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Undo / redo edits to a PDF file using Java (Android)

The PDFTron SDK has a low-level facility for undo and redo operations. It is a API that applies to any edits made to a particular document (not just annotations). This sample Java (Android) code shows how to use PDFTron SDK to walk back and forth on a fully general, bit-exact list of document states. Saving changes in a mode that is not 'incremental' will wipe out the undo-redo state list; the API will not be able to access old snapshots anymore. See the undoing and redoing guide for more information.

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// Copyright (c) 2001-2019 by PDFTron Systems Inc. All Rights Reserved.
// Consult legal.txt regarding legal and license information.

import com.pdftron.common.Matrix2D;
import com.pdftron.pdf.Element;
import com.pdftron.pdf.ElementBuilder;
import com.pdftron.pdf.ElementWriter;
import com.pdftron.pdf.Image;
import com.pdftron.pdf.PDFDoc;
import com.pdftron.pdf.Page;
import com.pdftron.sdf.DocSnapshot;
import com.pdftron.sdf.ResultSnapshot;
import com.pdftron.sdf.SDFDoc;
import com.pdftron.sdf.UndoManager;

import java.util.ArrayList;

// The following sample illustrates how to use the UndoRedo API.
public class UndoRedoTest extends PDFNetSample {

	private static OutputListener mOutputListener;

	private static ArrayList<String> mFileList = new ArrayList<>();

    public UndoRedoTest() {

	public void run(OutputListener outputListener) {;
		mOutputListener = outputListener;
            // The first step in every application using PDFNet is to initialize the
            // library and set the path to common PDF resources. The library is usually
            // initialized only once, but calling Initialize() multiple times is also fine.

			// Open the PDF document.
			PDFDoc doc = new PDFDoc(Utils.getAssetTempFile(INPUT_PATH + "newsletter.pdf").getAbsolutePath());

			UndoManager undo_manager = doc.getUndoManager();

			// Take a snapshot to which we can undo after making changes.
			ResultSnapshot snap0 = undo_manager.takeSnapshot();

			DocSnapshot snap0_state = snap0.currentState();
			Page page = doc.pageCreate();	// Start a new page

			ElementBuilder bld = new ElementBuilder();		// Used to build new Element objects
			ElementWriter writer = new ElementWriter();		// Used to write Elements to the page	
			writer.begin(page);		// Begin writing to this page

			// ----------------------------------------------------------
			// Add JPEG image to the file
			Image img = Image.create(doc, Utils.getAssetTempFile(INPUT_PATH + "peppers.jpg").getAbsolutePath());
			Element element = bld.createImage(img, new Matrix2D(200, 0, 0, 250, 50, 500));

			writer.end();	// Finish writing to the page

			// Take a snapshot after making changes, so that we can redo later (after undoing first).
			ResultSnapshot snap1 = undo_manager.takeSnapshot();

			if (snap1.previousState().equals(snap0_state))
				mOutputListener.println("snap1 previous state equals snap0_state; previous state is correct");
			DocSnapshot snap1_state = snap1.currentState();"addimage.pdf", mFileList).getAbsolutePath(), SDFDoc.SaveMode.INCREMENTAL, null);

			if (undo_manager.canUndo())
				ResultSnapshot undo_snap;
				undo_snap = undo_manager.undo();"addimage_undone.pdf", mFileList).getAbsolutePath(), SDFDoc.SaveMode.INCREMENTAL, null);

				DocSnapshot undo_snap_state = undo_snap.currentState();

				if (undo_snap_state.equals(snap0_state))
					mOutputListener.println("undo_snap_state equals snap0_state; undo was successful");
				if (undo_manager.canRedo())
					ResultSnapshot redo_snap = undo_manager.redo();"addimage_redone.pdf", mFileList).getAbsolutePath(), SDFDoc.SaveMode.INCREMENTAL, null);

					if (redo_snap.previousState().equals(undo_snap_state))
						mOutputListener.println("redo_snap previous state equals undo_snap_state; previous state is correct");
					DocSnapshot redo_snap_state = redo_snap.currentState();
					if (redo_snap_state.equals(snap1_state))
						mOutputListener.println("Snap1 and redo_snap are equal; redo was successful");
					mOutputListener.println("Problem encountered - cannot redo.");
				mOutputListener.println("Problem encountered - cannot undo.");

            // Calling Terminate when PDFNet is no longer in use is a good practice, but
            // is not required.
		catch (Exception e) 

		for (String file : mFileList) {


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