filebrowser: Improve performance a lot (speed of file list appearing)

Probably improvement for #1120 #1176

app/src/main/java/net/gsantner/opoc/ui/FilesystemViewerAdapter.java

@@ -36,6 +36,7 @@
 import net.gsantner.markor.R;
 import net.gsantner.opoc.util.ContextUtils;
 import net.gsantner.opoc.util.FileUtils;
+import net.gsantner.opoc.util.FileWithCachedData;
 
 import java.io.File;
 import java.io.FilenameFilter;
@@ -468,6 +469,8 @@ public void loadFolder(final File folder) {
             @Override
             public void run() {
                 synchronized (LOAD_FOLDER_SYNC_OBJECT) {
+                    ArrayList<File> oldAdapterData = new ArrayList<>();
+                    Collections.copy(_adapterData, oldAdapterData);
                     _currentFolder = folder;
                     _adapterData.clear();
                     _virtualMapping.clear();
@@ -546,6 +549,15 @@ public void run() {
                         }
                     }
 
+                    // Optimization - convert found File's to FileWithCachedData
+                    // Sorting invokes a lot of filesystem i/o calls which do consume much time
+                    // Changing sort order: Reuse information if available
+                    for (int i = 0; i < _adapterData.size(); i++) {
+                        final File o = _adapterData.remove(i);
+                        final int at = oldAdapterData.indexOf(o);
+                        _adapterData.add(i, at >= 0 ? oldAdapterData.get(at) : new FileWithCachedData(o));
+                    }
+
                     try {
                         Collections.sort(_adapterData, FilesystemViewerAdapter.this);
                     } catch (IllegalArgumentException ignored) {

app/src/main/java/net/gsantner/opoc/util/FileWithCachedData.java

@@ -0,0 +1,142 @@
+//
+// License of this file FileWithCachedData.java: Public Domain
+// Created by Gregor Santner, 2021 - https://gsantner.net
+//
+
+package net.gsantner.opoc.util;
+
+import android.support.annotation.NonNull;
+import android.support.annotation.Nullable;
+
+import java.io.File;
+import java.net.URI;
+
+@SuppressWarnings({"unused", "RedundantSuppression"})
+public class FileWithCachedData extends File {
+    private Integer cHashCode;
+    private Boolean cCanRead, cCanWrite, cExists, cIsAbsolute, cIsDirectory, cIsFile;
+    private Long cLastModified, cLength;
+    private String cAbsolutePath;
+    private File cAbsoluteFile;
+    private String[] cList;
+
+    public FileWithCachedData(@NonNull String pathname) {
+        super(pathname);
+    }
+
+    public FileWithCachedData(@Nullable String parent, @NonNull String child) {
+        super(parent, child);
+    }
+
+    public FileWithCachedData(@Nullable File parent, @NonNull String child) {
+        super(parent, child);
+    }
+
+    public FileWithCachedData(@NonNull URI uri) {
+        super(uri);
+    }
+
+    public FileWithCachedData(@NonNull File f) {
+        super(f.getPath());
+    }
+
+
+    @NonNull
+    @Override
+    public synchronized String getAbsolutePath() {
+        if (cAbsolutePath == null) {
+            cAbsolutePath = super.getAbsolutePath();
+        }
+        return cAbsolutePath;
+    }
+
+    @NonNull
+    @Override
+    public File getAbsoluteFile() {
+        if (cAbsoluteFile == null) {
+            cAbsoluteFile = super.getAbsoluteFile();
+        }
+        return cAbsoluteFile;
+    }
+
+    @Override
+    public boolean isAbsolute() {
+        if (cIsAbsolute == null) {
+            cIsAbsolute = super.isAbsolute();
+        }
+        return cIsAbsolute;
+    }
+
+    @Override
+    public boolean canRead() {
+        if (cCanRead == null) {
+            cCanRead = super.canRead();
+        }
+        return cCanRead;
+    }
+
+    @Override
+    public boolean canWrite() {
+        if (cCanWrite == null) {
+            cCanWrite = super.canWrite();
+        }
+        return cCanWrite;
+    }
+
+    @Override
+    public boolean exists() {
+        if (cExists == null) {
+            cExists = super.exists();
+        }
+        return cExists;
+    }
+
+    @Override
+    public boolean isDirectory() {
+        if (cIsDirectory == null) {
+            cIsDirectory = super.isDirectory();
+        }
+        return cIsDirectory;
+    }
+
+    @Override
+    public boolean isFile() {
+        if (cIsFile == null) {
+            cIsFile = super.isFile();
+        }
+        return cIsFile;
+    }
+
+    @Override
+    public long lastModified() {
+        if (cLastModified == null) {
+            cLastModified = super.lastModified();
+        }
+        return cLastModified;
+    }
+
+    @Override
+    public long length() {
+        if (cLength == null) {
+            cLength = super.length();
+        }
+        return cLength;
+    }
+
+    @Nullable
+    @Override
+    public String[] list() {
+        if (cList == null) {
+            cList = super.list();
+        }
+        return cList;
+    }
+
+    @Override
+    public int hashCode() {
+        if (cHashCode == null) {
+            cHashCode = super.hashCode();
+        }
+        return cHashCode;
+    }
+}