Concept export messages (untested)

app/src/main/java/eu/faircode/email/FragmentFolders.java

@@ -20,12 +20,14 @@ package eu.faircode.email;
 */
 
 import android.app.Dialog;
+import android.content.ContentResolver;
 import android.content.Context;
 import android.content.DialogInterface;
 import android.content.Intent;
 import android.content.SharedPreferences;
 import android.graphics.Color;
 import android.graphics.Rect;
+import android.net.Uri;
 import android.os.Bundle;
 import android.text.TextUtils;
 import android.view.LayoutInflater;
@@ -55,17 +57,27 @@ import androidx.swiperefreshlayout.widget.SwipeRefreshLayout;
 
 import com.google.android.material.floatingactionbutton.FloatingActionButton;
 import com.google.android.material.snackbar.Snackbar;
+import com.sun.mail.mbox.MboxProvider;
 
 import org.json.JSONException;
 import org.json.JSONObject;
 
+import java.io.File;
+import java.io.FileInputStream;
 import java.io.IOException;
+import java.io.OutputStream;
 import java.text.NumberFormat;
+import java.util.ArrayList;
 import java.util.Calendar;
 import java.util.Collections;
 import java.util.List;
+import java.util.Properties;
 
+import javax.mail.Folder;
+import javax.mail.Message;
 import javax.mail.MessagingException;
+import javax.mail.Session;
+import javax.mail.Store;
 
 import static android.app.Activity.RESULT_OK;
 
@@ -100,6 +112,7 @@ public class FragmentFolders extends FragmentBase {
     static final int REQUEST_EMPTY_FOLDER = 2;
     static final int REQUEST_DELETE_FOLDER = 3;
     static final int REQUEST_EXECUTE_RULES = 4;
+    static final int REQUEST_EXPORT_MESSAGES = 5;
 
     @Override
     public void onCreate(Bundle savedInstanceState) {
@@ -603,6 +616,10 @@ public class FragmentFolders extends FragmentBase {
                     if (resultCode == RESULT_OK && data != null)
                         onExecuteRules(data.getBundleExtra("args"));
                     break;
+                case REQUEST_EXPORT_MESSAGES:
+                    if (resultCode == RESULT_OK && data != null)
+                        onExportMessages(data.getData());
+                    break;
             }
         } catch (Throwable ex) {
             Log.e(ex);
@@ -833,6 +850,86 @@ public class FragmentFolders extends FragmentBase {
         }.execute(this, args, "folder:rules");
     }
 
+    private void onExportMessages(Uri uri) {
+        long id = getArguments().getLong("selected_folder", -1L);
+
+        Bundle args = new Bundle();
+        args.putLong("id", id);
+        args.putParcelable("uri", uri);
+
+        new SimpleTask<Void>() {
+            @Override
+            protected void onPreExecute(Bundle args) {
+                ToastEx.makeText(getContext(), R.string.title_executing, Toast.LENGTH_LONG).show();
+            }
+
+            @Override
+            protected void onPostExecute(Bundle args) {
+                ToastEx.makeText(getContext(), R.string.title_completed, Toast.LENGTH_LONG).show();
+            }
+
+            @Override
+            protected Void onExecute(Context context, Bundle args) throws Throwable {
+                long fid = args.getLong("id");
+                Uri uri = args.getParcelable("uri");
+
+                if (!"content".equals(uri.getScheme())) {
+                    Log.w("Export uri=" + uri);
+                    throw new IllegalArgumentException(context.getString(R.string.title_no_stream));
+                }
+
+                DB db = DB.getInstance(context);
+                List<Long> ids = db.message().getMessageIdsByFolder(fid);
+                if (ids == null)
+                    ids = new ArrayList<>();
+
+                File tmp = new File(context.getCacheDir(), "export_" + fid + ".mbox");
+                tmp.createNewFile();
+
+                System.setProperty("mail.mbox.locktype", "none");
+
+                Properties iprops = new Properties();
+                Session isession = Session.getDefaultInstance(iprops);
+                isession.addProvider(new MboxProvider());
+
+                Store istore = isession.getStore("mbox");
+                istore.connect();
+
+                try {
+                    Folder ifolder = istore.getDefaultFolder().getFolder(tmp.getAbsolutePath());
+                    ifolder.open(Folder.READ_WRITE);
+
+                    for (long id : ids) {
+                        EntityMessage message = db.message().getMessage(id);
+                        if (message == null)
+                            continue;
+
+                        Message imessage = MessageHelper.from(context, message, null, isession, false);
+                        ifolder.appendMessages(new Message[]{imessage});
+                    }
+
+                } finally {
+                    istore.close();
+                }
+
+                ContentResolver resolver = context.getContentResolver();
+                try (OutputStream out = resolver.openOutputStream(uri)) {
+                    Helper.copy(new FileInputStream(tmp), out);
+                }
+
+                if (!BuildConfig.DEBUG)
+                    tmp.delete();
+
+                return null;
+            }
+
+            @Override
+            protected void onException(Bundle args, Throwable ex) {
+                Log.unexpectedError(getParentFragmentManager(), ex);
+            }
+        }.execute(this, args, "folder:export");
+    }
+
     public static class FragmentDialogApply extends FragmentDialogBase {
         @NonNull
         @Override