adding the undo/redo functionality

app/src/main/kotlin/com/simplemobiletools/notes/activities/MainActivity.kt

@@ -132,6 +132,8 @@ class MainActivity : SimpleActivity(), ViewPager.OnPageChangeListener {
         when (item.itemId) {
             R.id.open_note -> displayOpenNoteDialog()
             R.id.save_note -> saveNote()
+            R.id.undo -> undo()
+            R.id.redo -> redo()
             R.id.new_note -> displayNewNoteDialog()
             R.id.rename_note -> displayRenameDialog()
             R.id.share -> shareText()
@@ -287,11 +289,11 @@ class MainActivity : SimpleActivity(), ViewPager.OnPageChangeListener {
 
     private fun openFile() {
         FilePickerDialog(this) {
-            openFile(it, true, {
+            openFile(it, true) {
                 OpenFileDialog(this, it.path) {
                     addNewNote(it)
                 }
-            })
+            }
         }
     }
 
@@ -524,6 +526,14 @@ class MainActivity : SimpleActivity(), ViewPager.OnPageChangeListener {
         invalidateOptionsMenu()
     }
 
+    private fun undo() {
+        mAdapter?.undo(view_pager.currentItem)
+    }
+
+    private fun redo() {
+        mAdapter?.redo(view_pager.currentItem)
+    }
+
     private fun getNoteIndexWithId(id: Int): Int {
         for (i in 0 until mNotes.count()) {
             if (mNotes[i].id == id) {

app/src/main/kotlin/com/simplemobiletools/notes/adapters/NotesPagerAdapter.kt

@@ -42,6 +42,10 @@ class NotesPagerAdapter(fm: FragmentManager, val notes: List<Note>, val activity
 
     fun focusEditText(position: Int) = fragments[position]?.focusEditText()
 
+    fun undo(position: Int) = fragments[position]?.undo()
+
+    fun redo(position: Int) = fragments[position]?.redo()
+
     override fun finishUpdate(container: ViewGroup) {
         try {
             super.finishUpdate(container)

app/src/main/kotlin/com/simplemobiletools/notes/fragments/NoteFragment.kt

@@ -4,7 +4,9 @@ import android.graphics.Typeface
 import android.os.Bundle
 import android.support.v4.app.Fragment
 import android.text.Editable
+import android.text.Selection
 import android.text.TextWatcher
+import android.text.style.UnderlineSpan
 import android.text.util.Linkify
 import android.util.TypedValue
 import android.view.Gravity
@@ -22,12 +24,18 @@ import com.simplemobiletools.notes.extensions.getTextSize
 import com.simplemobiletools.notes.extensions.updateWidget
 import com.simplemobiletools.notes.helpers.*
 import com.simplemobiletools.notes.models.Note
+import com.simplemobiletools.notes.models.TextHistory
+import com.simplemobiletools.notes.models.TextHistoryItem
 import kotlinx.android.synthetic.main.fragment_note.*
 import kotlinx.android.synthetic.main.fragment_note.view.*
 import kotlinx.android.synthetic.main.note_view_horiz_scrollable.view.*
 import java.io.File
 
+// text history handling taken from https://gist.github.com/zeleven/0cfa738c1e8b65b23ff7df1fc30c9f7e
 class NoteFragment : Fragment() {
+    private var textHistory = TextHistory()
+
+    private var isUndoOrRedo = false
     private var noteId = 0
     lateinit var note: Note
     lateinit var view: ViewGroup
@@ -161,11 +169,69 @@ class NoteFragment : Fragment() {
         notes_counter.text = words.count { it.isNotEmpty() }.toString()
     }
 
-    private var textWatcher: TextWatcher = object : TextWatcher {
+    fun undo() {
-        override fun beforeTextChanged(s: CharSequence, start: Int, count: Int, after: Int) {
+        val edit = textHistory.getPrevious() ?: return
+
+        val text = view.notes_view.editableText
+        val start = edit.start
+        val end = start + if (edit.after != null) edit.after.length else 0
+
+        isUndoOrRedo = true
+        text.replace(start, end, edit.before)
+        isUndoOrRedo = false
+
+        for (span in text.getSpans(0, text.length, UnderlineSpan::class.java)) {
+            text.removeSpan(span)
         }
 
-        override fun onTextChanged(s: CharSequence?, start: Int, before: Int, count: Int) {
+        Selection.setSelection(text, if (edit.before == null) {
+            start
+        } else {
+            start + edit.before.length
+        })
+    }
+
+    fun redo() {
+        val edit = textHistory.getNext() ?: return
+
+        val text = view.notes_view.editableText
+        val start = edit.start
+        val end = start + if (edit.before != null) edit.before.length else 0
+
+        isUndoOrRedo = true
+        text.replace(start, end, edit.after)
+        isUndoOrRedo = false
+
+        for (o in text.getSpans(0, text.length, UnderlineSpan::class.java)) {
+            text.removeSpan(o)
+        }
+
+        Selection.setSelection(text, if (edit.after == null) {
+            start
+        } else {
+            start + edit.after.length
+        })
+    }
+
+    fun isUndoAvailable() = textHistory.position > 0
+
+    fun isRedoAvailable() = textHistory.position < textHistory.history.size
+
+    private var textWatcher: TextWatcher = object : TextWatcher {
+        private var beforeChange: CharSequence? = null
+        private var afterChange: CharSequence? = null
+
+        override fun beforeTextChanged(s: CharSequence, start: Int, count: Int, after: Int) {
+            if (!isUndoOrRedo) {
+                beforeChange = s.subSequence(start, start + count)
+            }
+        }
+
+        override fun onTextChanged(s: CharSequence, start: Int, before: Int, count: Int) {
+            if (!isUndoOrRedo) {
+                afterChange = s.subSequence(start, start + count)
+                textHistory.add(TextHistoryItem(start, beforeChange!!, afterChange!!))
+            }
         }
 
         override fun afterTextChanged(editable: Editable) {

app/src/main/kotlin/com/simplemobiletools/notes/models/TextHistory.kt

@@ -0,0 +1,35 @@
+package com.simplemobiletools.notes.models
+
+import java.util.*
+
+class TextHistory {
+    var position = 0
+    val history = LinkedList<TextHistoryItem>()
+
+    fun getPrevious(): TextHistoryItem? {
+        if (position == 0) {
+            return null
+        }
+        position--
+        return history[position]
+    }
+
+    fun getNext(): TextHistoryItem? {
+        if (position >= history.size) {
+            return null
+        }
+
+        val item = history[position]
+        position++
+        return item
+    }
+
+    fun add(item: TextHistoryItem) {
+        while (history.size > position) {
+            history.removeLast()
+        }
+
+        history.add(item)
+        position++
+    }
+}

app/src/main/kotlin/com/simplemobiletools/notes/models/TextHistoryItem.kt

@@ -0,0 +1,3 @@
+package com.simplemobiletools.notes.models
+
+data class TextHistoryItem(val start: Int, val before: CharSequence?, val after: CharSequence?)