cache 30 images per folder for faster loading the second time

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

@@ -130,6 +130,7 @@ class MainActivity : SimpleActivity(), DirectoryAdapter.DirOperationsListener {
         if (mIsGettingDirs)
             return
 
+        mIsGettingDirs = true
         val token = object : TypeToken<List<Directory>>() {}.type
         val dirs = Gson().fromJson<ArrayList<Directory>>(config.directories, token) ?: ArrayList<Directory>(1)
         if (dirs.size == 0) {
@@ -138,7 +139,6 @@ class MainActivity : SimpleActivity(), DirectoryAdapter.DirOperationsListener {
             gotDirectories(dirs)
         }
 
-        mIsGettingDirs = true
         GetDirectoriesAsynctask(applicationContext, mIsPickVideoIntent || mIsGetVideoContentIntent, mIsPickImageIntent || mIsGetImageContentIntent) {
             gotDirectories(it)
         }.execute()

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

@@ -13,6 +13,8 @@ import android.view.MenuItem
 import com.bumptech.glide.Glide
 import com.bumptech.glide.request.animation.GlideAnimation
 import com.bumptech.glide.request.target.SimpleTarget
+import com.google.gson.Gson
+import com.google.gson.reflect.TypeToken
 import com.simplemobiletools.commons.extensions.*
 import com.simplemobiletools.gallery.R
 import com.simplemobiletools.gallery.adapters.MediaAdapter
@@ -164,6 +166,14 @@ class MediaActivity : SimpleActivity(), MediaAdapter.MediaOperationsListener {
             return
 
         mIsGettingMedia = true
+        val token = object : TypeToken<List<Medium>>() {}.type
+        val media = Gson().fromJson<ArrayList<Medium>>(config.loadFolderMedia(mPath), token) ?: ArrayList<Medium>(1)
+        if (media.size == 0) {
+            media_holder.isRefreshing = true
+        } else {
+            gotMedia(media)
+        }
+
         GetMediaAsynctask(applicationContext, mPath, mIsGetVideoIntent, mIsGetImageIntent, mShowAll) {
             gotMedia(it)
         }.execute()
@@ -277,6 +287,13 @@ class MediaActivity : SimpleActivity(), MediaAdapter.MediaOperationsListener {
 
         mMedia = media
         initializeGallery()
+        storeFolder()
+    }
+
+    private fun storeFolder() {
+        val subList = mMedia.subList(0, Math.min(30, mMedia.size))
+        val json = Gson().toJson(subList)
+        config.saveFolderMedia(mPath, json)
     }
 
     override fun refreshItems() {

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

@@ -68,6 +68,12 @@ class Config(context: Context) : BaseConfig(context) {
 
     fun getIsFolderHidden(path: String) = hiddenFolders.contains(path)
 
+    fun saveFolderMedia(path: String, json: String) {
+        prefs.edit().putString(SAVE_FOLDER_PREFIX + path, json).apply()
+    }
+
+    fun loadFolderMedia(path: String) = prefs.getString(SAVE_FOLDER_PREFIX + path, "")
+
     var hiddenFolders: MutableSet<String>
         get() = prefs.getStringSet(HIDDEN_FOLDERS, HashSet<String>())
         set(hiddenFolders) = prefs.edit().remove(HIDDEN_FOLDERS).putStringSet(HIDDEN_FOLDERS, hiddenFolders).apply()

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

@@ -14,6 +14,7 @@ val DIR_COLUMN_CNT = "dir_column_cnt"
 val MEDIA_COLUMN_CNT = "media_column_cnt"
 val SHOW_ALL = "show_all"   // display images and videos from all folders together
 val SHOW_MEDIA = "show_media"
+val SAVE_FOLDER_PREFIX = "folder_"
 
 // sorting
 val SORT_BY_NAME = 1