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Undo / redo edits to a PDF file using Kotlin

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 Kotlin 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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package com.pdftron.android.pdfnetsdksamples.samples

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

import com.pdftron.android.pdfnetsdksamples.OutputListener
import com.pdftron.android.pdfnetsdksamples.PDFNetSample
import com.pdftron.android.pdfnetsdksamples.R
import com.pdftron.android.pdfnetsdksamples.util.Utils
import com.pdftron.common.Matrix2D
import com.pdftron.pdf.ElementBuilder
import com.pdftron.pdf.ElementWriter
import com.pdftron.pdf.Image
import com.pdftron.pdf.PDFDoc
import com.pdftron.sdf.ResultSnapshot
import com.pdftron.sdf.SDFDoc
import java.util.*

//---------------------------------------------------------------------------------------
// The following sample illustrates how to use the UndoRedo API.
//---------------------------------------------------------------------------------------
class UndoRedoTest : PDFNetSample() {
    init {
        setTitle(R.string.sample_undoredo_title)
        setDescription(R.string.sample_undoredo_description)
    }

    override fun run(outputListener: OutputListener?) {
        super.run(outputListener)
        mOutputListener = outputListener
        mFileList.clear()
        printHeader(outputListener!!)
        try {
            // 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.
            val doc = PDFDoc(Utils.getAssetTempFile(PDFNetSample.INPUT_PATH + "newsletter.pdf")!!.absolutePath)

            val undo_manager = doc.undoManager

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

            val snap0_state = snap0.currentState()

            val page = doc.pageCreate()    // Start a new page

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

            // ----------------------------------------------------------
            // Add JPEG image to the file
            val img = Image.create(doc, Utils.getAssetTempFile(PDFNetSample.INPUT_PATH + "peppers.jpg")!!.absolutePath)
            val element = bld.createImage(img, Matrix2D(200.0, 0.0, 0.0, 250.0, 50.0, 500.0))
            writer.writePlacedElement(element)

            writer.end()    // Finish writing to the page
            doc.pagePushFront(page)

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

            if (snap1.previousState().equals(snap0_state)) {
                mOutputListener!!.println("snap1 previous state equals snap0_state; previous state is correct")
            }

            val snap1_state = snap1.currentState()

            doc.save(Utils.createExternalFile("addimage.pdf", mFileList).absolutePath, SDFDoc.SaveMode.INCREMENTAL, null)

            if (undo_manager.canUndo()) {
                val undo_snap: ResultSnapshot
                undo_snap = undo_manager.undo()

                doc.save(Utils.createExternalFile("addimage_undone.pdf", mFileList).absolutePath, SDFDoc.SaveMode.INCREMENTAL, null)

                val 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()) {
                    val redo_snap = undo_manager.redo()

                    doc.save(Utils.createExternalFile("addimage_redone.pdf", mFileList).absolutePath, 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")
                    }

                    val redo_snap_state = redo_snap.currentState()

                    if (redo_snap_state.equals(snap1_state)) {
                        mOutputListener!!.println("Snap1 and redo_snap are equal; redo was successful")
                    }
                } else {
                    mOutputListener!!.println("Problem encountered - cannot redo.")
                }
            } else {
                mOutputListener!!.println("Problem encountered - cannot undo.")
            }

            // Calling Terminate when PDFNet is no longer in use is a good practice, but
            // is not required.
        } catch (e: Exception) {
            mOutputListener!!.printError(e.stackTrace)
        }

        for (file in mFileList) {
            addToFileList(file)
        }
        printFooter(outputListener)
    }

    companion object {

        private var mOutputListener: OutputListener? = null

        private val mFileList = ArrayList<String>()
    }

}
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