add a dialog for selecting folder thumbnail grouping

app/src/main/kotlin/com/simplemobiletools/gallery/activities/MediaActivity.kt

@@ -217,6 +217,7 @@ class MediaActivity : SimpleActivity(), MediaAdapter.MediaOperationsListener {
             R.id.open_camera -> launchCamera()
             R.id.folder_view -> switchToFolderView()
             R.id.change_view_type -> changeViewType()
+            R.id.group -> showGroupByDialog()
             R.id.hide_folder -> tryHideFolder()
             R.id.unhide_folder -> unhideFolder()
             R.id.exclude_folder -> tryExcludeFolder()
@@ -416,6 +417,27 @@ class MediaActivity : SimpleActivity(), MediaAdapter.MediaOperationsListener {
         }
     }
 
+    private fun showGroupByDialog() {
+        val items = arrayListOf(
+                RadioItem(GROUP_BY_NONE, getString(R.string.do_not_group_files)),
+                RadioItem(GROUP_BY_LAST_MODIFIED, getString(R.string.by_last_modified)),
+                RadioItem(GROUP_BY_DATE_TAKEN, getString(R.string.by_date_taken)),
+                RadioItem(GROUP_BY_FILE_TYPE, getString(R.string.by_file_type)),
+                RadioItem(GROUP_BY_EXTENSION, getString(R.string.by_extension))
+        )
+
+        if (mShowAll) {
+            items.add(RadioItem(GROUP_BY_FOLDER, getString(R.string.by_folder)))
+        }
+
+        RadioGroupDialog(this, items, config.groupBy) {
+            config.groupBy = it as Int
+            invalidateOptionsMenu()
+            media_grid.adapter = null
+            setupAdapter()
+        }
+    }
+
     private fun tryHideFolder() {
         if (config.wasHideFolderTooltipShown) {
             hideFolder()

app/src/main/kotlin/com/simplemobiletools/gallery/helpers/Config.kt

@@ -326,4 +326,8 @@ class Config(context: Context) : BaseConfig(context) {
     var wereFavoritesPinned: Boolean
         get() = prefs.getBoolean(WERE_FAVORITES_PINNED, false)
         set(wereFavoritesPinned) = prefs.edit().putBoolean(WERE_FAVORITES_PINNED, wereFavoritesPinned).apply()
+
+    var groupBy: Int
+        get() = prefs.getInt(GROUP_BY, GROUP_BY_NONE)
+        set(groupBy) = prefs.edit().putInt(GROUP_BY, groupBy).apply()
 }

app/src/main/kotlin/com/simplemobiletools/gallery/helpers/Constants.kt

@@ -50,6 +50,7 @@ const val WAS_OTG_HANDLED = "was_otg_handled"
 const val TEMP_SKIP_DELETE_CONFIRMATION = "temp_skip_delete_confirmation"
 const val BOTTOM_ACTIONS = "bottom_actions"
 const val WERE_FAVORITES_PINNED = "were_favorites_pinned"
+const val GROUP_BY = "group_by"
 
 // slideshow
 const val SLIDESHOW_INTERVAL = "slideshow_interval"
@@ -112,3 +113,10 @@ const val TYPE_RAWS = 8
 const val LOCAITON_INTERNAL = 1
 const val LOCATION_SD = 2
 const val LOCATION_OTG = 3
+
+const val GROUP_BY_NONE = 0
+const val GROUP_BY_LAST_MODIFIED = 1
+const val GROUP_BY_DATE_TAKEN = 2
+const val GROUP_BY_FILE_TYPE = 3
+const val GROUP_BY_EXTENSION = 4
+const val GROUP_BY_FOLDER = 5

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

@@ -31,6 +31,10 @@
         android:icon="@drawable/ic_camera"
         android:title="@string/open_camera"
         app:showAsAction="ifRoom"/>
+    <item
+        android:id="@+id/group"
+        android:title="@string/group_by"
+        app:showAsAction="never"/>
     <item
         android:id="@+id/change_view_type"
         android:title="@string/change_view_type"