Implement support for Markdown preview in TextEditorActivity

Uses Commonmark for Markdown parsing and rendering.

Due to the way Commonmark works, it cannot support content streaming and update in realtime; user needs to let TextEditorActivity to load as much content as wish.

Author: TranceLove

app/build.gradle

@@ -261,6 +261,10 @@ dependencies {
 
     implementation libs.gson
     implementation libs.amaze.trashBin
+
+    implementation libs.commonmark
+    implementation libs.commonmark.ext.gfm.tables
+    implementation libs.commonmark.ext.gfm.strikethrough
 }
 
 kotlin {

app/src/main/java/com/amaze/filemanager/asynchronous/asynctasks/texteditor/read/FileWindowReader.kt

@@ -64,6 +64,7 @@ class FileWindowReader(
      * 4. Snaps the end to the last newline (unless at file end)
      * 5. Returns the decoded string and actual byte range consumed
      */
+    @Suppress("LongMethod")
     fun readWindow(
         byteOffset: Long,
         maxChars: Int,
@@ -161,7 +162,7 @@ class FileWindowReader(
      * past any continuation bytes (10xxxxxx pattern).
      */
     private fun snapToCharBoundary(offset: Long): Long {
-        if (offset <= 0L || offset >= fileSize) return offset.coerceIn(0L, fileSize)
+        if (offset !in 1..<fileSize) return offset.coerceIn(0L, fileSize)
 
         val buf = ByteBuffer.allocate(1)
         var pos = offset
@@ -226,16 +227,14 @@ class FileWindowReader(
                 channel = channel,
                 fileSize = size,
                 closeable =
-                    object : Closeable {
-                        override fun close() {
-                            try {
-                                fis.close()
-                            } catch (_: Exception) {
-                            }
-                            try {
-                                pfd.close()
-                            } catch (_: Exception) {
-                            }
+                    Closeable {
+                        try {
+                            fis.close()
+                        } catch (_: Exception) {
+                        }
+                        try {
+                            pfd.close()
+                        } catch (_: Exception) {
                         }
                     },
             )

app/src/main/java/com/amaze/filemanager/asynchronous/asynctasks/texteditor/read/ReadTextFileCallable.java

@@ -45,7 +45,7 @@
 
 public class ReadTextFileCallable implements Callable<ReturnedValueOnReadFile> {
 
-  public static final int MAX_FILE_SIZE_CHARS = 50 * 1024;
+  public static final int MAX_FILE_SIZE_CHARS = 100 * 1024;
 
   private final ContentResolver contentResolver;
   private final EditableFileAbstraction fileAbstraction;

app/src/main/java/com/amaze/filemanager/ui/activities/texteditor/MarkdownHtmlGenerator.kt

@@ -0,0 +1,97 @@
+/*
+ * Copyright (C) 2014-2024 Arpit Khurana <arpitkh96@gmail.com>, Vishal Nehra <vishalmeham2@gmail.com>,
+ * Emmanuel Messulam<emmanuelbendavid@gmail.com>, Raymond Lai <airwave209gt at gmail.com> and Contributors.
+ *
+ * This file is part of Amaze File Manager.
+ *
+ * Amaze File Manager is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+package com.amaze.filemanager.ui.activities.texteditor
+
+import org.commonmark.ext.gfm.strikethrough.StrikethroughExtension
+import org.commonmark.ext.gfm.tables.TablesExtension
+import org.commonmark.parser.Parser
+import org.commonmark.renderer.html.HtmlRenderer
+
+/**
+ * Utility for Markdown preview in the text editor.
+ * Pure functions with no Android dependencies — easy to unit-test.
+ */
+object MarkdownHtmlGenerator {
+    /**
+     * Returns `true` if [fileName] ends with `.md` or `.markdown` (case-insensitive).
+     */
+    @JvmStatic
+    fun isMarkdownFile(fileName: String?): Boolean {
+        if (fileName == null) return false
+        val lower = fileName.lowercase()
+        return lower.endsWith(".md") || lower.endsWith(".markdown")
+    }
+
+    /**
+     * Parse Markdown source text and return the rendered HTML body fragment.
+     */
+    @JvmStatic
+    fun renderToHtml(markdownSource: String): String {
+        val extensions = listOf(TablesExtension.create(), StrikethroughExtension.create())
+        val parser = Parser.builder().extensions(extensions).build()
+        val document = parser.parse(markdownSource)
+        val renderer = HtmlRenderer.builder().extensions(extensions).build()
+        return renderer.render(document)
+    }
+
+    /**
+     * Wrap an HTML body fragment with a full HTML document including theme-aware CSS.
+     *
+     * FIXME: Move template to strings.xml
+     * FIXME: Use Android/Material native Color constants for easy maintenance
+     *
+     * @param bodyHtml the rendered Markdown HTML fragment
+     * @param isDarkTheme true for dark/black theme, false for light theme
+     * @return a complete HTML document string
+     */
+    @JvmStatic
+    fun wrapWithBaseHtml(
+        bodyHtml: String,
+        isDarkTheme: Boolean,
+    ): String {
+        val bgColor = if (isDarkTheme) "#1a1a1a" else "#ffffff"
+        val textColor = if (isDarkTheme) "#e0e0e0" else "#212121"
+        val linkColor = if (isDarkTheme) "#82b1ff" else "#1565c0"
+        val codeBg = if (isDarkTheme) "#2d2d2d" else "#f5f5f5"
+        val borderColor = if (isDarkTheme) "#444444" else "#dddddd"
+        val quoteColor = if (isDarkTheme) "#aaaaaa" else "#666666"
+
+        return """<!DOCTYPE html>
+<html><head><meta charset="UTF-8">
+<meta name="viewport" content="width=device-width, initial-scale=1.0">
+<style>
+body { font-family: sans-serif; padding: 16px; margin: 0; background-color: $bgColor; color: $textColor; line-height: 1.6; word-wrap: break-word; }
+a { color: $linkColor; }
+pre { background-color: $codeBg; padding: 12px; border-radius: 4px; overflow-x: auto; }
+code { background-color: $codeBg; padding: 2px 4px; border-radius: 2px; font-size: 90%; }
+pre code { padding: 0; background: none; }
+blockquote { border-left: 4px solid $borderColor; margin: 0; padding: 0 16px; color: $quoteColor; }
+table { border-collapse: collapse; width: 100%; }
+th, td { border: 1px solid $borderColor; padding: 8px; text-align: left; }
+th { background-color: $codeBg; }
+img { max-width: 100%; height: auto; }
+hr { border: none; border-top: 1px solid $borderColor; }
+h1, h2, h3, h4, h5, h6 { margin-top: 24px; margin-bottom: 16px; }
+</style></head><body>
+$bodyHtml
+</body></html>"""
+    }
+}

app/src/main/java/com/amaze/filemanager/ui/activities/texteditor/TextEditorActivity.java

@@ -66,6 +66,7 @@
 import android.view.animation.AnimationUtils;
 import android.view.inputmethod.EditorInfo;
 import android.view.inputmethod.InputMethodManager;
+import android.webkit.WebView;
 import android.widget.ScrollView;
 import android.widget.Toast;
 
@@ -86,12 +87,14 @@ public class TextEditorActivity extends ThemedActivity
   private Typeface inputTypefaceMono;
   private androidx.appcompat.widget.Toolbar toolbar;
   ScrollView scrollView;
+  private WebView markdownWebView;
 
   private SearchTextTask searchTextTask;
   private static final String KEY_MODIFIED_TEXT = "modified";
   private static final String KEY_INDEX = "index";
   private static final String KEY_ORIGINAL_TEXT = "original";
   private static final String KEY_MONOFONT = "monofont";
+  private static final String KEY_MARKDOWN_PREVIEW = "markdown_preview";
 
   private ConstraintLayout searchViewLayout;
   public AppCompatImageButton upButton;
@@ -134,6 +137,8 @@ public void onCreate(Bundle savedInstanceState) {
     }
     mainTextView = findViewById(R.id.textEditorMainEditText);
     scrollView = findViewById(R.id.textEditorScrollView);
+    markdownWebView = findViewById(R.id.textEditorMarkdownWebView);
+    markdownWebView.getSettings().setJavaScriptEnabled(false);
 
     final Uri uri = getIntent().getData();
     if (uri != null) {
@@ -178,6 +183,11 @@ public void onCreate(Bundle savedInstanceState) {
       if (savedInstanceState.getBoolean(KEY_MONOFONT)) {
         mainTextView.setTypeface(inputTypefaceMono);
       }
+      // Restore markdown preview state
+      if (savedInstanceState.getBoolean(KEY_MARKDOWN_PREVIEW, false)) {
+        viewModel.setMarkdownPreviewEnabled(true);
+        toggleMarkdownPreview(true);
+      }
       // Restore windowed mode state after rotation
       if (viewModel.isWindowed()) {
         setReadOnly();
@@ -203,6 +213,7 @@ protected void onSaveInstanceState(@NonNull Bundle outState) {
     outState.putInt(KEY_INDEX, mainTextView.getScrollY());
     outState.putString(KEY_ORIGINAL_TEXT, viewModel.getOriginal());
     outState.putBoolean(KEY_MONOFONT, inputTypefaceMono.equals(mainTextView.getTypeface()));
+    outState.putBoolean(KEY_MARKDOWN_PREVIEW, viewModel.getMarkdownPreviewEnabled());
   }
 
   private void checkUnsavedChanges() {
@@ -309,6 +320,11 @@ public boolean onPrepareOptionsMenu(Menu menu) {
     // Hide search in windowed mode (search only works on in-memory text)
     menu.findItem(R.id.find).setVisible(!windowed);
 
+    // Show markdown preview item only for .md/.markdown files
+    MenuItem markdownItem = menu.findItem(R.id.markdown_preview);
+    markdownItem.setVisible(isMarkdownFile());
+    markdownItem.setChecked(viewModel.getMarkdownPreviewEnabled());
+
     menu.findItem(R.id.monofont).setChecked(inputTypefaceMono.equals(mainTextView.getTypeface()));
     return super.onPrepareOptionsMenu(menu);
   }
@@ -362,6 +378,11 @@ public boolean onOptionsItemSelected(MenuItem item) {
     } else if (item.getItemId() == R.id.monofont) {
       item.setChecked(!item.isChecked());
       mainTextView.setTypeface(item.isChecked() ? inputTypefaceMono : inputTypefaceDefault);
+    } else if (item.getItemId() == R.id.markdown_preview) {
+      boolean newState = !item.isChecked();
+      item.setChecked(newState);
+      viewModel.setMarkdownPreviewEnabled(newState);
+      toggleMarkdownPreview(newState);
     } else {
       return false;
     }
@@ -661,7 +682,6 @@ private void observeWindowContent() {
 
               // Determine an anchor: find the text line near the middle of the current viewport
               int oldScrollY = scrollView.getScrollY();
-              Layout oldLayout = mainTextView.getLayout();
 
               // Replace text (TextWatcher will fire but windowed-mode guard skips modification
               // tracking)
@@ -673,7 +693,6 @@ private void observeWindowContent() {
                     Layout newLayout = mainTextView.getLayout();
                     if (newLayout == null) return;
 
-                    TextEditorActivityViewModel.Direction direction;
                     // Infer direction from old scroll position
                     int viewportHeight = scrollView.getHeight();
                     if (oldScrollY > viewportHeight / 2) {
@@ -728,4 +747,46 @@ public void initWindowedScrollListener() {
 
     scrollView.getViewTreeObserver().addOnScrollChangedListener(windowedScrollListener);
   }
+
+  // ── Markdown Preview Helpers ────────────────────────────────────────
+
+  /** Returns true if the currently opened file has a Markdown extension (.md or .markdown). */
+  private boolean isMarkdownFile() {
+    EditableFileAbstraction file = viewModel.getFile();
+    if (file == null) return false;
+    return MarkdownHtmlGenerator.isMarkdownFile(file.name);
+  }
+
+  /**
+   * Toggle between Markdown preview (WebView) and the normal EditText editor.
+   *
+   * @param enabled true to show the WebView with rendered Markdown; false to show EditText
+   */
+  private void toggleMarkdownPreview(boolean enabled) {
+    if (enabled) {
+      renderMarkdownToWebView();
+      scrollView.setVisibility(View.GONE);
+      markdownWebView.setVisibility(View.VISIBLE);
+    } else {
+      markdownWebView.setVisibility(View.GONE);
+      scrollView.setVisibility(View.VISIBLE);
+    }
+    invalidateOptionsMenu();
+  }
+
+  /**
+   * Parse the current EditText content as Markdown using commonmark, render to HTML, and load it
+   * into the WebView.
+   */
+  private void renderMarkdownToWebView() {
+    String markdownSource = "";
+    if (mainTextView.getText() != null) {
+      markdownSource = mainTextView.getText().toString();
+    }
+
+    String bodyHtml = MarkdownHtmlGenerator.renderToHtml(markdownSource);
+    boolean isDark = getAppTheme().equals(AppTheme.DARK) || getAppTheme().equals(AppTheme.BLACK);
+    String fullHtml = MarkdownHtmlGenerator.wrapWithBaseHtml(bodyHtml, isDark);
+    markdownWebView.loadDataWithBaseURL(null, fullHtml, "text/html", "UTF-8", null);
+  }
 }

app/src/main/java/com/amaze/filemanager/ui/activities/texteditor/TextEditorActivityViewModel.kt

@@ -66,6 +66,11 @@ class TextEditorActivityViewModel : ViewModel() {
 
     var file: EditableFileAbstraction? = null
 
+    // ── Markdown preview state ────────────────────────────────────────
+
+    /** Whether Markdown preview mode is currently enabled. */
+    var markdownPreviewEnabled = false
+
     // ── Sliding window state ──────────────────────────────────────────
 
     /** Whether the editor is in windowed (read-only) mode for large files. */

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

@@ -43,23 +43,36 @@
 
     </LinearLayout>
 
-    <ScrollView
-        android:id="@+id/textEditorScrollView"
+    <FrameLayout
+        android:id="@+id/textEditorContentFrame"
         android:layout_width="match_parent"
-        android:layout_height="match_parent"
-        android:fillViewport="true">
+        android:layout_height="match_parent">
 
-        <androidx.appcompat.widget.AppCompatEditText
-            android:id="@+id/textEditorMainEditText"
+        <ScrollView
+            android:id="@+id/textEditorScrollView"
             android:layout_width="match_parent"
-            android:layout_height="wrap_content"
-            android:fontFamily="sans-serif-light"
-            android:gravity="start|top"
-            android:inputType="textCapSentences|textMultiLine"
-            android:lineSpacingExtra="1dp"
-            android:padding="16dp"
-            android:textSize="14sp" />
+            android:layout_height="match_parent"
+            android:fillViewport="true">
 
-    </ScrollView>
+            <androidx.appcompat.widget.AppCompatEditText
+                android:id="@+id/textEditorMainEditText"
+                android:layout_width="match_parent"
+                android:layout_height="wrap_content"
+                android:fontFamily="sans-serif-light"
+                android:gravity="start|top"
+                android:inputType="textCapSentences|textMultiLine"
+                android:lineSpacingExtra="1dp"
+                android:padding="16dp"
+                android:textSize="14sp" />
+
+        </ScrollView>
+
+        <WebView
+            android:id="@+id/textEditorMarkdownWebView"
+            android:layout_width="match_parent"
+            android:layout_height="match_parent"
+            android:visibility="gone" />
+
+    </FrameLayout>
 
 </androidx.appcompat.widget.LinearLayoutCompat>

app/src/main/res/menu/text.xml

@@ -42,4 +42,10 @@
         android:title="@string/monofont"
         android:checkable="true"
         app:showAsAction="never" />
+    <item
+        android:id="@+id/markdown_preview"
+        android:title="@string/markdown_preview"
+        android:checkable="true"
+        android:visible="false"
+        app:showAsAction="never" />
 </menu>

app/src/main/res/values-ja/strings.xml

@@ -479,6 +479,7 @@
     <string name="copy_low_memory">メモリ不足。メモリを解放してください</string>
     <string name="cloud_token_lost">トークンを失われた。もう一度サインインしてください。</string>
     <string name="monofont">等幅フォント</string>
+    <string name="markdown_preview">Markdown プレビュー</string>
     <string name="showHeaders">ヘッダーを表示する</string>
 
 	<!--FROM HERE ON STRINGS ARE EXCLUSIVELY USED IN plurals.xml-->

app/src/main/res/values-uk/strings.xml

@@ -479,6 +479,7 @@
     <string name="copy_low_memory">Не вистачає пам\'яті, будь ласка звільніть оперативну пам\'ять</string>
     <string name="cloud_token_lost">Token втрачено, будь ласка авторизуйтесь знову</string>
     <string name="monofont">Моноширинний шрифт</string>
+    <string name="markdown_preview">Попередній перегляд Markdown</string>
     <string name="showHeaders">Відображати заголовки</string>
 
 	<!--FROM HERE ON STRINGS ARE EXCLUSIVELY USED IN plurals.xml-->