android best way to implement undo/redo on ArrayList

Question

sorry about the general nature of this question but i don't want to start with any assumptions for fear of missing the big picture

i have a document editing kind of app (music notation) and want to implement undo and redo. all the relevant data is held in this

static  ArrayList <TTEvt> mEvList;

in my windows/MFC app, i just serialize the data structure and put it on a stack. it uses lots of memory but easy and foolproof.

so i was wondering what's the bast way to save and restore my ArrayList in android?

thanks

Answer 1

so here is the working multilevel undo/redo code. it works fine but it is sloooow. i took the gzip out which helps a bit but it basically makes my UI unusable. but at least it works, i can try optimize from here.

    static LinkedList<byte[]> undoStack = new LinkedList<byte[]>();
    static LinkedList<byte[]> redoStack = new LinkedList<byte[]>();

    public static void addUndoCheckpoint() {

        long start = System.currentTimeMillis();

        byte[] byteBuf = null;
        try {
            ByteArrayOutputStream baos = new ByteArrayOutputStream();
            // GZIPOutputStream gzipOut = new GZIPOutputStream(baos);
            ObjectOutputStream objectOut = new ObjectOutputStream(baos);
            for (TTEvt ev : Doc.mEvList)
                objectOut.writeObject(ev);
            objectOut.close();
            byteBuf = baos.toByteArray();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

        undoStack.push(byteBuf);

        if (undoStack.size() > 10)
            undoStack.removeLast(); // limit size

        redoStack.clear();

        long end = System.currentTimeMillis();
        Log.d("MainView", "addUndoCheckpoint time=" + (end - start) + "mS");
    }

    public static void doUndo() {

        if (undoStack.size() == 0)
            return;

        // push current state onto redo stack first
        byte[] byteBuf = null;
        try {
            ByteArrayOutputStream baos = new ByteArrayOutputStream();
            // GZIPOutputStream gzipOut = new GZIPOutputStream(baos);
            ObjectOutputStream objectOut = new ObjectOutputStream(baos);
            for (TTEvt ev : Doc.mEvList)
                objectOut.writeObject(ev);
            objectOut.close();
            byteBuf = baos.toByteArray();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        redoStack.push(byteBuf); // push current state onto redo stack
        if (redoStack.size() > 10)
            redoStack.removeLast();

        // now undo
        mEvList.clear();
        byteBuf = undoStack.pop();

        ObjectInputStream objectIn = null;
        try {
            ByteArrayInputStream bais = new ByteArrayInputStream(byteBuf);
            // GZIPInputStream gzipIn;
            // gzipIn = new GZIPInputStream(bais);
            objectIn = new ObjectInputStream(bais);
            while (true) {
                TTEvt ev = (TTEvt) objectIn.readObject();
                if (ev == null)
                    break;
                Doc.mEvList.add(ev);
            }
            objectIn.close();
        } catch (IOException e) {
            // this is the normal exit
            if (objectIn != null) {
                try {
                    objectIn.close();
                } catch (IOException e1) {
                    // TODO Auto-generated catch block
                    e1.printStackTrace();
                }
            }
            // e.printStackTrace();
        } catch (ClassNotFoundException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

        MainView.forceTotalRedraw();
    }

    public static void doRedo() {

        if (redoStack.size() == 0)
            return;

        // push current state onto undo stack first so we can undo the redo
        byte[] byteBuf = null;
        try {
            ByteArrayOutputStream baos = new ByteArrayOutputStream();
            // GZIPOutputStream gzipOut = new GZIPOutputStream(baos);
            ObjectOutputStream objectOut = new ObjectOutputStream(baos);
            for (TTEvt ev : Doc.mEvList)
                objectOut.writeObject(ev);
            objectOut.close();
            byteBuf = baos.toByteArray();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        undoStack.push(byteBuf); // push current state onto redo stack
        if (undoStack.size() > 10)
            undoStack.removeLast();

        // now redo
        mEvList.clear();
        byteBuf = redoStack.pop();

        ObjectInputStream objectIn = null;
        try {
            ByteArrayInputStream bais = new ByteArrayInputStream(byteBuf);
            // GZIPInputStream gzipIn;
            // gzipIn = new GZIPInputStream(bais);
            objectIn = new ObjectInputStream(bais);
            while (true) {
                TTEvt ev = (TTEvt) objectIn.readObject();
                if (ev == null)
                    break;
                Doc.mEvList.add(ev);
            }
            objectIn.close();
        } catch (IOException e) {
            // this is the normal exit
            if (objectIn != null) {
                try {
                    objectIn.close();
                } catch (IOException e1) {
                    // TODO Auto-generated catch block
                    e1.printStackTrace();
                }
            }
            // e.printStackTrace();
        } catch (ClassNotFoundException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

        MainView.forceTotalRedraw();

    }
}

note that your should call addUndoCheckpoint BEFORE you change ArrayList. i'm currently working on a version that you can call after you change ArrayList. this has the advantage that you can do addUndoCheckpoint in the background and not slow down the UI.