshot of the same post on this
branch](https://github.com/user-attachments/assets/fa99964d-a057-4727-b9f0-1251a199d5f8)

app/src/main/java/com/keylesspalace/tusky/TabPreferenceActivity.kt

@@ -40,11 +40,11 @@ import com.keylesspalace.tusky.databinding.ActivityTabPreferenceBinding
 import com.keylesspalace.tusky.databinding.DialogAddHashtagBinding
 import com.keylesspalace.tusky.entity.MastoList
 import com.keylesspalace.tusky.network.MastodonApi
+import com.keylesspalace.tusky.util.hashtagPattern
 import com.keylesspalace.tusky.util.unsafeLazy
 import com.keylesspalace.tusky.util.viewBinding
 import com.keylesspalace.tusky.util.visible
 import dagger.hilt.android.AndroidEntryPoint
-import java.util.regex.Pattern
 import javax.inject.Inject
 import kotlinx.coroutines.Dispatchers
 import kotlinx.coroutines.launch
@@ -71,10 +71,6 @@ class TabPreferenceActivity : BaseActivity(), ItemInteractionListener, ListSelec
         resources.getDimension(R.dimen.selected_drag_item_elevation)
     }
 
-    private val hashtagRegex by unsafeLazy {
-        Pattern.compile("([\\w_]*[\\p{Alpha}_][\\w_]*)", Pattern.CASE_INSENSITIVE)
-    }
-
     private val onFabDismissedCallback = object : OnBackPressedCallback(false) {
         override fun handleOnBackPressed() {
             toggleFab(false)
@@ -285,7 +281,7 @@ class TabPreferenceActivity : BaseActivity(), ItemInteractionListener, ListSelec
 
     private fun validateHashtag(input: CharSequence?): Boolean {
         val trimmedInput = input?.trim() ?: ""
-        return trimmedInput.isNotEmpty() && hashtagRegex.matcher(trimmedInput).matches()
+        return trimmedInput.isNotEmpty() && hashtagPattern.matcher(trimmedInput).matches()
     }
 
     private fun updateAvailableTabs() {

app/src/main/java/com/keylesspalace/tusky/adapter/StatusBaseViewHolder.java

@@ -121,6 +121,7 @@ public static class Key {
     protected final ConstraintLayout statusContainer;
     private final TextView translationStatusView;
     private final Button untranslateButton;
+    private final TextView trailingHashtagView;
 
 
     private final NumberFormat numberFormat = NumberFormat.getNumberInstance();
@@ -183,6 +184,7 @@ protected StatusBaseViewHolder(@NonNull View itemView) {
 
         translationStatusView = itemView.findViewById(R.id.status_translation_status);
         untranslateButton = itemView.findViewById(R.id.status_button_untranslate);
+        trailingHashtagView = itemView.findViewById(R.id.status_trailing_hashtags_content);
 
         this.avatarRadius48dp = itemView.getContext().getResources().getDimensionPixelSize(R.dimen.avatar_radius_48dp);
         this.avatarRadius36dp = itemView.getContext().getResources().getDimensionPixelSize(R.dimen.avatar_radius_36dp);
@@ -284,7 +286,7 @@ private void setTextVisible(boolean sensitive,
 
         if (expanded) {
             CharSequence emojifiedText = CustomEmojiHelper.emojify(content, emojis, this.content, statusDisplayOptions.animateEmojis());
-            LinkHelper.setClickableText(this.content, emojifiedText, mentions, tags, listener);
+            LinkHelper.setClickableText(this.content, emojifiedText, mentions, tags, listener, this.trailingHashtagView);
             for (int i = 0; i < mediaLabels.length; ++i) {
                 updateMediaLabel(i, sensitive, true);
             }
@@ -295,6 +297,9 @@ private void setTextVisible(boolean sensitive,
             }
         } else {
             hidePoll();
+            if (trailingHashtagView != null) {
+                trailingHashtagView.setVisibility(View.GONE);
+            }
             LinkHelper.setClickableMentions(this.content, mentions, listener);
         }
         if (TextUtils.isEmpty(this.content.getText())) {

app/src/main/java/com/keylesspalace/tusky/components/notifications/StatusNotificationViewHolder.kt

@@ -346,7 +346,7 @@ internal class StatusNotificationViewHolder(
             emojifiedText,
             statusViewData.actionable.mentions,
             statusViewData.actionable.tags,
-            listener
+            listener,
         )
         val emojifiedContentWarning: CharSequence = statusViewData.status.spoilerText.emojify(
             statusViewData.actionable.emojis,

app/src/main/java/com/keylesspalace/tusky/components/viewthread/edits/ViewEditsAdapter.kt

@@ -130,7 +130,7 @@ class ViewEditsAdapter(
             emojifiedText,
             emptyList(),
             emptyList(),
-            listener
+            listener,
         )
 
         if (edit.poll == null) {

app/src/main/java/com/keylesspalace/tusky/util/LinkHelper.kt

@@ -52,6 +52,7 @@ import com.keylesspalace.tusky.interfaces.LinkListener
 import com.keylesspalace.tusky.settings.PrefKeys
 import java.net.URI
 import java.net.URISyntaxException
+import java.util.regex.Pattern
 
 fun getDomain(urlString: String?): String {
     val host = urlString?.toUri()?.host
@@ -70,27 +71,91 @@ fun getDomain(urlString: String?): String {
  * @param content containing text with mentions, links, or hashtags
  * @param mentions any '@' mentions which are known to be in the content
  * @param listener to notify about particular spans that are clicked
+ * @param trailingHashtagView a text view to fill with trailing / out-of-band hashtags
  */
 fun setClickableText(
     view: TextView,
     content: CharSequence,
     mentions: List<Mention>,
     tags: List<HashTag>?,
-    listener: LinkListener
+    listener: LinkListener,
+    trailingHashtagView: TextView? = null,
 ) {
     val spannableContent = markupHiddenUrls(view, content)
+    val (endOfContent, trailingHashtags) = when {
+        trailingHashtagView == null || tags.isNullOrEmpty() -> Pair(spannableContent.length, emptyList())
+        else -> getTrailingHashtags(spannableContent)
+    }
+    var inlineHashtagSpanCount = 0
 
     view.text = spannableContent.apply {
         styleQuoteSpans(view)
-        getSpans(0, spannableContent.length, URLSpan::class.java).forEach { span ->
+        getSpans(0, endOfContent, URLSpan::class.java).forEach { span ->
+            if (get(getSpanStart(span)) == '#') {
+                inlineHashtagSpanCount += 1
+            }
             setClickableText(span, this, mentions, tags, listener)
         }
-    }
+    }.subSequence(0, endOfContent).trimEnd()
+
     view.movementMethod = NoTrailingSpaceLinkMovementMethod
+
+    val showHashtagBar = (trailingHashtags.isNotEmpty() || inlineHashtagSpanCount != tags?.size)
+    // I don't _love_ setting the visibility here, but the alternative is to duplicate the logic in other places
+    trailingHashtagView?.visible(showHashtagBar)
+
+    if (showHashtagBar) {
+        trailingHashtagView?.apply {
+            text = SpannableStringBuilder().apply {
+                tags?.forEachIndexed { index, tag ->
+                    val text = "#${tag.name}"
+                    append(text, getCustomSpanForTag(text, tags, URLSpan(tag.url), listener), 0)
+                    if (index != tags.lastIndex) {
+                        append(" ")
+                    }
+                }
+            }
+        }
+    }
+}
+
+private val trailingHashtagExpression by unsafeLazy {
+    Pattern.compile("""$WORD_BREAK_EXPRESSION(#$HASHTAG_EXPRESSION$WORD_BREAK_FROM_SPACE_EXPRESSION+)*""", Pattern.CASE_INSENSITIVE)
+}
+
+/**
+ * Find the "trailing" hashtags in spanned content
+ * These are hashtags in lines consisting *only* of hashtags at the end of the post
+ */
+@VisibleForTesting
+internal fun getTrailingHashtags(content: Spanned): Pair<Int, List<HashTag>> {
+    // split() instead of lines() because we need to be able to account for the length of the removed delimiter
+    val trailingContentLength = content.split('\r', '\n').asReversed().takeWhile { line ->
+        line.isBlank() || trailingHashtagExpression.matcher(line).matches()
+    }.sumOf { it.length + 1 } // length + 1 to include the stripped line ending character
+
+    return when (trailingContentLength) {
+        0 -> Pair(content.length, emptyList())
+        else -> {
+            val trailingContentOffset = content.length - trailingContentLength
+            Pair(
+                trailingContentOffset,
+                content.getSpans(trailingContentOffset, content.length, URLSpan::class.java)
+                    .filter { content[content.getSpanStart(it)] == '#' } // just in case
+                    .map { spanToHashtag(content, it) }
+            )
+        }
+    }
 }
 
+// URLSpan("#tag", url) -> Hashtag("tag", url)
+private fun spanToHashtag(content: Spanned, span: URLSpan) = HashTag(
+    content.subSequence(content.getSpanStart(span) + 1, content.getSpanEnd(span)).toString(),
+    span.url,
+)
+
 @VisibleForTesting
-fun markupHiddenUrls(view: TextView, content: CharSequence): SpannableStringBuilder {
+internal fun markupHiddenUrls(view: TextView, content: CharSequence): SpannableStringBuilder {
     val spannableContent = SpannableStringBuilder(content)
     val originalSpans = spannableContent.getSpans(0, content.length, URLSpan::class.java)
     val obscuredLinkSpans = originalSpans.filter {

app/src/main/java/com/keylesspalace/tusky/util/StringUtils.kt

@@ -3,10 +3,16 @@
 package com.keylesspalace.tusky.util
 
 import android.text.Spanned
+import java.util.regex.Pattern
 import kotlin.random.Random
 
 private const val POSSIBLE_CHARS = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ"
 
+const val WORD_BREAK_EXPRESSION = """(^|$|[^\p{L}\p{N}_])"""
+const val WORD_BREAK_FROM_SPACE_EXPRESSION = """(^|$|\s)"""
+const val HASHTAG_EXPRESSION = "([\\w_]*[\\p{Alpha}_][\\w_]*)"
+val hashtagPattern = Pattern.compile(HASHTAG_EXPRESSION, Pattern.CASE_INSENSITIVE or Pattern.MULTILINE)
+
 fun randomAlphanumericString(count: Int): String {
     val chars = CharArray(count)
     for (i in 0 until count) {

app/src/main/res/layout/item_status.xml

@@ -338,6 +338,24 @@
         app:layout_constraintTop_toBottomOf="@id/status_poll_button"
         tools:text="7 votes • 7 hours remaining" />
 
+    <com.keylesspalace.tusky.view.ClickableSpanTextView
+        android:id="@+id/status_trailing_hashtags_content"
+        android:layout_width="0dp"
+        android:layout_height="wrap_content"
+        android:layout_marginTop="8dp"
+        android:layout_marginEnd="14dp"
+        android:hyphenationFrequency="full"
+        android:importantForAccessibility="no"
+        android:textColor="?android:textColorPrimary"
+        android:textIsSelectable="true"
+        android:textSize="?attr/status_text_medium"
+        android:singleLine="true"
+        android:ellipsize="end"
+        app:layout_constraintTop_toBottomOf="@id/status_poll_description"
+        app:layout_constraintEnd_toEndOf="parent"
+        app:layout_constraintStart_toStartOf="@id/status_display_name"
+        tools:text="#one #two #three #four #five #six #seven #eight #nine #ten" />
+
     <ImageButton
         android:id="@+id/status_reply"
         style="@style/TuskyImageButton"
@@ -350,7 +368,7 @@
         app:layout_constraintEnd_toStartOf="@id/status_inset"
         app:layout_constraintHorizontal_chainStyle="spread_inside"
         app:layout_constraintStart_toStartOf="@id/status_display_name"
-        app:layout_constraintTop_toBottomOf="@id/status_poll_description"
+        app:layout_constraintTop_toBottomOf="@id/status_trailing_hashtags_content"
         app:srcCompat="@drawable/ic_reply_24dp"
         tools:ignore="NegativeMargin" />
 

app/src/main/res/layout/item_status_detailed.xml

@@ -295,6 +295,23 @@
         app:layout_constraintTop_toBottomOf="@id/status_poll_button"
         tools:text="7 votes • 7 hours remaining" />
 
+    <com.keylesspalace.tusky.view.ClickableSpanTextView
+        android:id="@+id/status_trailing_hashtags_content"
+        android:layout_width="0dp"
+        android:layout_height="wrap_content"
+        android:layout_marginTop="8dp"
+        android:layout_marginStart="14dp"
+        android:layout_marginEnd="14dp"
+        android:hyphenationFrequency="full"
+        android:importantForAccessibility="no"
+        android:textColor="?android:textColorPrimary"
+        android:textIsSelectable="true"
+        android:textSize="?attr/status_text_medium"
+        app:layout_constraintTop_toBottomOf="@id/status_poll_description"
+        app:layout_constraintEnd_toEndOf="parent"
+        app:layout_constraintStart_toStartOf="parent"
+        tools:text="#one #two #three #four #five #six #seven #eight #nine #ten" />
+
     <TextView
         android:id="@+id/status_meta_info"
         android:layout_width="0dp"
@@ -309,7 +326,7 @@
         android:textSize="?attr/status_text_medium"
         app:layout_constraintLeft_toLeftOf="parent"
         app:layout_constraintRight_toRightOf="parent"
-        app:layout_constraintTop_toBottomOf="@id/status_poll_description"
+        app:layout_constraintTop_toBottomOf="@id/status_trailing_hashtags_content"
         tools:text="21 Dec 2018 18:45" />
 
     <View

app/src/test/java/com/keylesspalace/tusky/util/LinkHelperTest.kt

@@ -315,6 +315,74 @@ class LinkHelperTest {
         }
     }
 
+    @Test
+    fun `get trailing hashtags with empty content returns empty list`() {
+        assert(getTrailingHashtags(SpannableStringBuilder("")).second.isEmpty())
+    }
+
+    @Test
+    fun `get trailing hashtags with no hashtags returns empty list`() {
+        assert(getTrailingHashtags(SpannableStringBuilder("some untagged content")).second.isEmpty())
+    }
+
+    @Test
+    fun `get trailing hashtags with all inline hashtags returns empty list`() {
+        assert(getTrailingHashtags(SpannableStringBuilder("some #inline #tagged #content")).second.isEmpty())
+    }
+
+    @Test
+    fun `get trailing hashtags with one tag`() {
+        val content = SpannableStringBuilder("some content followed by tags:\n").apply {
+            tags.first().let { append("#${it.name}", URLSpan(it.url), 0) }
+        }
+
+        val (_, trailingHashtags) = getTrailingHashtags(content)
+        assertEquals(tags.first().name, trailingHashtags.single().name)
+        assertEquals(tags.first().url, trailingHashtags.single().url)
+    }
+
+    @Test
+    fun `get trailing hashtags with multiple tags`() {
+        for (separator in listOf(" ", "\t", "\n", "\r\n")) {
+            val content = SpannableStringBuilder("some content followed by tags:\n").apply {
+                for (tag in tags) {
+                    append(separator)
+                    append("#${tag.name}", URLSpan(tag.url), 0)
+                    append(separator)
+                }
+            }
+
+            val (_, trailingHashtags) = getTrailingHashtags(content)
+            assertEquals(tags.size, trailingHashtags.size)
+            tags.forEachIndexed { index, tag ->
+                assertEquals(tag.name, trailingHashtags[index].name)
+                assertEquals(tag.url, trailingHashtags[index].url)
+            }
+        }
+    }
+
+    @Test
+    fun `get trailing hashtags ignores inline tags`() {
+        for (separator in listOf(" ", "\t", "\n", "\r\n")) {
+            val content = SpannableStringBuilder("some content with inline tag ").apply {
+                append("#inline", URLSpan("https://example.com/tag/inline"), 0)
+                append(" followed by trailing tags\n")
+                for (tag in tags) {
+                    append(separator)
+                    append("#${tag.name}", URLSpan(tag.url), 0)
+                    append(separator)
+                }
+            }
+
+            val (_, trailingHashtags) = getTrailingHashtags(content)
+            assertEquals(tags.size, trailingHashtags.size)
+            tags.forEachIndexed { index, tag ->
+                assertEquals(tag.name, trailingHashtags[index].name)
+                assertEquals(tag.url, trailingHashtags[index].url)
+            }
+        }
+    }
+
     @RunWith(Parameterized::class)
     class UrlMatchingTests(private val url: String, private val expectedResult: Boolean) {
         companion object {